Catalyst 3550 Multilayer Switch Software Configuration Guide, 12.1(4)EA1
Configuring IP Unicast Routing
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Configuring IP Unicast Routing

Table Of Contents

Configuring IP Unicast Routing

Understanding Routing

Steps for Configuring Routing

Configuring IP Addressing

Default Addressing Configuration

Assigning IP Addresses to Network Interfaces

Use of Subnet Zero

Classless Routing

Configuring Address Resolution Methods

Define a Static ARP Cache

Set ARP Encapsulation

Enable Proxy ARP

Configure HP Probe Proxy

Routing Assistance When IP Routing is Disabled

Proxy ARP

Default Gateway

ICMP Router Discovery Protocol (IRDP)

Configuring Broadcast Packet Handling

Enabling Directed Broadcast-to-Physical Broadcast Translation

Forwarding UDP Broadcast Packets and Protocols

Establishing an IP Broadcast Address

Flooding IP Broadcasts

Monitoring and Maintaining IP Addressing

Enabling IP Routing

Configuring RIP

RIP Authentication

Summary Addresses and Split Horizon

Configuring IGRP

Load Balancing and Traffic Distribution Control

Split Horizon

Configuring OSPF

OSPF Interface Parameters

OSPF Area Parameters

Other OSPF Behavior Parameters

Change LSA Group Pacing

Loopback Interface

Monitoring OSPF

Configuring EIGRP

EIGRP Router Mode Commands

EIGRP Interface Mode Commands

Configure EIGRP Route Authentication

Monitoring and Maintaining EIGRP

Configuring Protocol-Independent Features

Configuring Cisco Express Forwarding

Configuring the Number of Equal-Cost Routing Paths

Configuring Static Routes

Specifying Default Routes

Specifying a Default Network

Redistributing Routing Information

Filtering Routing Information

Setting Passive Interfaces

Controlling Advertising and Processing in Routing Updates

Filtering Sources of Routing Information

Managing Authentication Keys

Monitoring and Maintaining the IP Network


Configuring IP Unicast Routing


This chapter describes how to configure IP unicast routing on your multilayer switch. To use this feature, you must have the enhanced multilayer switch image installed on your switch.


Note For more detailed IP unicast configuration information, refer to the Cisco IOS IP and IP Routing Configuration Guide for Release 12.1. For complete syntax and usage information for the commands used in this chapter, refer to the Cisco IOS IP and IP Routing Command Reference for Release 12.1.


This chapter consists of these sections:

Understanding Routing

Steps for Configuring Routing

Configuring IP Addressing

Enabling IP Routing

Configuring RIP

Configuring IGRP

Configuring OSPF

Configuring EIGRP

Configuring Protocol-Independent Features

Monitoring and Maintaining the IP Network


Note When configuring routing parameters on the switch, to allocate system resources to maximize the number of unicast routes allowed, you can use the sdm prefer routing global configuration command to set the Switch Database Management feature to the routing template. For more information on the SDM templates, refer to the "Optimizing System Resources for User-Selected Features" section.


Understanding Routing

Network devices in different VLANs cannot communicate with one another without a Layer 3 device (router) to route traffic between the VLANs. Routers can perform routing in three different ways:

By using default routing

By using preprogrammed static routes for the traffic

By dynamically calculating routes by using a routing protocol

Default routing refers to sending traffic with a destination unknown to the router to a default outlet or destination.

Static routing forwards packets from predetermined ports through a single path into and out of a network. Static routing is secure and uses little bandwidth, but does not automatically respond to changes in the network, such as link failure, and therefore, might result in unreachable destinations. As networks grow, static routing becomes a labor-intensive liability.

Dynamic routing protocols are used by routers to dynamically calculate the best route for forwarding traffic. There are two types of dynamic routing protocols:

Distance-vector protocols maintain routing tables with distance values of networked resources, and routers periodically pass these tables to their neighbors. Distance-vector protocols use one or a series of metrics for calculating the best routes. These protocols are easy to configure and use.

Routers using link-state protocols maintain a complex database of network topology, based on the exchange of link-state advertisements (LSAs) between routers. LSAs are triggered by an event in the network, which speeds up the convergence time or time required to respond to these changes. Link-state protocols respond quickly to topology changes, but require greater bandwidth and more resources than distance-vector protocols.

Distance-vector protocols supported by the Catalyst 3550 switch are Routing Information Protocol (RIP), which uses a single distance metric (cost) to determine the best path, and Interior Gateway Routing Protocol (IGRP), which uses a series of metrics. The switch also supports the Open Shortest Path First (OSPF) link-state protocol and Enhanced IGRP (EIGRP), which adds some link-state routing features to traditional IGRP to improve efficiency.

In some network environments, VLANs are associated with individual networks or subnetworks. In an IP network, each subnetwork is mapped to an individual VLAN. Configuring VLANs helps control the size of the broadcast domain and keeps local traffic local. However, when an end station in one VLAN needs to communicate with an end station in another VLAN, inter-VLAN communication is required. This communication is supported by inter-VLAN routing. You configure one or more routers to route traffic to the appropriate destination VLAN.

Figure 20-1 shows a basic routing topology. Switch A is in VLAN 10, and Switch B is in VLAN 20. The router has an interface in each VLAN.

Figure 20-1 Routing Topology Example

When Host A in VLAN 10 needs to communicate with Host B in VLAN 10, it sends a packet addressed to that host. Switch A forwards the packet directly to Host B, without sending it to the router.

When Host A sends a packet to Host C in VLAN 20, Switch A forwards the packet to the router, which receives the traffic on the VLAN 10 interface. The router checks the routing table, determines the correct outgoing interface, and forwards the packet on the VLAN 20 interface to Switch B. Switch B receives the packet and forwards it to Host C.

Steps for Configuring Routing

By default, IP routing is disabled on the Catalyst 3550 switch, and you must enable it before routing can take place. For detailed IP routing configuration information, refer to the Cisco IOS IP and IP Routing Configuration Guide for Release 12.1.

In the following procedures, the specified interface must be one of these Layer 3 interfaces:

A routed port: a physical port configured as a Layer 3 port by using the no switchport interface configuration command.

A switch virtual interface (SVI): a VLAN interface created by using the interface vlan vlan_id global configuration command and by default a Layer 3 interface.

An EtherChannel port channel in Layer 3 mode: a port-channel logical interface created by using the interface port-channel port-channel-number global configuration command and binding the Ethernet interface into the channel group. For more information, see the "Configuring Layer 3 EtherChannels" section.


Note On a Catalyst 3550-12T switch, we recommend configuring a maximum total of 16 SVIs and routed ports at one time to allow enough system resources to support other features. If requirements exceed the system hardware allocation, processing overflow is sent to the CPU, degrading performance. Refer to the "Optimizing System Resources for User-Selected Features" section for more information about feature combinations.


All Layer 3 interfaces must have IP addresses assigned to them. Refer to the "Assigning IP Addresses to Network Interfaces" section.

Configuring routing consists of several main procedures:

To support VLAN interfaces, create and configure VLANs on the switch, and assign VLAN membership to Layer 2 interfaces. For more information, see "Creating and Maintaining VLANs."

Configure Layer 3 interfaces.

Enable IP routing on the switch.

Assign IP addresses to the Layer 3 interfaces.

Enable selected routing protocols on the switch.

Configure routing protocol parameters (optional).

Configuring IP Addressing

A required task for configuring IP routing is to assign IP addresses to Layer 3 network interfaces to enable the interfaces and allow communication with the hosts on those interfaces that use IP. These sections describe how to configure various IP addressing features. Assigning IP addresses to the interface is required; the other procedures are optional.

Default Addressing Configuration

Assigning IP Addresses to Network Interfaces

Configuring Address Resolution Methods

Routing Assistance When IP Routing is Disabled

Configuring Broadcast Packet Handling

Monitoring and Maintaining IP Addressing

Default Addressing Configuration

Table 20-1 shows the default addressing configuration.

Table 20-1 Default Addressing Configuration 

Feature
Default Setting

IP address

None defined.

ARP

No permanent entries in the Address Resolution Protocol (ARP) cache.

Encapsulation: Standard Ethernet-style ARP.

Timeout: 14400 seconds (4 hours).

IP broadcast address

255.255.255.255 (all ones).

IP classless routing

Enabled.

IP default gateway

Disabled.

IP directed broadcast

Disabled (all IP directed broadcasts are dropped).

IP domain

Domain list: No domain names defined.

Domain lookup: Enabled.

Domain name: Enabled.

IP forward-protocol

If a helper address is defined or User Datagram Protocol (UDP) flooding is configured, UDP forwarding is enabled on default ports.

Any-local-broadcast: Disabled.

Spanning Tree Protocol (STP): Disabled.

Turbo-flood: Disabled.

IP helper address

Disabled.

IP host

Disabled.

IRDP

Disabled.

Defaults when enabled:

Broadcast IDRP advertisements.

Maximum interval between advertisements: 600 seconds.

Minimum interval between advertisements: 0.75 times max interval

Preference: 0.

IP probe proxy

Disabled.

IP proxy ARP

Enabled.

IP routing

Disabled.

IP subnet-zero

Disabled.


Assigning IP Addresses to Network Interfaces

An IP address identifies a location to which IP packets can be sent. Some IP addresses are reserved for special uses and cannot be used for host, subnet, or network addresses. Table 20-2 lists ranges of IP addresses and shows which are reserved and which are available for use. RFC 1166, "Internet Numbers," contains the official description of IP addresses.

Table 20-2 Reserved and Available IP Addresses  

Class
Address or Range
Status

A

0.0.0.0
1.0.0.0 to 126.0.0.0
127.0.0.0

Reserved
Available
Reserved

B

128.0.0.0 to 191.254.0.0
191.255.0.0

Available
Reserved

C

192.0.0.0
192.0.1.0 to 223.255.254
223.255.255.0

Reserved
Available
Reserved

D

224.0.0.0 to 239.255.255.255

Multicast group addresses

E

240.0.0.0 to 255.255.255.254
255.255.255.255

Reserved
Broadcast


An interface can have one primary IP address. A mask identifies the bits that denote the network number in an IP address. When you use the mask to subnet a network, the mask is referred to as a subnet mask. To receive an assigned network number, contact your Internet service provider.

Beginning in privileged EXEC mode, follow these steps to assign an IP address and a network mask to a Layer 3 interface:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

interface interface-id

Enter interface configuration mode, and specify the Layer 3 interface to configure.

Step 3 

no switchport

Remove the interface from Layer 2 configuration mode (if it is a physical interface).

Step 4 

ip address ip_address subnet_mask

Configure the IP address and IP subnet mask.

Step 5 

no shutdown

Enable the interface.

Step 6 

end

Exit configuration mode.

Step 7 

show interfaces [interface-id]

show ip interfaces [interface-id]

show running-config interfaces [interface-id]

Verify the configuration.

Step 8 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

Use the no ip address command to remove an IP address or to disable IP processing.

This example shows how to configure an IP address on Gigabit Ethernet interface 0/10:

Switch# configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Switch(config)# interface gigabitethernet0/10
Switch(config)# no switchport
Switch(config-if)# ip address 10.1.2.3 255.255.0.0 
Switch(config-if)# no shutdown
Switch(config-if)# end 
Switch#

This example uses the show interfaces command to display the interface IP address configuration and status of Gigabit Ethernet interface 0/10:

Switch# show interfaces gigabitethernet0/10
GigabitEthernet0/10 is up, line protocol is up
  Hardware is Gigabit Ethernet, address is 0002.4b29.2e00 (bia 0002
  Internet address is 40.5.121.10/24
  MTU 1500 bytes, BW 100000 Kbit, DLY 10 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Full-duplex, 100Mb/s
  input flow-control is off, output flow-control is off
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input never, output 00:00:04, output hang never
  Last clearing of "show interface" counters never
  Queueing strategy: fifo
  Output queue 0/40, 0 drops; input queue 0/75, 0 drops
  5 minute input rate 0 bits/sec, 0 packets/sec
  5 minute output rate 0 bits/sec, 0 packets/sec
     0 packets input, 0 bytes, 0 no buffer
     Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 input packets with dribble condition detected
     30745 packets output, 3432096 bytes, 0 underruns
     0 output errors, 0 collisions, 6 interface resets
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier
     0 output buffer failures, 0 output buffers swapped out

This example uses the show ip interface command to display the detailed configuration and status of Gigabit Ethernet interface 0/10:

Switch# show ip interface gigabitethernet0/10
GigabitEthernet0/10 is up, line protocol is down
  Internet address is 10.1.2.3/16
  Broadcast address is 255.255.255.255
  Address determined by non-volatile memory
  MTU is 1500 bytes
  Helper address is not set
  Directed broadcast forwarding is disabled
  Multicast reserved groups joined: 224.0.0.1 224.0.0.2 224.0.0.13
  Outgoing access list is stan1
  Inbound  access list is 23
  Proxy ARP is enabled
  Local Proxy ARP is disabled
  Security level is default
  Split horizon is enabled
  ICMP redirects are always sent
  ICMP unreachables are always sent
  ICMP mask replies are never sent
  IP fast switching is enabled
  IP fast switching on the same interface is disabled
  IP Flow switching is disabled
  IP CEF switching is enabled
  IP Null turbo vector
  IP multicast fast switching is enabled
  IP multicast distributed fast switching is disabled
  IP route-cache flags are Fast, CEF
  Router Discovery is disabled
  IP output packet accounting is disabled
  IP access violation accounting is disabled
  TCP/IP header compression is disabled
  RTP/IP header compression is disabled
  Probe proxy name replies are disabled
  Policy routing is disabled
  Network address translation is enabled, interface in domain outside
  WCCP Redirect outbound is disabled
  WCCP Redirect exclude is disabled
  BGP Policy Mapping is disabled 

This example uses the show running-config command to display the interface IP address configuration of Gigabit Ethernet interface 0/10:

Switch# show running-config interfaces gigabitethernet0/10 
Building configuration...

Current configuration : 189 bytes
!
interface GigabitEthernet0/10
 description CubeB
 no switchport
 ip address 10.1.2.3 255.255.0.0
 ip access-group 23 in
 ip access-group stan1 out
 ip pim sparse-dense-mode
 ip cgmp
end 

Use of Subnet Zero

Subnetting with a subnet address of zero is strongly discouraged because of the problems that can arise if a network and a subnet have the same addresses. For example, if network 131.108.0.0 is subnetted as 255.255.255.0, subnet zero would be written as 131.108.0.0, which is the same as the network address.

You can use the all ones subnet (131.108.255.0) and even though it is discouraged, you can enable the use of subnet zero if you need the entire subnet space for your IP address.

Beginning in privileged EXEC mode, follow these steps to enable subnet zero:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

ip subnet-zero

Enable the use of subnet zero for interface addresses and routing updates.

Step 3 

end

Exit configuration mode.

Step 4 

show running-config

(Optional) Verify the setting.

Step 5 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

Use the no ip subnet-zero command to restore the default and disable the use of subnet zero.

This is an example of a partial output from the show running-config command used to verify IP subnet zero setting.

Switch# show running-config
Building configuration...

Current configuration : 7454 bytes
!
version 12.1
no service pad
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname Perdido1
!
!
<output truncated>
ip subnet-zero
ip routing
no ip domain-lookup
ip domain-name a,b,c
ip name-server 12.10.13.14 

<output truncated>

Classless Routing

By default, classless routing behavior is enabled on the switch when it is configured to route. With classless routing, if a router receives packets for a subnet of a network with no default route, the router forwards the packet to the best supernet route. A supernet consists of contiguous blocks of Class C address spaces used to simulate a single, larger address space and is designed to relieve the pressure on the rapidly depleting Class B address space.

In Figure 20-2, classless routing is enabled. When the host sends a packet to 120.20.4.1, instead of discarding the packet, the router forwards it to the best supernet route. If you disable classless routing and a router receives packets destined for a subnet of a network with no network default route, the router discards the packet.

Figure 20-2 IP Classless Routing

In Figure 20-3, the router in network 128.20.0.0 is connected to subnets 128.20.1.0, 128.20.2.0, and 128.20.3.0. If the host sends a packet to 120.20.4.1, because there is no network default route, the router discards the packet.

Figure 20-3 No IP Classless Routing

To prevent the switch from forwarding packets destined for unrecognized subnets to the best supernet route possible, you can disable classless routing behavior.

Beginning in privileged EXEC mode, follow these steps to disable classless routing:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

no ip classless

Disable classless routing behavior.

Step 3 

end

Exit configuration mode.

Step 4 

show running-config

(Optional) Verify the setting.

Step 5 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

To restore the default and have the switch forward packets destined for a subnet of a network with no network default route to the best supernet route possible, use the ip classless global configuration command.

Configuring Address Resolution Methods

You can control interface-specific handling of IP by using address resolution. A device using IP can have both a local address or MAC address, which uniquely defines the device on its local segment or LAN, and a network address, which identifies the network to which the device belongs. The local address or MAC address is known as a data link address because it is contained in the data link layer (Layer 2) section of the packet header and is read by data link (Layer 2) devices. To communicate with a device on Ethernet, the software must determine the MAC address of the device. The process of determining the MAC address from an IP address is called address resolution. The process of determining the IP address from the MAC address is called reverse address resolution.

The switch can use these forms of address resolution:

Address Resolution Protocol (ARP) is used to associate IP address with MAC addresses. Taking an IP address as input, ARP determines the associated MAC address and then stores the IP address/MAC address association in an ARP cache for rapid retrieval. Then the IP datagram is encapsulated in a link-layer frame and sent over the network. Encapsulation of IP datagrams and ARP requests or replies on IEEE 802 networks other than Ethernet is specified by the Subnetwork Access Protocol (SNAP).

Proxy ARP helps hosts with no routing tables determine the MAC addresses of hosts on other networks or subnets. If the switch (router) receives an ARP request for a host that is not on the same interface as the ARP request sender, and if the router has all of its routes to the host through other interfaces, it generates a proxy ARP packet giving its own local data link address. The host that sent the ARP request then sends its packets to the router, which forwards them to the intended host.

HP Probe is a protocol developed by Hewlett-Packard Company (HP) for use on IEEE-802.3 networks.

Catalyst 3550 switches also use the Reverse Address Resolution Protocol (RARP), which works the same as ARP except that the RARP request packets requests an IP address instead of a local MAC address. Using RARP requires a RARP server on the same network segment as the router interface. Use the interface configuration command ip rarp-server address to identify the server.

For more information on RARP, refer to the Cisco IOS Configuration Fundamentals Configuration Guide for Release 12.1.

You can perform these tasks to configure address resolution:

Define a Static ARP Cache

Set ARP Encapsulation

Enable Proxy ARP

Configure HP Probe Proxy

Define a Static ARP Cache

ARP and other address resolution protocols provide dynamic mapping between IP addresses and MAC addresses. Because most hosts support dynamic address resolution, you usually do not need to specify static ARP cache entries. If you must define a static ARP cache entry, you can do so globally, which installs a permanent entry in the ARP cache that the switch uses to translate IP addresses into MAC addresses. Optionally, you can also specify that the switch respond to ARP requests as if it were the owner of the specified IP address. If you do not want the ARP entry to be permanent, you can specify a timeout period for the ARP entry.

Beginning in privileged EXEC mode, follow these steps to provide static mapping between IP addresses and MAC addresses:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

arp ip-address hardware-address type

Globally associate an IP address with a MAC (hardware) address in the ARP cache. Type defines the encapsulation type:

arpa for Ethernet interfaces

snap for Token Ring and FDDI interfaces

Step 3 

arp ip-address hardware-address type alias

(Optional) Specify that the switch respond to ARP requests as if it were the owner of the specified IP address.

Step 4 

interface interface-id

Enter interface configuration mode, and specify the interface to configure.

Step 5 

arp timeout seconds

(Optional) Set the length of time an ARP cache entry will stay in the cache.

Step 6 

end

Return to privileged EXEC mode.

Step 7 

show interface [interface-id]

(Optional) Verify the type of ARP and the timeout value used on all interfaces or a specific interface.

Step 8 

show arp

(Optional) View the contents of the ARP cache.

Step 9 

show ip arp

(Optional) Verify IP ARP entries.

Step 10 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

To remove an entry from the ARP cache, use the no arp ip-address hardware-address type global configuration command. To remove all nonstatic entries from the ARP cache, use the clear arp-cache privileged EXEC command.

This is a sample output from the show arp privileged EXEC command.

Switch# show arp
Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet  10.1.2.3                -   0002.4b29.2e00  ARPA   GigabitEthernet0/10
Internet  172.20.136.9          120   0030.19c6.54e1  ARPA   Vlan1
Internet  172.20.250.42         149   0030.19c6.54e1  ARPA   Vlan1
Internet  120.20.30.1             -   0002.4b29.2e00  ARPA   Vlan27
Internet  172.20.139.152        101   0030.19c6.54e1  ARPA   Vlan1
Internet  172.20.139.130        205   0030.19c6.54e1  ARPA   Vlan1
Internet  172.20.141.225        186   0030.19c6.54e1  ARPA   Vlan1
Internet  172.20.135.204        169   0002.4b29.4400  ARPA   Vlan1
Internet  172.20.135.202          -   0002.4b29.2e00  ARPA   Vlan1
Internet  172.20.135.197        172   0002.4b28.ce80  ARPA   Vlan1
Internet  172.20.135.196        156   0002.4b28.ce00  ARPA   Vlan1

Note For the Catalyst 3550 switch, the output from the show arp command and the show ip arp command would usually be the same.


Set ARP Encapsulation

By default, Ethernet ARP encapsulation (represented by the arpa keyword) is enabled on an IP interface. You can change the encapsulation methods to SNAP or HP Probe, as required by your network.

When you set HP Probe encapsulation, the Probe protocol always is used to attempt to resolve an Ethernet MAC address, and the switch communicates transparently with Hewlett-Packard IEEE-802.3 hosts that use this type of encapsulation. You must explicitly configure Probe for all interfaces that will use Probe.

Beginning in privileged EXEC mode, follow these steps to specify the ARP encapsulation type:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

interface interface-id

Enter interface configuration mode, and specify the Layer 3 interface to configure.

Step 3 

arp {arpa | probe | snap}

Specify the ARP encapsulation method:

arpa: Address Resolution Protocol

probe: HP Probe protocol

snap: Subnetwork Address Protocol

Step 4 

end

Return to privileged EXEC mode.

Step 5 

show interface [interface-id]

(Optional) Verify ARP encapsulation configuration on all interfaces or the specified interface.

Step 6 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

To disable an encapsulation type, use the no arp interface configuration command.

This is a sample output from the show interface interface-id privileged EXEC command displaying ARP encapsulation.

Switch# show interface gigabitethernet0/10
GigabitEthernet0/10 is up, line protocol is up
  Hardware is Gigabit Ethernet, address is 0002.4b29.2e00 (bia 0002
  Internet address is 40.5.121.10/24
  MTU 1500 bytes, BW 100000 Kbit, DLY 10 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Full-duplex, 100Mb/s
  input flow-control is off, output flow-control is off
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input never, output 00:00:04, output hang never
  Last clearing of "show interface" counters never
  Queueing strategy: fifo
  Output queue 0/40, 0 drops; input queue 0/75, 0 drops
  5 minute input rate 0 bits/sec, 0 packets/sec
  5 minute output rate 0 bits/sec, 0 packets/sec
     0 packets input, 0 bytes, 0 no buffer
     Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 input packets with dribble condition detected
     30745 packets output, 3432096 bytes, 0 underruns
     0 output errors, 0 collisions, 6 interface resets
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier
     0 output buffer failures, 0 output buffers swapped out

Enable Proxy ARP

By default, the switch uses proxy ARP to help hosts determine MAC addresses of hosts on other networks or subnets.

Beginning in privileged EXEC mode, follow these steps to enable proxy ARP if it has been disabled:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

interface interface-id

Enter interface configuration mode, and specify the Layer 3 interface to configure.

Step 3 

ip proxy-arp

Enable proxy ARP on the interface.

Step 4 

end

Return to privileged EXEC mode.

Step 5 

show ip interface [interface-id]

(Optional) Verify the configuration on the interface or all interfaces.

Step 6 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

To disable proxy ARP on the interface, use the no ip proxy-arp command.

This is an example of the show ip interface privileged EXEC command for Gigabit Ethernet interface 0.3, where proxy ARP is enabled.

Switch# show ip interface gigabitethernet0/3 
GigabitEthernet0/3 is up, line protocol is down
  Internet address is 10.1.3.59/24
  Broadcast address is 255.255.255.255
  Address determined by setup command
  MTU is 1500 bytes
  Helper address is not set
  Directed broadcast forwarding is disabled
  Multicast reserved groups joined: 224.0.0.1 224.0.0.2
  Outgoing access list is not set
  Inbound  access list is not set
  Proxy ARP is enabled
  Local Proxy ARP is disabled
  Security level is default
  Split horizon is enabled
  ICMP redirects are always sent
  ICMP unreachables are always sent
  ICMP mask replies are never sent
  IP fast switching is enabled
  IP fast switching on the same interface is disabled
  IP Flow switching is disabled
  IP CEF switching is enabled
  IP CEF Fast switching turbo vector
  IP multicast fast switching is enabled
  IP multicast distributed fast switching is disabled
  IP route-cache flags are Fast, CEF
  Router Discovery is enabled
  IP output packet accounting is disabled
  IP access violation accounting is disabled
  TCP/IP header compression is disabled
  RTP/IP header compression is disabled
  Probe proxy name replies are disabled
  Policy routing is disabled
  Network address translation is disabled
  WCCP Redirect outbound is disabled
  WCCP Redirect exclude is disabled
  BGP Policy Mapping is disabled 

Configure HP Probe Proxy

HP Probe Proxy support allows the switch to respond to HP Probe Proxy name requests, typically used at sites with Hewlett Packard equipment.

Beginning in privileged EXEC mode, follow these steps to enable configure HP Probe Proxy:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

interface interface-id

Enter interface configuration mode, and specify the Layer 3 interface to configure.

Step 3 

ip probe proxy

Enable the switch to respond to HP Probe Proxy name requests.

Step 4 

exit

Return to global configuration mode.

Step 5 

ip hp-host hostname ip-address

Enter the host name and IP address of an IP host (for which the router is acting as a proxy).

Step 6 

end

Return to privileged EXEC mode.

Step 7 

show ip interface [interface-id]

(Optional) Verify the configuration on the interface or all interfaces.

Step 8 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

To disable HP Probe Proxy, use the no ip probe proxy interface configuration command. To remove a host name, use the no ip hp-host hostname ip-address global configuration command.

Routing Assistance When IP Routing is Disabled

These mechanisms allow the switch to learn about routes to other networks when it does not have IP routing enabled:

Proxy ARP

Default Gateway

ICMP Router Discovery Protocol (IRDP)

Proxy ARP

Proxy ARP, the most common method for learning about other routes, enables an Ethernet host with no routing information to communicate with hosts on other networks or subnets. The host assumes that all hosts are on the same local Ethernet and that they can use ARP to determine their MAC addresses. If a switch receives an ARP request for a host that is not on the same network as the sender, the switch evaluates whether it has the best route to that host. If it does, it sends an ARP reply packet with its own Ethernet MAC address, and the host that sent the request sends the packet to the switch, which forwards it to the intended host. Proxy ARP treats all networks as if they are local and performs ARP requests for every IP address.

Proxy ARP is enabled by default. To enable it after it has been disabled, see the "Enable Proxy ARP" section. Proxy ARP works as long as other routers support it.

Default Gateway

Another method for locating routes is to define a default router or default gateway. All nonlocal packets are sent to this router, which either routes them appropriately or sends an IP Control Message Protocol (ICMP) redirect message back, defining which local router the host should use. The switch caches the redirect messages and forwards each packet as efficiently as possible. A limitation of this method is that there is no means of detecting when the default router has gone down or is unavailable.

Beginning in privileged EXEC mode, follow these steps to define a default gateway (router) when IP routing is disabled:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

ip default-gateway ip-address

Set up a default gateway (router).

Step 3 

end

Return to privileged EXEC mode.

Step 4 

show ip redirects

(Optional) Display the address of the default gateway router to verify the setting.

Step 5 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

Use the no ip default-gateway global configuration command to disable this function.

This example shows how to set and verify a default gateway:

Switch(config)# ip default-gateway 10.1.5.59
Switch(config)# end 
Switch# show ip redirect
Default gateway is 10.1.5.59

Host               Gateway           Last Use    Total Uses  Interface
ICMP redirect cache is empty 

ICMP Router Discovery Protocol (IRDP)

Router discovery allows the switch to dynamically learn about routes to other networks using IRDP. IRDP allows hosts to locate routers. When operating as a client, the switch generates router discovery packets. When operating as a host, the switch receives router discovery packets. The switch can also listen to Routing Information Protocol (RIP) and Interior Gateway Routing Protocol (IGRP) routing updates and use this information to infer locations of routers. The switch does not actually store the routing tables sent by routing devices; it merely keeps track of which systems are sending the data. The advantage of using IRDP is that it allows each router to specify both a priority and the time after which a device is assumed to be down if no further packets are received.

Each device discovered becomes a candidate for the default router, and a new highest-priority router is selected when a higher priority router is discovered, when the current default router is declared down, or when a TCP connection is about to time out because of excessive retransmissions.

The only required task for IRDP routing on an interface is to enable IRDP processing on that interface. When enabled, the default parameters apply. You can optionally change any of these parameters.

Beginning in privileged EXEC mode, follow these steps to enable and configure IRDP on an interface:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

interface interface-id

Enter interface configuration mode, and specify the Layer 3 interface to configure.

Step 3 

ip irdp

Enable IRDP processing on the interface.

Step 4 

ip irdp multicast

(Optional) Send IRDP advertisements to the multicast address (224.0.0.1) instead of IP broadcasts.

Note This command allows for compatibility with Sun Microsystems Solaris, which requires IRDP packets to be sent out as multicasts. Many implementations cannot receive these multicasts; ensure end-host ability before using this command.

Step 5 

ip irdp holdtime seconds

(Optional) Set the IRDP period for which advertisements are valid. Default is three times the maxadvertinterval value. It must be greater than maxadvertinterval and cannot be greater than 9000 seconds. If you change the maxadvertinterval value, this value also changes.

Step 6 

ip irdp maxadvertinterval seconds

(Optional) Set the IRDP maximum interval between advertisements. The default is 600 seconds.

Step 7 

ip irdp minadvertinterval seconds

(Optional) Set the IRDP minimum interval between advertisements. The default is 0.75 times the maxadvertinterval. If you change the maxadvertinterval, this value changes to the new default (0.75 of maxadvertinterval).

Step 8 

ip irdp preference number

(Optional) Set a device IRDP preference level. The allowed range is -231 to 231. The default is 0. A higher value increases the router preference level.

Step 9 

ip irdp address address [number]

(Optional) Specify an IRDP address and preference to proxy-advertise.

Step 10 

end

Return to privileged EXEC mode.

Step 11 

show ip irdp

(Optional) Verify settings by displaying IRDP values.

Step 12 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

If you change the maxadvertinterval value, the holdtime and minadvertinterval values also change, so it is important to first change the maxadvertinterval value, before manually changing either the holdtime or minadvertinterval values.

Use the no ip irdp interface configuration command to disable IRDP routing.

This example shows IRDP routing enabled on Gigabit Ethernet interface 0/3:

Switch# show ip irdp
Vlan1 has router discovery disabled

Vlan2 has router discovery disabled

GigabitEthernet0/1 has router discovery disabled

GigabitEthernet0/2 has router discovery disabled

GigabitEthernet0/3 has router discovery enabled

Advertisements will occur between every 450 and 600 seconds.
Advertisements are sent with broadcasts.
Advertisements are valid for 1800 seconds.
Default preference will be 0.
GigabitEthernet0/4 has router discovery disabled

Port-channel1 has router discovery disabled

Configuring Broadcast Packet Handling

After configuring IP interface address, you can choose to enable routing and configure one or more routing protocols, or you can configure the way the switch responds to network broadcasts. A broadcast is a data packet destined for all hosts on a physical network. The switch supports two kinds of broadcasting:

A directed broadcast packet is sent to a specific network or series of networks. A directed broadcast address includes the network or subnet fields.

A flooded broadcast packet is sent to every network.


Note You can also limit broadcast, unicast, and multicast traffic on Layer 2 interfaces by using the interface mode switchport broadcast, switchport unicast, and switchport multicast commands. For more information, see "Configuring Traffic Suppression and Traffic Control."


Routers provide some protection from broadcast storms by limiting their extent to the local cable. Bridges (including intelligent bridges), because they are Layer 2 devices, forward broadcasts to all network segments, thus propagating broadcast storms. The best solution to the broadcast storm problem is to use a single broadcast address scheme on a network. In most modern IP implementations, you can set the address to be used as the broadcast address. Many implementations, including the one in the Catalyst 3550 switch, support several addressing schemes for forwarding broadcast messages.

Perform the tasks in these sections to enable these schemes:

Enabling Directed Broadcast-to-Physical Broadcast Translation

Forwarding UDP Broadcast Packets and Protocols

Establishing an IP Broadcast Address

Flooding IP Broadcasts

Enabling Directed Broadcast-to-Physical Broadcast Translation

By default, IP directed broadcasts are dropped; they are not forwarded. Dropping IP-directed broadcasts makes routers less susceptible to denial-of-service attacks.

You can enable forwarding of IP-directed broadcasts on an interface where the broadcast becomes a physical (MAC-layer) broadcast. Only those protocols configured by using the ip forward-protocol global configuration command are forwarded.

You can specify an access list to control which broadcasts are forwarded. When an access list is specified, only those IP packets permitted by the access list are eligible to be translated from directed broadcasts to physical broadcasts. For more information on access lists, see "Configuring Network Security with ACLs."

Beginning in privileged EXEC mode, follow these steps to enable forwarding of IP-directed broadcasts on an interface:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

interface interface-id

Enter interface configuration mode, and specify the interface to configure.

Step 3 

ip directed-broadcast [access-list-number]

Enable directed broadcast-to-physical broadcast translation on the interface. You can include an access list to control which broadcasts are forwarded. When an access list is specified, only IP packets permitted by the access list are eligible to be translated.

Step 4 

exit

Return to global configuration mode.

Step 5 

ip forward-protocol {udp [port] | nd | sdns}

Specify which protocols and ports the router forwards when forwarding broadcast packets.

udp: Forward UPD datagrams.

port: (Optional) Destination port that controls which UDP services are forwarded.

nd: Forward ND datagrams.

sdns: Forward SDNS datagrams

Step 6 

show ip interface [interface-id]

show running-config

(Optional) Verify the configuration on the interface or all interfaces.

Step 7 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

Use the no ip directed-broadcast interface configuration command to disable translation of directed broadcast to physical broadcasts. Use the no ip forward-protocol global configuration command to remove a protocol or port.

Forwarding UDP Broadcast Packets and Protocols

User Datagram Protocol (UDP) is an IP host-to-host layer protocol, as is TCP. UDP provides a low-overhead, connectionless session between two end systems and does not provide for acknowledgment of received datagrams. Network hosts occasionally use UDP broadcasts to determine address, configuration, and name information. If such a host is on a network segment that does not include a server, UDP broadcasts are normally not forwarded. You can remedy this situation by configuring an interface on a router to forward certain classes of broadcasts to a helper address. You can use more than one helper address per interface.

You can specify a UDP destination port to control which UDP services are forwarded. You can specify multiple UDP protocols. You can also specify the Network Disk (ND) protocol, which is used by older diskless Sun workstations and the network security protocol SDNS.

By default, both UDP and ND forwarding are enabled if a helper address has been defined for an interface. The description for the ip forward-protocol interface configuration command in the Cisco IOS IP and IP Routing Command Reference for Release 12.1 lists the ports that are forwarded by default if you do not specify any UDP ports.

If you do not specify any UDP ports when you configure the forwarding of UDP broadcasts, you are configuring the router to act as a BOOTP forwarding agent. BOOTP packets carry Dynamic Host Configuration Protocol (DHCP) information.

Beginning in privileged EXEC mode, follow these steps to enable forwarding UDP broadcast packets on an interface and specify the destination address:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

interface interface-id

Enter interface configuration mode, and specify the Layer 3 interface to configure.

Step 3 

ip helper-address address

Enable forwarding and specify the destination address for forwarding UDP broadcast packets, including BOOTP.

Step 4 

exit

Return to global configuration mode.

Step 5 

ip forward-protocol {udp [port] | nd | sdns}

Specify which protocols the router forwards when forwarding broadcast packets.

udp: Forward UPD datagrams.

port: (Optional) Destination port that controls which UDP services are forwarded.

nd: Forward ND datagrams.

sdns: Forward SDNS datagrams

Step 6 

end

Return to privileged EXEC mode.

Step 7 

show ip interface [interface-id]

show running-config

(Optional) Verify the configuration on the interface or all interfaces.

Step 8 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

Use the no ip helper-address interface configuration command to disable the forwarding of broadcast packets to specific addresses. Use the no ip forward-protocol global configuration command to remove a protocol or port.

Establishing an IP Broadcast Address

The most popular IP broadcast address (and the default) is an address consisting of all ones (255.255.255.255). However, the switch can be configured to generate any form of IP broadcast address.

Beginning in privileged EXEC mode, follow these steps to set the IP broadcast address on an interface:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

interface interface-id

Enter interface configuration mode and specify the interface to configure.

Step 3 

ip broadcast-address ip-address

Enter a broadcast address different from the default, for example 128.1.255.255.

Step 4 

end

Return to privileged EXEC mode.

Step 5 

show ip interface [interface-id]

(Optional) Verify the broadcast address on the interface or all interfaces.

Step 6 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

To restore the default IP broadcast address, use the no ip broadcast-address command.

Flooding IP Broadcasts

You can allow IP broadcasts to be flooded throughout your internetwork in a controlled fashion by using the database created by the bridging STP. Using this feature also prevents loops. To support this capability, bridging must be configured on each interface that is to participate in the flooding. If bridging is not configured on an interface, it still can receive broadcasts. However, the interface never forwards broadcasts it receives, and the router never uses that interface to send broadcasts received on a different interface.

Packets that are forwarded to a single network address using the IP helper-address mechanism can be flooded. Only one copy of the packet is sent on each network segment.

To be considered for flooding, packets must meet these criteria. (Note that these are the same conditions used to consider packet forwarding using IP helper addresses.)

The packet must be a MAC-level broadcast.

The packet must be an IP-level broadcast.

The packet must be a TFTP, DNS, Time, NetBIOS, ND, or BOOTP packet, or a UDP specified by the ip forward-protocol udp global configuration command.

The time-to-live (TTL) value of the packet must be at least two.

A flooded UDP datagram is given the destination address specified with the ip broadcast-address interface configuration command on the output interface. The destination address can be set to any address. Thus, the destination address might change as the datagram propagates through the network. The source address is never changed. The TTL value is decremented.

When a flooded UDP datagram is sent out an interface (and the destination address possibly changed), the datagram is handed to the normal IP output routines and is, therefore, subject to access lists, if they are present on the output interface.

Beginning in privileged EXEC mode, follow these steps to use the bridging spanning-tree database to flood UDP datagrams:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

ip forward-protocol spanning-tree

Use the bridging spanning-tree database to flood UDP datagrams.

Step 3 

end

Return to privileged EXEC mode.

Step 4 

show running-config

(Optional) Verify the configuration.

Step 5 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

Use the no ip forward-protocol spanning-tree global configuration command to disable the flooding of IP broadcasts.

In the Catalyst 3550 switch, the majority of packets are forwarded in hardware; most packets do not go through the switch CPU. For those packets that do go to the CPU, you can speed up spanning tree-based UDP flooding by a factor of about four to five times by using turbo-flooding. This feature is supported over Ethernet interfaces configured for ARP encapsulation.

Beginning in privileged EXEC mode, follow these steps to increase spanning-tree-based flooding:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode

Step 2 

ip forward-protocol turbo-flood

Use the spanning-tree database to speed up flooding of UDP datagrams.

Step 3 

end

Return to privileged EXEC mode.

Step 4 

show running-config

(Optional) Verify the configuration.

Step 5 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

To disable this feature, use the no ip forward-protocol turbo-flood global configuration command.

Monitoring and Maintaining IP Addressing

When the contents of a particular cache, table, or database have become or are suspected to be invalid, you can remove all its contents by using the clear privileged EXEC commands. Table 20-3 lists the commands for clearing contents.

Table 20-3 Clearing Caches, Tables and Databases

Command
Purpose

clear arp-cache

Clear the IP ARP cache and the fast-switching cache.

clear host {name | *}

Remove one or all entries from the host name and the address cache.

clear ip route {network [mask] |*}

Remove one or more routes from the IP routing table.


You can display specific statistics, such as the contents of IP routing tables, caches, and databases; the reachability of nodes; and the routing path that packets are taking through the network. Table 20-4 lists the privileged EXEC commands for displaying statistics.

Table 20-4 Displaying Caches, Tables, and Databases  

Command
Purpose

show arp

Display the entries in the ARP table.

show hosts

Display the default domain name, style of lookup service, name server hosts, and the cached list of host names and addresses.

show ip aliases

Display IP addresses mapped to TCP ports (aliases).

show ip arp

Display the IP ARP cache.

show ip interface [interface-id]

Display the status of interfaces.

show ip irdp

Display IRDP values.

show ip masks address

Display the masks used for network addresses and the number of subnets using each mask.

show ip redirects

Display the address of a default gateway.

show ip route [address [mask]] | [protocol]

Display the current state of the routing table.

show ip route summary

Display the current state of the routing table in summary form.


These are examples of output displays for displaying caches, tables, and databases.

Switch# show arp
Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet  10.1.2.3                -   0002.4b29.2e00  ARPA   GigabitEthernet0/10
Internet  172.20.136.9          178   0030.19c6.54e1  ARPA   Vlan1
Internet  172.20.250.42         207   0030.19c6.54e1  ARPA   Vlan1
Internet  120.20.30.1             -   0002.4b29.2e00  ARPA   Vlan27
Internet  172.20.139.152        159   0030.19c6.54e1  ARPA   Vlan1
Internet  172.20.139.130         24   0030.19c6.54e1  ARPA   Vlan1
Internet  172.20.141.225          5   0030.19c6.54e1  ARPA   Vlan1
Internet  172.20.135.204        227   0002.4b29.4400  ARPA   Vlan1
Internet  172.20.135.202          -   0002.4b29.2e00  ARPA   Vlan1
Internet  172.20.135.197        230   0002.4b28.ce80  ARPA   Vlan1
Internet  172.20.135.196        214   0002.4b28.ce00  ARPA   Vlan1
Internet  172.20.135.193         58   0030.19c6.54e1  ARPA   Vlan1

Switch# show hosts
Default domain is a,b,c
Name/address lookup uses static mappings

Host                     Flags      Age Type   Address(es)

Switch# show ip aliases
Address Type             IP Address      Port
Interface                10.1.2.3
Interface                120.20.30.1
Interface                172.20.135.202

Switch# show ip masks
Supernet masks  Reference count
255.252.0.0     1
0.0.0.0         1

Switch# show ip redirects
Default gateway is 172.20.135.193

Host               Gateway           Last Use    Total Uses  Interface
ICMP redirect cache is empty

Switch# show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
       * - candidate default, U - per-user static route, o - ODR
       P - periodic downloaded static route
Gateway of last resort is 172.20.135.193 to network 0.0.0.0

S*   0.0.0.0/0 [1/0] via 172.20.135.193
C    172.20.0.0/14 is directly connected, Vlan1
Perdido1#show ip route summary
IP routing table name is Default-IP-Routing-Table(0)
Route Source    Networks    Subnets     Overhead    Memory (bytes)
connected       1           0           64          144
static          1           0           64          144
rip             0           0           0           0
Total           2           0           128         288 

Switch # show ip interface
Vlan1 is up, line protocol is up
  Internet address is 172.20.142.153/25
  Broadcast address is 255.255.255.255
  Address determined by non-volatile memory
  MTU is 1500 bytes
  Helper address is not set
  Directed broadcast forwarding is disabled
  Outgoing access list is not set
  Inbound  access list is not set
  Proxy ARP is enabled
  Local Proxy ARP is disabled
  Security level is default
  Split horizon is enabled
  ICMP redirects are always sent
  ICMP unreachables are always sent
  ICMP mask replies are never sent
  IP fast switching is enabled
  IP fast switching on the same interface is disabled
  IP Flow switching is disabled
  IP CEF switching is enabled
  IP CEF Fast switching turbo vector
  IP multicast fast switching is disabled
  IP multicast distributed fast switching is disabled
  IP route-cache flags are CEF
  Router Discovery is disabled
  IP output packet accounting is disabled
  IP access violation accounting is disabled
  TCP/IP header compression is disabled
  RTP/IP header compression is disabled
  Probe proxy name replies are disabled
  Policy routing is disabled
  Network address translation is disabled
  WCCP Redirect outbound is disabled
  WCCP Redirect exclude is disabled
  BGP Policy Mapping is disabled
GigabitEthernet0/1 is up, line protocol is up
  Internet protocol processing disabled
GigabitEthernet0/2 is up, line protocol is down
  Internet protocol processing disabled 

Enabling IP Routing

By default, the switch is in Layer 2 switching mode and IP routing is disabled. To use the Layer 3 capabilities of the switch, you must enable IP routing.

Beginning in privileged EXEC mode, follow these steps to configure IP routing:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

ip routing

Enable IP routing.

Step 3 

router ip_routing_protocol

Specify an IP routing protocol. This step might include other commands, such as specifying the networks to route with the network command. Refer to sections later in this chapter on specific protocols and to the Cisco IOS IP and IP Routing Configuration Guide for Release 12.1.

Step 4 

end

Return to privileged EXEC mode.

Step 5 

show running-config

(Optional) Verify the configuration.

Step 6 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

Use the no ip routing global configuration command to disable routing.

This example shows how to enable IP routing using RIP as the routing protocol:

Switch# configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Switch(config)# ip routing 
Switch(config)# router rip 
Switch(config-router)# network 10.0.0.0 
Switch(config-router)# end 

You can now set up parameters for the selected routing protocols. Refer to the appropriate section:

Configuring RIP

Configuring IGRP

Configuring OSPF

Configuring EIGRP

You can also configure nonprotocol-specific features:

Configuring Protocol-Independent Features

Configuring RIP

The Routing Information Protocol (RIP) is an interior gateway protocol (IGP) created for use in small, homogeneous networks. It is a distance-vector routing protocol that uses broadcast User Datagram Protocol (UDP) data packets to exchange routing information. The protocol is documented in RFC 1058. You can find detailed information about RIP in IP Routing Fundamentals, published by Cisco Press.

Using RIP, the switch sends routing information updates (advertisements) every 30 seconds. If a router does not receive an update from another router for 180 seconds or more, it marks the routes served by that router as unusable. If there is still no update after 240 seconds, the router removes all routing table entries for the non-updating router.

RIP uses hop counts to rate the value of different routes. The hop count is the number of routers that can be traversed in a route. A directly connected network has a hop count of zero; a network with a hop count of 16 is unreachable. This small range (0 to 15) makes RIP unsuitable for large networks.

If the router has a default network path, RIP advertises a route that links the router to the pseudonetwork 0.0.0.0. The 0.0.0.0 network does not exist; it is treated by RIP as a network to implement the default routing feature. The switch advertises the default network if a default was learned by RIP or if the router has a gateway of last resort and RIP is configured with a default metric. RIP sends updates to the interfaces in specified networks. If an interface's network is not specified, it is not advertised in any RIP update.

Table 20-5 shows the default RIP configuration.

Table 20-5 Default RIP Configuration 

Feature
Default Setting

Auto summary

Enabled.

Default-information originate

Disabled.

Default metric

Built-in; automatic metric translations.

IP RIP authentication key-chain

No authentication.

Authentication mode: clear text.

IP RIP receive version

According to the version command.

IP RIP send version

According to the version command.

IP RIP triggered

According to the version command.

IP split horizon

Varies with media.

Neighbor

None defined.

Network

None specified.

Offset list

Disabled.

Output delay

0 milliseconds.

Timers basic

Update: 30 seconds.

Invalid: 180 seconds.

Hold-down: 180 seconds.

Flush: 240 seconds.

Validate-update-source

Enabled.

Version

Receives RIP version 1 and version 2 packets;
sends version 1 packets.


For protocol-independent features that also apply to RIP, see the "Configuring Protocol-Independent Features" section.

To configure RIP, you enable RIP routing for a network and optionally configure other parameters.

Beginning in privileged EXEC mode, follow these steps to enable and configure RIP:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

ip routing

Enable IP routing. (Required only if IP routing is disabled.)

Step 3 

router rip

Enable a RIP routing process. This step changes the mode to router configuration.

Step 4 

network network number

Associate a network with a RIP routing process. You can specify multiple network commands. RIP routing updates are sent and received only through interfaces on these networks.

Step 5 

neighbor ip-address

(Optional) Define a neighboring router with which to exchange routing information. This step allows routing updates from RIP (normally a broadcast protocol) to reach nonbroadcast networks.

Step 6 

offset list [access-list number | name] {in | out} offset [type number]

(Optional) Apply an offset list to routing metrics to increase incoming and outgoing metrics to routes learned through RIP. You can limit the offset list with an access list or an interface.

Step 7 

timers basic update invalid holddown flush

(Optional) Adjust routing protocol timers.

update—the time (in seconds) between sending of routing updates. The default is 30 seconds.

invalid—the timer interval (in seconds) after which a route is declared invalid. The default is 180 seconds.

holddown—the time (in seconds) that must pass before a route is removed from the routing table. The default is 180 seconds.

flush—the amount of time (in seconds) for which routing updates are postponed. The default is 240 seconds.

Step 8 

version {1 | 2}

(Optional) Configure the switch to receive and send only RIP Version 1 or RIP version 2 packets. By default, the switch receives Version 1 and 2 but sends only Version 1.
You can also use the interface commands ip rip {send | receive} version 1 | 2 | 1 2} to control what versions are used for sending and receiving on interfaces.

Step 9 

no auto summary

(Optional) Disable automatic summarization. By default, the switch summarizes subprefixes when crossing classful network boundaries. Disable summarization (RIP version 2 only) to advertise subnet and host routing information to classful network boundaries.

Step 10 

no validate-update-source

(Optional) Disable validation of the source IP address of incoming RIP routing updates. By default, the switch validates the source IP address of incoming RIP routing updates and discards the update if the source address is not valid. Under normal circumstances, disabling this feature is not recommended. However, if you have a router that is off-network and you want to receive its updates, you can use this command.

Step 11 

output-delay delay

(Optional) Add interpacket delay for RIP updates sent.
By default, packets in a multiple-packet RIP update have no delay added between packets. If you are sending packets to a lower-speed device, you can add an interpacket delay in the range of 8 to 50 milliseconds.

Step 12 

end

Return to privileged EXEC mode.

Step 13 

show ip protocols

(Optional) Verify the configuration.

Step 14 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

To turn off the RIP routing process, use the no router rip global configuration command.

To display the parameters and current state of the active routing protocol process, use the show ip protocols privileged EXEC command. This is sample output from the show ip protocols command, showing RIP processes:

Switch# show ip protocols
Routing Protocol is "rip"
  Sending updates every 30 seconds, next due in 19 seconds
  Invalid after 180 seconds, hold down 180, flushed after 240
  Outgoing update filter list for all interfaces is
  Incoming update filter list for all interfaces is
  Redistributing: rip
  Default version control: send version 1, receive any version
    Interface             Send  Recv  Triggered RIP  Key-chain
    Vlan1                 1     1 2
    Vlan2                 1     1 2 
    GigabitEthernet0/2    1     1 2
    GigabitEthernet0/3    1     1 2                  CHAIN 
  Automatic network summarization is in effect
  Maximum path: 4
  Routing for Networks:
    10.0.0.0
  Routing Information Sources:
    Gateway         Distance      Last Update
  Distance: (default is 120) 
<output truncated>

Use the show ip rip database privileged EXEC command to display summary address entries in the RIP database. This example shows output with a summary address entry for route 12.11.0.0/16, with three child routes active:

Switch# show ip rip database
0.0.0.0/0    auto-summary
0.0.0.0/0    redistributed
    [0] via 0.0.0.0,
172.20.0.0/14    directly connected, Vlan1

This is sample output of the show ip rip database command with a prefix and mask:

Switch# show ip rip database 172.19.86.0 255.255.255.0 172.19.86.0/24
    [1] via 172.19.67.38, 00:00:25, Serial0
    [1] via 172.19.70.36, 00:00:14, Serial1

RIP Authentication

RIP version 1 does not support authentication. If you are sending and receiving RIP Version 2 packets, you can enable RIP authentication on an interface. The key chain determines the set of keys that can be used on the interface. If a key chain is not configured, no authentication is performed, not even the default. Therefore, you must also perform the tasks in the "Managing Authentication Keys" section.

The switch supports two modes of authentication on interfaces for which RIP authentication is enabled: plain text and MD5. The default is plain text.

Beginning in privileged EXEC mode, follow these steps to configure RIP authentication on an interface:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

interface interface-id

Enter interface configuration mode, and specify the interface to configure.

Step 3 

ip rip authentication key-chain name-of-chain

Enable RIP authentication.

Step 4 

ip rip authentication mode [text | md5}

Configure the interface to use MD5 digest authentication or plain text authentication (the default).

Step 5 

end

Return to privileged EXEC mode.

Step 6 

show running-config interfaces [interface-id]

(Optional) Verify the configuration.

Step 7 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

To restore clear text authentication, use the no ip rip authentication mode command. To prevent authentication, use the no ip rip authentication key-chain command.

This example shows how to verify the setting by using the show running-configuration interface privileged EXEC command.

Switch# show running-config interface gigabitethernet0/3
Building configuration...

Current configuration : 158 bytes
!
interface GigabitEthernet0/3
 no switchport
 ip address 10.1.3.59 255.255.255.0
 ip directed-broadcast
 ip irdp
 ip rip authentication key-chain CHAIN
end 

Summary Addresses and Split Horizon

Routers connected to broadcast-type IP networks and using distance-vector routing protocols normally use the split-horizon mechanism to reduce the possibility of routing loops. Split horizon blocks information about routes from being advertised by a router on any interface from which that information originated. This feature usually optimizes communication among multiple routers, especially when links are broken.


Note In general, disabling split horizon is not recommended unless you are certain that your application requires it to properly advertise routes.


If you want to configure an interface running RIP to advertise a summarized local IP address pool on a network access server for dial-up clients, use the ip summary-address rip interface configuration command.


Note If split horizon is enabled, neither autosummary nor interface IP summary addresses are advertised.


Beginning in privileged EXEC mode, follow these steps to set an interface to advertise a summarized local IP address and to disable split horizon on the interface:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

interface interface-id

Enter interface configuration mode, and specify the Layer 3 interface to configure.

Step 3 

ip address ip_address subnet_mask

Configure the IP address and IP subnet.

Step 4 

ip summary-address rip ip address ip-network mask

Configure the IP address to be summarized and the IP network mask.

Step 5 

no ip split horizon

Disable split horizon on the interface.

Step 6 

end

Return to privileged EXEC mode.

Step 7 

show ip interface interface-id

(Optional) Verify the configuration.

Step 8 

copy running-config startup-config

(Optional) Save your configuration changes to NVRAM.

To disable IP summarization, use the no ip summary-address rip router configuration command.

In this example, the major net is 10.0.0.0. The summary address 10.2.0.0 overrides the autosummary address of 10.0.0.0 so that 10.2.0.0 is advertised out interface Gigabit Ethernet 0.2, and 10.0.0.0 is not advertised. In the example, if the interface is still in Layer 2 mode (the default), you must enter a no switchport interface configuration command before entering the ip address command.


Note If split horizon is enabled, neither autosummary nor interface summary addresses (those configured with the ip summary-address rip command) are advertised.


Switch(config)# router rip
Switch(config-router)# interface gi0/2
Switch(config-if)# ip address 10.1.5.1 255.255.255.0
Switch(config-if)# ip summary-address rip 10.2.0.0 255.255.0.0
Switch(config-if)# no ip split-horizon
 
Switch(config)# router rip
Switch(config-router)# network 10.0.0.0
Switch(config-router)# neighbor 2.2.2.2 peer-group mygroup

Configuring IGRP

Interior Gateway Routing Protocol (IGRP) is a dynamic, distance-vector routing, proprietary Cisco protocol for routing in an autonomous system that contains large, arbitrarily complex networks with diverse bandwidth and delay characteristics. IGRP uses a combination of user-configurable metrics, including internetwork delay, bandwidth, reliability, and load. IGRP also advertises types of routes: interior, system, and exterior, as shown in Figure 20-4.

Interior routes are routes between subnets in the network attached to a router interface. If the network attached to a router is not subnetted, IGRP does not advertise interior routes.

System routes are routes to networks within an autonomous system. The router derives system routes from directly connected network interfaces and system route information provided by other IGRP-speaking routers or access servers. System routes do not include subnet information.

Exterior routes are routes to networks outside the autonomous system that are considered when identifying a gateway of last resort. The router chooses a gateway of last resort from the list of exterior routes that IGRP provides if it does not have a better route for a packet and the destination is not a connected network. If the autonomous system has more than one connection to an external network, different routers can choose different exterior routers as the gateway of last resort.

Figure 20-4 Interior, System, and Exterior Routes

By default, a router running IGRP sends an update broadcast every 90 seconds and declares a route inaccessible if it does not receive an update from the first router in the route within three update periods (270 seconds). After seven update periods (630 seconds), the route is removed from the routing table.

Table 20-6 shows the default IGRP configuration.

Table 20-6 Default IGRP Configuration 

Feature
Default Setting

IP split horizon

Varies with media.

Metric holddown

Disabled.

Metric maximum-hops

100 hops.

Neighbor

None defined.

Network

None specified.

Offset-list

Disabled.

Set metric

None set in route map.

Timers basic

Update: 90 seconds.

Invalid: 270 seconds.

Hold-down: 280 seconds.

Flush: 630 seconds.

Sleeptime: 0 milliseconds.

Traffic-share

Distributed proportionately to the ratios of the metrics.


Routers running IGRP use flash and poison-reverse updates to speed up the convergence of the routing algorithm. Flash updates are updates sent before the standard interval, notifying other routers of a metric change. Poison-reverse updates are intended to defeat larger routing loops caused by increases in routing metrics. The poison-reverse updates are sent to remove a route and place it in hold-down, which keeps new routing information from being used for a certain period of time.

Load Balancing and Traffic Distribution Control

IGRP can simultaneously use an asymmetric set of paths for a given destination. This unequal-cost load balancing allows traffic to be distributed among up to four unequal-cost paths to provide greater overall throughput and reliability.

Alternate path variance (that is, the difference in desirability between the primary and alternate paths) determines the feasibility of a potential route. An alternate route is feasible if the next router in the path is closer to the destination (has a lower metric value) than the router being used, and if the metric for the entire alternate path is within the variance. Only feasible paths are used for load balancing and are included in the routing table. These conditions limit the number of load balancing occurrences, but ensure that the dynamics of the network remain stable.

These general rules apply to IGRP unequal-cost load balancing:

IGRP accepts up to four paths for a given destination network.

The local best metric must be greater than the metric learned from the next router; that is, the next hop router must be closer (have a smaller metric value) to the destination than the local best metric.

The alternative path metric must be within the specified variance of the local best metric. The multiplier times the local best metric for the destination must be greater than or equal to the metric through the next router.

If these conditions are met, the route is determined to be feasible and can be added to the routing table.

By default, the amount of variance is set to one (equal-cost load balancing). Use the variance router configuration command to define how much worse an alternate path can be before that path is disallowed.

If variance is configured as described in the preceding section, IGRP or Enhanced IGRP distributes traffic among multiple routes of unequal cost to the same destination. If you want faster convergence to alternate routes, but you do not want to send traffic across inferior routes in the normal case, you might prefer to have no traffic flow along routes with higher metrics. Use the traffic-share router configuration command to control distribution of traffic among multiple routes of unequal cost.


Note For more information and examples, refer to the Cisco IOS IP and IP Routing Configuration Guide for Release 12.1.


Beginning in privileged EXEC mode, follow these steps to configure IGRP. Configuring the routing process is required; other steps are optional:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

router igrp autonomous-system

Enable an IGRP routing process. This command changes the mode to router configuration. The autonomous system number identifies the routes to other IGRP routers and tags routing information.

Step 3 

network network-number

Associate networks with an IGRP routing process. IGRP sends updates to the interfaces in the specified networks. If an interface's network is not specified, it is not advertised in any IGRP update. It is not necessary to have a registered autonomous system number, but if you do have a registered number, we recommend that you use it to identify your process.

Step 4 

offset list [access-list number | name]
{
in | out} offset [type number]

(Optional) Apply an offset list to routing metrics to increase incoming and outgoing metrics to routes learned through IGRP. You can limit the offset list with an access list or an interface.

Step 5 

neighbor ip-address

(Optional) Define a neighboring router with which to exchange routing information. This step allows routing updates from RIP (normally a broadcast protocol) to reach nonbroadcast network.

Step 6 

metric weights tos k1 k2 k3 k4 k5

(Optional) Adjust the IGRP metric. By default, the IGRP composite metric is a 23-bit quantity that is the sum of the segment delays and the lowest segment bandwidth for a given route.

tos—type of services; the default is 0.

k1-k5—constants that convert a metric vector into a scalar quantity. Defaults for k1 and k3 are 1; all others are 0.

Step 7 

timers basic update invalid holddown flush [sleeptime]

(Optional) Adjust routing protocol timers.

update—the time (in seconds) between sending of routing updates. The default is 90 seconds.

invalid—the timer interval (in seconds) after which a route is declared invalid. The default is 270 seconds.

holddown—the time (in seconds) during which routing information about better paths is suppressed. The default is 280 seconds.

flush—the time (in seconds) that must pass before a route is removed from the routing table. The default is 630 seconds.

sleeptime—interval in milliseconds for postponing routing updates. The default is 0.

Step 8 

no metric holddown

(Optional) Disable the IGRP hold-down period. The route to a network is placed in holddown if the router learns that the network is farther away than previously known or is down. Holddown keeps new routing information from being used for a certain period of time. This can prevent routing loops caused by slow convergence. It is sometimes advantageous to disable holddown to increase the network's ability to quickly respond to topology changes; this command provides this function.

Use the metric holddown command if other routers or access servers within the IGRP autonomous system are not configured with the no metric holddown command. If all routers are not configured the same way, you increase the possibility of routing loops.

Step 9 

metric maximum-hops hops

(Optional) Configure the maximum network diameter. Routes with hop counts exceeding this diameter are not advertised. The default is 100 hops; the maximum is 255 hops.

Step 10 

no validate-update-source

(Optional) Disable validation of the source IP address of incoming RIP routing updates. By default, the switch validates the source IP address of incoming RIP routing updates and discards the update if the source address is not valid.

Step 11 

variance multiplier

(Optional) Define the variance associated with a particular path to enable unequal-cost load balancing if desired, balancing traffic across all feasible paths to converge to a new path if a path should fail. The multiplier can be from 1 to 128; the default is 1 (equal-cost load balancing).

Step 12 

traffic-share {balanced | min}

(Optional) Distribute traffic proportionately to the ratios of metrics (balanced) or by the minimum-cost route (min).

Step 13 

end

Return to privileged EXEC mode.

Step 14 

show ip protocols

(Optional) Verify the configuration.

Step 15 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

To shut down an IGRP routing process, use the no router igrp command.

This example configures a router for IGRP and assigns autonomous system 109. The network commands show the networks directly connected to the router.

Switch(config)# router igrp 109
Switch(config-router)# network 131.108.0.0
Switch(config-router)# network 192.31.7.0

This example verifies the IGRP configuration.

Switch# show ip protocols
<output truncated>
Routing Protocol is "igrp 109"
  Sending updates every 90 seconds, next due in 52 seconds
  Invalid after 270 seconds, hold down 280, flushed after 630
  Outgoing update filter list for all interfaces is
  Incoming update filter list for all interfaces is
  Default networks flagged in outgoing updates
  Default networks accepted from incoming updates
  IGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
  IGRP maximum hopcount 100
  IGRP maximum metric variance 1
  Redistributing: igrp 109
  Maximum path: 4
  Routing for Networks:
    131.108.0.0
    183.31.0.0
  Routing Information Sources:
    Gateway         Distance      Last Update
  Distance: (default is 100) 

Split Horizon

Routers connected to broadcast-type IP networks and using distance-vector routing protocols normally use the split-horizon mechanism to reduce the possibility of routing loops. Split horizon blocks information about routes from being advertised by a router on any interface from which that information originated. This feature can optimize communication among multiple routers, especially when links are broken.


Note In general, we do not recommend disabling split horizon unless you are certain that your application requires it to properly advertise routes.


Beginning in privileged EXEC mode, follow these steps to disable split horizon on the interface:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

interface interface-id

Enter interface configuration mode, and specify the interface to configure.

Step 3 

ip address ip_address subnet_mask

Configure the IP address and IP subnet.

Step 4 

no ip split horizon

Disable split horizon on the interface.

Step 5 

end

Return to privileged EXEC mode.

Step 6 

show ip interface interface-id

(Optional) Verify the configuration.

Step 7 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

To enable the split horizon mechanism, use the ip split-horizon interface configuration command.

Configuring OSPF

This section briefly describes how to configure Open Shortest Path First (OSPF). For a complete description of the OSPF commands, refer to the "OSPF Commands" chapter of the Cisco IOS IP and IP Routing Command Reference for Release 12.1.


Note OSPF classifies different media into broadcast, nonbroadcast, and point-to-point networks. The Catalyst 3550 switch supports only broadcast networks (Ethernet, Token Ring, and FDDI).


OSPF is an Interior Gateway Protocol (IGP) designed expressly for IP networks, supporting IP subnetting and tagging of externally derived routing information. OSPF also allows packet authentication and uses IP multicast when sending and receiving packets. The Cisco implementation supports RFC 1253, Open Shortest Path First (OSPF) management information base (MIB).

For protocol-independent features that include OSPF, see the "Configuring Protocol-Independent Features" section.

The Cisco implementation conforms to the OSPF Version 2 specifications with these key features:

Stub areas—Definition of stub areas is supported.

Route redistribution—Routes learned through any IP routing protocol can be redistributed into another IP routing protocol. At the intradomain level, this means that OSPF can import routes learned through IGRP and RIP. OSPF routes can also be exported into IGRP and RIP.

Authentication—Plain text and MD5 authentication among neighboring routers within an area is supported.

Routing interface parameter—Configurable parameters supported include interface output cost, retransmission interval, interface transmit delay, router priority, router dead and hello intervals, and authentication key.

Virtual links—Virtual links are supported.

Not-so-stubby-area (NSSA)—RFC 1587.

OSPF typically requires coordination among many internal routers, area border routers (ABRs) connected to multiple areas, and autonomous system boundary routers (ASBRs). The minimum configuration would use all default parameter values, no authentication, and interfaces assigned to areas. If you customize your environment, you must ensure coordinated configuration of all routers.

Table 20-7 shows the default OSPF configuration.

Table 20-7 Default OSPF Configuration 

Feature
Default Setting

Interface parameters

Cost: No default cost predefined.

Retransmit interval: 5 seconds.

Transmit delay: 1 second.

Priority: 1.

Hello interval: 10 seconds.

Dead interval: 4 times the hello interval.

No authentication.

No password specified.

MD5 authentication disabled.

Area

Authentication type: 0 (no authentication).

Default cost: 1.

Range: Disabled.

Stub: No stub area defined.

NSSA: No NSSA area defined.

Auto cost

100 Mbps.

Default-information originate

Disabled. When enabled, the default metric setting is 10, and the external route type default is Type 2.

Default metric

Built-in, automatic metric translation, as appropriate for each routing protocol.

Distance OSPF

dist1 (all routes within an area): 110.
dist2 (all routes from one area to another): 110.
and dist3 (routes from other routing domains): 110.

OSPF database filter

Disabled. All outgoing link-state advertisements (LSAs) are flooded to the interface.

IP OSPF name lookup

Disabled.

Log adjacency changes

Enabled.

Neighbor

None specified.

Neighbor database filter

Disabled. All outgoing LSAs are flooded to the neighbor.

Network area

Disabled.

Router ID

No OSPF routing process defined.

Summary address

Disabled.

Timers LSA group pacing

240 seconds.

Timers spf

spf delay: 5 seconds.

spf-holdtime: 10 seconds.

Virtual link:

No area ID or router ID defined.

Hello interval: 10 seconds.

Retransmit interval: 5 seconds.

Transmit delay: 1 second.

Dead interval: 40 seconds.

Authentication key: no key predefined.

Message-digest key (MD5): no key predefined.


Enabling OSPF requires that you create an OSPF routing process, specify the range of IP addresses to be associated with the routing process, and assign area IDs to be associated with that range.

Beginning in privileged EXEC mode, follow these steps to enable OSPF:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

router ospf process-id

Enable OSPF routing, changing to router configuration mode. The process ID is an internally used identification parameter that is locally assigned and can be any positive integer. Each OSPF routing process has a unique value.

Step 3 

network address wildcard-mask area area-id

Define an interface on which OSPF runs and the area ID for that interface. You can use the wildcard-mask to use a single command to define one or more multiple interfaces to be associated with a specific OSPF area. The area ID can be a decimal value or an IP address.

Step 4 

end

Return to privileged EXEC mode.

Step 5 

show ip protocols

(Optional) Verify the configuration.

Step 6 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

To terminate an OSPF routing process, use the no router ospf process-id command.

This example configures an OSPF routing process and assigns a process number of 109:

Switch(config)# router ospf 109
Switch(config-router)# network 131.108.0.0 255.255.255.0 area 24

This example verifies the OSPF process ID.

Switch# show ip protocols
<output truncated>
Routing Protocol is "ospf 109"
  Invalid after 0 seconds, hold down 0, flushed after 0
  Outgoing update filter list for all interfaces is
  Incoming update filter list for all interfaces is
  Redistributing: ospf 109
  Maximum path: 4
  Routing for Networks:
    131.108.0.0/24 Area 24
  Routing Information Sources:
    Gateway         Distance      Last Update
  Distance: (default is 110) 

OSPF Interface Parameters

You can use the ip ospf interface configuration commands to modify interface-specific OSPF parameters. You are not required to modify any of these parameters, but some interface parameters (hello interval, dead interval, and authentication key) must be consistent across all routers in an attached network. If you modify these parameters, be sure all routers in the network have compatible values.


Note The ip ospf commands are all optional.


Beginning in privileged EXEC mode, follow these steps to modify OSPF interface parameters:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

interface interface-id

Enter interface configuration mode, and specify the Layer 3 interface to configure.

Step 3 

ip ospf cost

Explicitly specify the cost of sending a packet on the interface.

Step 4 

ip ospf retransmit-interval seconds

Specify the number of seconds between link state advertisement transmissions. The range is 1 to 65535 seconds. The default is 5 seconds.

Step 5 

ip ospf transmit-delay seconds

Set the estimated number of seconds to transmit a link state update packet. The range is 1 to 65535 seconds. The default is 1 second.

Step 6 

ip ospf priority number

Set priority to help determine the OSPF designated router for a network. The range is from 0 to 255. The default is 1.

Step 7 

ip ospf hello-interval seconds

Set the number of seconds between hello packets sent on an OSPF interface. The value must be the same for all nodes on a network. The range is 1 to 65535 seconds. The default is 10 seconds.

Step 8 

ip ospf dead-interval seconds

Set the number of seconds since a device hello packet was seen before its neighbors declare the OSPF router to be down. The value must be the same for all nodes on a network. The range is 1 to 65535 seconds. The default is 4 times the hello interval.

Step 9 

ip ospf authentication-key key

Assign a password to be used by neighboring OSPF routers. The password can be any string of keyboard-entered characters up to 8 bytes in length. All neighboring routers on the same network must have the same password to exchange OSPF information.

Step 10 

ip ospf message digest-key keyid md5 key

Enable MDS authentication.

keyid—an identifier from 1 to 255.

key—an alphanumeric password of up to 16 bytes.

Step 11 

ip ospf database-filter all out

Block flooding of OSPF LSA packets to the interface. By default, OSPF floods new LSAs over all interfaces in the same area, except the interface on which the LSA arrives.

Step 12 

end

Return to privileged EXEC mode.

Step 13 

show ip ospf interface [interface-name]

Display OSPF-related interface information.

Step 14 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

Use the no form of these commands to remove the configured parameter value or return to the default value.

This example shows an output from the show ip ospf interface privileged EXEC command:

Switch# show ip ospf interface
Vlan1 is up, line protocol is up
  Internet Address 172.20.135.202/14, Area 1
  Process ID 1, Router ID 172.20.135.202, Network Type BROADCAST, Cost: 1
  Transmit Delay is 1 sec, State DR, Priority 1
  Designated Router (ID) 172.20.135.202, Interface address 172.20.135.202
  No backup designated router on this network
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    Hello due in 00:00:02
  Index 1/1, flood queue length 0
  Next 0x0(0)/0x0(0)
  Last flood scan length is 0, maximum is 0
  Last flood scan time is 0 msec, maximum is 0 msec
  Neighbor Count is 0, Adjacent neighbor count is 0
  Suppress hello for 0 neighbor(s) 

OSPF Area Parameters

You can optionally configure several OSPF area parameters. These parameters include authentication for password-based protection against unauthorized access to an area, stub areas, and not-so-stubby-areas (NSSAs). Stub areas are areas into which information on external routes is not sent. Instead, the area border router (ABR) generates a default external route into the stub area for destinations outside the autonomous system (AS). An NSSA does not flood all LSAs from the core into the area, but can import AS external routes within the area by redistribution.

Route summarization is the consolidation of advertised addresses into a single summary route to be advertised by other areas. If network numbers are contiguous, you can use the area range command to configure the ABR to advertise a summary route that covers all networks in the range.


Note The OSPF area commands are all optional.


Beginning in privileged EXEC mode, follow these steps to configure area parameters:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

router ospf process-id

Enable OSPF routing, which changes to router configuration mode.

Step 3 

area area-id authentication

Allow password-based protection against unauthorized access to the identified area. The identifier can be either a decimal value or an IP address.

Step 4 

area area-id authentication message-digest

Enable MD5 authentication on the area.

Step 5 

area area-id stub [no-summary]

Define an area as a stub area. The no-summary keyword prevents an ABR from sending summary link advertisements into the stub area.

Step 6 

area area-id nssa [no-redistribution] [default-information-originate]

Defines an area as a not-so-stubby-area. Every router within the same area must agree that the area is NSSA.

no-redistribution—keyword used when the router is a NSSA ABR and you want the redistribute command to import routes into normal areas, but not into the NSSA.

default-information-originate—keyword used on an ABR to allow importing type 7 LSAs into the NSSA area.

Step 7 

area area-id range address mask

Specify an address range for which a single route is advertised. Use this command only with area border routers.

Step 8 

end

Return to privileged EXEC mode.

Step 9 

show ip ospf [process-id]

show ip ospf [process-id [area-id]] database

(Optional) Display information about the OSPF routing process in general or for a specific process ID to verify configuration.

(Optional) Display lists of information related to the OSPF database for a specific router.

Step 10 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

Use the no form of these commands to remove the configured parameter value or to return to the default value.

These examples are results of the show ip ospf database and show ip ospf privileged EXEC commands:

Switch # show ip ospf database

       OSPF Router with ID (172.20.135.202) (Process ID 1)

                Router Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum Link count
172.20.135.202  172.20.135.202  455         0x80000009 0x83C2   1 

Switch# show ip ospf
 Routing Process "ospf 1" with ID 172.20.135.202 and Domain ID 0.0.0.1
 Supports only single TOS(TOS0) routes
 Supports opaque LSA
 SPF schedule delay 5 secs, Hold time between two SPFs 10 secs
 Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs
 Number of external LSA 0. Checksum Sum 0x0
 Number of opaque AS LSA 0. Checksum Sum 0x0
 Number of DCbitless external and opaque AS LSA 0
 Number of DoNotAge external and opaque AS LSA 0
 Number of areas in this router is 1. 1 normal 0 stub 0 nssa
 External flood list length 0
    Area 1
        Number of interfaces in this area is 1
        Area has no authentication
        SPF algorithm executed 1 times
        Area ranges are
        Number of LSA 1. Checksum Sum 0x83C2
        Number of opaque link LSA 0. Checksum Sum 0x0
        Number of DCbitless LSA 0
        Number of indication LSA 0
        Number of DoNotAge LSA 0
        Flood list length 0

Other OSPF Behavior Parameters

You can optionally configure other OSPF parameters in router configuration mode.

Route summarization: When redistributing routes from other protocols as described in the "Redistributing Routing Information" section, each route is advertised individually in an external LSA. To help decrease the size of the OSPF link state database, you can use the summary-address command to advertise a single router for all the redistributed routes included in a specified network address and mask.

Virtual links: In OSPF, all areas must be connected to a backbone area. You can establish a virtual link in case of a backbone-continuity break by configuring two Area Border Routers as endpoints of a virtual link. Configuration information includes the identity of the other virtual endpoint (the other ABR) and the nonbackbone link that the two routers have in common (the transit area). Virtual links cannot be configured through a stub area.

Default route: When you specifically configure redistribution of routes into an OSPF routing domain, the route automatically becomes an autonomous system boundary router (ASBR). You can force the ASBR to generate a default route into the OSPF routing domain.

Domain Name Server (DNS) names for use in all OSPF show command displays makes it easier to identify a router than displaying it by router ID or neighbor ID.

Default Metrics: OSPF calculates the OSPF metric for an interface according to the bandwidth of the interface. The metric is calculated as ref-bw divided by bandwidth, where ref is 10 by default, and bandwidth (bw) is determined by the bandwidth command. For multiple links with high bandwidth, you can specify a larger number to differentiate the cost on those links.

Administrative distance is a rating of the trustworthiness of a routing information source, an integer between 0 and 255, with a higher value meaning a lower trust rating. An administrative distance of 255 means the routing information source cannot be trusted at all and should be ignored. OSPF uses three different administrative distances: routes within an area (interarea), routes to another area (interarea), and routes from another routing domain learned through redistribution (external). You can change any of the distance values.

Passive interfaces: Because interfaces between two devices on an Ethernet represent only one network segment, to prevent OSPF from sending hello packets for the transmitting interface, you must configure the transmitting device to be a passive interface. Both devices can identify each other through the hello packet for the receiving interface.

Route calculation timers: You can configure the delay time between when OSPF receives a topology change and when it starts the shortest path first (SPF) calculation and the hold time between two SPF calculations.

Log neighbor changes: You can configure the router to send a syslog message when an OSPF neighbor state changes, providing a high-level view of changes in the router without using the debug ip osfp adjacency debug command.

Beginning in privileged EXEC mode, follow these steps to configure these OSPF parameters:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

router ospf process-id

Enable OSPF routing, changing to router configuration mode.

Step 3 

summary-address address mask

(Optional) Specify an address and IP subnet mask for redistributed routes so that only one summary route is advertised.

Step 4 

area area-id virtual-link router-id [hello-interval seconds] [retransmit-interval seconds] [trans] [[authentication-key key] | message-digest-key keyid md5 key]]

(Optional) Establish a virtual link and set its parameters. See the "OSPF Interface Parameters" section for parameter definitions and Table 20-7 for virtual link defaults.

Step 5 

default-information originate [always] [metric metric-value] [metric-type type-value] [route-map map-name]

(Optional) Force the ASBR to generate a default route into the OSPF routing domain. Parameters are all optional.

Step 6 

ip ospf name-lookup

(Optional) Configure DNS name lookup. The default is disabled.

Step 7 

ip auto-cost reference-bandwidth ref-bw

(Optional) Specify an address range for which a single route will be advertised. Use this command only with area border routers.

Step 8 

distance ospf {[inter-area dist1] [inter-area dist2] [external dist3]}

(Optional) Change the OSPF distance values. The default distance for each type of route is 110. The range is 1 to 255.

Step 9 

passive-interface type number

(Optional) Suppress the sending of hello packets through the specified interface.

Step 10 

timers spf spf-delay spf-holdtime

(Optional) Configure route calculation timers.

spf-delay—an integer from 0 to 65535. The default is 5 seconds; 0 means no delay.

spf-holdtime—an integer from 0 to 65535. The default is 10 seconds; 0 means no delay.

Step 11 

ospf log-adj-changes

(Optional) Send syslog message when a neighbor state changes.

Step 12 

end

Return to privileged EXEC mode.

Step 13 

show ip ospf [process-id [area-id]] database

(Optional) Display lists of information related to the OSPF database for a specific router. Refer to the "Monitoring OSPF" section for some of the keyword options.

Step 14 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

Change LSA Group Pacing

The OSPF LSA group pacing feature allows the router to group OSPF LSAs and pace the refreshing, check-summing, and aging functions for more efficient router use. This feature is enabled by default with a 4-minute default pacing interval, and you will not usually need to modify this parameter. The optimum group pacing interval is inversely proportional to the number of LSAs the router is refreshing, check-summing, and aging. For example, if you have approximately 10,000 LSAs in the database, decreasing the pacing interval would benefit you. If you have a very small database (40 to 100 LSAs), increasing the pacing interval to 10 to 20 minutes might benefit you slightly.

Beginning in privileged EXEC mode, follow these steps to configure OSPF LSA pacing:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

router ospf process-id

Enable OSPF routing, changing to router configuration mode.

Step 3 

timers lsa-group-pacing seconds

Change the group pacing of LSAs.

Step 4 

end

Return to privileged EXEC mode.

Step 5 

show running-config

(Optional) Verify the configuration.

Step 6 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

To return to the default value, use the no timers lsa-group-pacing command.

Loopback Interface

OSPF uses the largest IP address configured on the interfaces as its router ID. If this interface is down or removed, the OSPF process must recalculate a new router ID and resend all its routing information out its interfaces. If a loopback interface is configured with an IP address, OSPF uses this IP address as its router ID, even if other interfaces have larger IP addresses. Because loopback interfaces never fail, this provides greater stability. OSPF automatically prefers a loopback interface over other interfaces, and it chooses the highest IP address among all loopback interfaces.

Beginning in privileged EXEC mode, follow these steps to configure a loopback interface:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

interface loopback 0

Create a loopback interface, which changes to interface configuration mode.

Step 3 

ip address address mask

Assign an IP address to this interface.

Step 4 

end

Return to privileged EXEC mode.

Step 5 

show ip interface

(Optional) Verify the configuration.

Step 6 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

Use the no interface loopback 0 command to disable the loopback interface.

Monitoring OSPF

You can display specific statistics such as the contents of IP routing tables, caches, and databases.

Table 20-8 lists some of the privileged EXEC commands for displaying statistics. Refer to the Cisco IOS IP and IP Routing Command Reference for Release 12.1 for more show ip ospf database command options and for explanations of fields in the resulting display.

Table 20-8 Show IP OSPF Statistics Commands

Command
Purpose

show ip ospf [process-id]

Display general information about OSPF routing processes.

show ip ospf [process-id] database [router] [link-state-id]

show ip ospf [process-id] database [router] [self-originate]

show ip ospf [process-id] database [router] [adv-router [ip-address]]

show ip ospf [process-id] database [network] [link-state-id]

show ip ospf [process-id] database [summary] [link-state-id]

show ip ospf [process-id] database [asbr-summary] [link-state-id]

show ip ospf [process-id] database [external] [link-state-id]

show ip ospf [process-id area-id] database [database-summary]

Display lists of information related to the OSPF database.

show ip ospf border-routes

Display the internal OSPF routing ABR and ASBR table entries.

show ip ospf interface [interface-name]

Display OSPF-related interface information.

show ip ospf neighbor [interface-name] [neighbor-id] detail

Display OSPF interface neighbor information.

show ip ospf virtual-links

Display OSPF-related virtual links information.


This is sample output from the show ip ospf command when a specific OSPF process ID is not entered:

Switch# show ip ospf
Routing Process "ospf 1" with ID 172.20.135.202 and Domain ID 0.0.0.1
 Supports only single TOS(TOS0) routes
 Supports opaque LSA
 SPF schedule delay 5 secs, Hold time between two SPFs 10 secs
 Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs
 Number of external LSA 0. Checksum Sum 0x0
 Number of opaque AS LSA 0. Checksum Sum 0x0
 Number of DCbitless external and opaque AS LSA 0
 Number of DoNotAge external and opaque AS LSA 0
 Number of areas in this router is 1. 1 normal 0 stub 0 nssa
 External flood list length 0
    Area 1
        Number of interfaces in this area is 1
        Area has no authentication
        SPF algorithm executed 1 times
        Area ranges are
        Number of LSA 1. Checksum Sum 0x39E7
        Number of opaque link LSA 0. Checksum Sum 0x0
        Number of DCbitless LSA 0
        Number of indication LSA 0
        Number of DoNotAge LSA 0
        Flood list length 0 

This is sample output from the show ip ospf database command when no arguments or keywords are used:

Switch# show ip ospf database

O OSPF Router with ID (172.20.135.202) (Process ID 1)


                Router Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum Link count
172.20.135.202  172.20.135.202  1065        0x8000002E 0x39E7   1 

This is sample output of the show ip ospf interface command when Gigabit Ethernet 0/1 is specified:

Switch# show ip ospf interface gigabitethernet0/1

GigabitEthernet 0/1 is up, line protocol is up
Internet Address 131.119.254.202, Mask 255.255.255.0, Area 0.0.0.0
AS 201, Router ID 192.77.99.1, Network Type BROADCAST, Cost: 10
Transmit Delay is 1 sec, State OTHER, Priority 1
Designated Router id 131.119.254.10, Interface address 131.119.254.10
Backup Designated router id 131.119.254.28, Interface addr 131.119.254.28
Timer intervals configured, Hello 10, Dead 60, Wait 40, Retransmit 5
Hello due in 0:00:05
Neighbor Count is 8, Adjacent neighbor count is 2
  Adjacent with neighbor 131.119.254.28  (Backup Designated Router)
  Adjacent with neighbor 131.119.254.10  (Designated Router)

This is sample output from the show ip ospf neighbor command showing a single line of summary information for each neighbor:

Switch# show ip ospf neighbor

   ID          Pri   State        Dead Time     Address         Interface
199.199.199.137 1    FULL/DR       0:00:31    160.89.80.37      Ethernet0
192.31.48.1     1    FULL/DROTHER  0:00:33    192.31.48.1       Fddi0
192.31.48.200   1    FULL/DROTHER  0:00:33    192.31.48.200     Fddi0
199.199.199.137 5    FULL/DR       0:00:33    192.31.48.189     Fddi0

This is sample output from the show ip ospf virtual-links command:

Switch# show ip ospf virtual-links

Virtual Link to router 160.89.101.2 is up
Transit area 0.0.0.1, via interface Ethernet0, Cost of using 10
Transmit Delay is 1 sec, State POINT_TO_POINT
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 0:00:08
Adjacency State FULL

Configuring EIGRP

Enhanced IGRP (EIGRP) is a Cisco proprietary enhanced version of the IGRP. Enhanced IGRP uses the same distance vector algorithm and distance information as IGRP; however, the convergence properties and the operating efficiency of Enhanced IGRP are significantly improved.

The convergence technology employs an algorithm referred to as the Diffusing Update Algorithm (DUAL), which guarantees loop-free operation at every instant throughout a route computation and allows all devices involved in a topology change to synchronize at the same time. Routers that are not affected by topology changes are not involved in recomputations.

IP EIGRP provides increased network width. With RIP, the largest possible width of your network is 15 hops. When IGRP is enabled, the largest possible width is 224 hops. Because the EIGRP metric is large enough to support thousands of hops, the only barrier to expanding the network is the transport-layer hop counter. EIGRP increments the transport control field only when an IP packet has traversed 15 routers and the next hop to the destination was learned through EIGRP. When a RIP route is being used as the next hop to the destination, the transport control field is incremented as usual.

EIGRP offers these features:

Fast convergence.

Incremental updates when the state of a destination changes, instead of sending the entire contents of the routing table, minimizing the bandwidth required for EIGRP packets.

Less CPU usage than IGRP because full update packets need not be processed each time they are received.

Protocol-independent neighbor discovery mechanism to learn about neighboring routers.

Variable-length subnet masks (VLSMs).

Arbitrary route summarization.

EIGRP scales to large networks.

Enhanced IGRP has these four basic components:

Neighbor discovery and recovery is the process that routers use to dynamically learn of other routers on their directly attached networks. Routers must also discover when their neighbors become unreachable or inoperative. Neighbor discovery and recovery is achieved with low overhead by periodically sending small hello packets. As long as hello packets are received, the Cisco IOS software can determine that a neighbor is alive and functioning. Once this status is determined, the neighboring routers can exchange routing information.

The reliable transport protocol is responsible for guaranteed, ordered delivery of EIGRP packets to all neighbors. It supports intermixed transmission of multicast and unicast packets. Some EIGRP packets must be sent reliably, and others need not be. For efficiency, reliability is provided only when necessary. For example, on a multiaccess network that has multicast capabilities (such as Ethernet), it is not necessary to send hellos reliably to all neighbors individually. Therefore, EIGRP sends a single multicast hello with an indication in the packet informing the receivers that the packet need not be acknowledged. Other types of packets (such as updates) require acknowledgment, which is shown in the packet. The reliable transport has a provision to send multicast packets quickly when there are unacknowledged packets pending. Doing so helps ensure that convergence time remains low in the presence of varying speed links.

The DUAL finite state machine embodies the decision process for all route computations. It tracks all routes advertised by all neighbors. DUAL uses the distance information (known as a metric) to select efficient, loop-free paths. DUAL selects routes to be inserted into a routing table based on feasible successors. A successor is a neighboring router used for packet forwarding that has a least-cost path to a destination that is guaranteed not to be part of a routing loop. When there are no feasible successors, but there are neighbors advertising the destination, a recomputation must occur. This is the process whereby a new successor is determined. The amount of time it takes to recompute the route affects the convergence time. Recomputation is processor-intensive; it is advantageous to avoid recomputation if it is not necessary. When a topology change occurs, DUAL tests for feasible successors. If there are feasible successors, it uses any it finds to avoid unnecessary recomputation.

The protocol-dependent modules are responsible for network layer protocol-specific tasks. An example is the IP EIGRP module, which is responsible for sending and receiving EIGRP packets that are encapsulated in IP. It is also responsible for parsing EIGRP packets and informing DUAL of the new information received. EIGRP asks DUAL to make routing decisions, but the results are stored in the IP routing table. EIGRP is also responsible for redistributing routes learned by other IP routing protocols.

Table 20-6 shows the default EIGRP configuration.

Table 20-9 Default EIGRP Configuration 

Feature
Default Setting

Auto summary

Enabled. Subprefixes are summarized to the classful network boundary when crossing classful network boundaries.

Default-information

Exterior routes are accepted and default information is passed between IGRP or EIGRP processes when doing redistribution.

Default metric

Only connected routes and interface static routes can be redistributed without a default metric. The metric includes:

Bandwidth: 0 or greater kbps.

Delay (tens of microseconds): 0 or any positive number that is a multiple of 39.1 nanoseconds.

Reliability: any number between 0 and 255 (255 means 100 percent reliability).

Loading: effective bandwidth as a number between 0 and 255 (255 is 100 percent loading).

MTU: maximum transmission unit size of the route in bytes. 0 or any positive integer.

Distance

Internal distance: 90.

External distance: 170.

EIGRP log-neighbor changes

Disabled. No adjacency changes logged.

IP authentication key-chain

No authentication provided.

IP authentication mode

No authentication provided.

IP bandwidth-percent

50 percent.

IP hello interval

For low-speed nonbroadcast multiaccess (NBMA) networks: 60 seconds; all other networks: 5 seconds.

IP hold-time

For low-speed NBMA networks: 180 seconds; all other networks: 15 seconds.

IP split-horizon

Enabled.

IP summary address

No summary aggregate addresses are predefined.

Metric weights

tos: 0; k1 and k3: 1; k2, k4, and k5: 0

Network

None specified.

Offset-list

Disabled.

Router EIGRP

Disabled.

Set metric

No metric set in the route map.

Traffic-share

Distributed proportionately to the ratios of the metrics.

Variance

1 (equal-cost load balancing).


To create an EIGRP routing process, you must enable EIGRP and associate networks. EIGRP sends updates to the interfaces in the specified networks. If you do not specify an interface network, it is not advertised in any EIGRP update.


Note If you have routers on your network that are configured for IGRP, and you want to change to EIGRP, you must designate transition routers that have both IGRP and EIGRP configured. In these cases, perform Steps 1 through 3 in the next section and also refer to the "Configuring IGRP" section. You must use the same autonomous system number in order for routes to be redistributed automatically.


EIGRP Router Mode Commands

Beginning in privileged EXEC mode, follow these steps to configure EIGRP. Configuring the routing process is required; other steps are optional:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

router eigrp autonomous-system

Enable an EIGRP routing process, changing to router configuration mode. The autonomous system number identifies the routes to other EIGRP routers and is used to tag routing information.

Step 3 

network network-number

Associate networks with an EIGRP routing process. EIGRP sends updates to the interfaces in the specified networks. If an interface's network is not specified, it is not advertised in any IGRP or EIGRP update.

Step 4 

eigrp log-neighbor-changes

(Optional) Enable logging of EIGRP neighbor changes to monitor routing system stability.

Step 5 

metric weights tos k1 k2 k3 k4 k5

(Optional) Adjust the EIGRP metric. Although the defaults have been carefully determined to provide excellent operation in most networks, you can adjust them.


Caution Determining metrics is complex and is not recommended without guidance from an experienced network designer.

Step 6 

offset list [access-list number | name] {in | out} offset [type number]

(Optional) Apply an offset list to routing metrics to increase incoming and outgoing metrics to routes learned through EIGRP. You can limit the offset list with an access list or an interface.

Step 7 

no auto-summary

(Optional) Disable automatic summarization of subnet routes into network-level routes.

Step 8 

ip summary-address eigrp autonomous-system-number address mask

(Optional) Configure a summary aggregate.

Step 9 

end

Return to privileged EXEC mode.

Step 10 

show ip protocols

(Optional) Verify the configuration.

Step 11 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

Use the no forms of these commands to disable the feature or return the setting to the default value.

This is an example of the output from the show ip protocols privileged EXEC command for EIGRP.

Switch# show ip protocols

<output truncated>

Routing Protocol is "eigrp 1"

Outgoing update filter list for all interfaces is

Incoming update filter list for all interfaces is

Default networks flagged in outgoing updates

Default networks accepted from incoming updates

EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0

EIGRP maximum hopcount 100

EIGRP maximum metric variance 1

Redistributing: eigrp 1, igrp 1

Automatic network summarization is in effect

Maximum path: 4

Routing for Networks:

172.20.0.0

Routing Information Sources:

Gateway Distance Last Update

Distance: internal 90 external 170

EIGRP Interface Mode Commands

Other optional EIGRP parameters can be configured on an interface basis.

Beginning in privileged EXEC mode, follow these steps:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

interface interface-id

Enter interface configuration mode, and specify the Layer 3 interface to configure.

Step 3 

ip bandwidth-percent eigrp percent

(Optional) Configure the percentage of bandwidth that can be used by EIGRP on an interface. The default is 50 percent.

Step 4 

ip summary-address eigrp autonomous-system-number address mask

(Optional) Configure a summary aggregate address for a specified interface (not usually necessary if auto-summary is enabled).

Step 5 

ip hello-interval eigrp autonomous-system-number seconds

(Optional) Change the hello time interval for an EIGRP routing process. The range is 1 to 65535 seconds. The default is 60 seconds for low-speed NBMA networks and 5 seconds for all other networks.

Step 6 

ip hold-time eigrp autonomous-system-number seconds

(Optional) Change the hold time interval for an EIGRP routing process. The range is 1 to 65535 seconds. The default is 180 seconds for low-speed NBMA networks and 15 seconds for all other networks.


Caution Do not adjust the hold time without consulting Cisco technical support.

Step 7 

no ip split-horizon eigrp autonomous-system-number

(Optional) Disable split horizon to allow route information to be advertised by a router out any interface from which that information originated.

Step 8 

show ip eigrp interfaces

(Optional) Display which interfaces EIGRP is active on and information about EIGRP relating to those interfaces.

Step 9 

end

Return to privileged EXEC mode.

Step 10 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

Use the no forms of these commands to disable the feature or return the setting to the default value.

This is an example of the show ip eigrp interface privileged EXEC command:

Switch# show ip eigrp interface
IP-EIGRP interfaces for process 1

                    Xmit Queue   Mean   Pacing Time   Multicast    Pending
Interface    Peers  Un/Reliable  SRTT   Un/Reliable   Flow Timer   Routes
Vl1            1        0/0      1000       0/10           0           0 

Configure EIGRP Route Authentication

EIGRP route authentication provides MD5 authentication of routing updates from the EIGRP routing protocol to prevent the introduction of unauthorized or false routing messages from unapproved sources.

Beginning in privileged EXEC mode, follow these steps to enable authentication:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

interface interface-id

Enter interface configuration mode, and specify the Layer 3 interface to configure.

Step 3 

ip authentication mode eigrp autonomous-system md5

Enable MD5 authentication in IP EIGRP packets.

Step 4 

ip authentication key-chain eigrp autonomous-system key-chain

Enable authentication of IP EIGRP packets.

Step 5 

exit

Return to global configuration mode.

Step 6 

key chain name-of-chain

Identify a key chain and enter key-chain configuration mode. Match the name configured in Step 4.

Step 7 

key number

In key-chain configuration mode, identify the key number.

Step 8 

key-string text

In key-chain key configuration mode, identify the key string.

Step 9 

accept-lifetime start-time {infinite | end-time | duration seconds}

(Optional) Specify the time period during which the key can be received.

The start-time and end-time syntax can be either hh:mm:ss Month date year or hh:mm:ss date Month year. The default is forever with the default start-time and the earliest acceptable date ss January 1, 1993. The default end-time and duration is infinite.

Step 10 

send-lifetime start-time {infinite | end-time | duration seconds}

(Optional) Specify the time period during which the key can be sent.

The start-time and end-time syntax can be either hh:mm:ss Month date year or hh:mm:ss date Month year. The default is forever with the default start-time and the earliest acceptable date is January 1, 1993. The default end-time and duration is infinite.

Step 11 

end

Return to privileged EXEC mode.

Step 12 

show key chain

(Optional) Display authentication key information.

Step 13 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

Use the no forms of these commands to disable the feature or to return the setting to the default value.

Monitoring and Maintaining EIGRP

You can display delete neighbors from the neighbor table. You can also display various EIGRP routing statistics. Table 20-10 lists the privileged EXEC commands for deleting neighbors and displaying statistics. Refer to Refer to the Cisco IOS IP and IP Routing Command Reference for Release 12.1 for explanations of fields in the resulting display.

Table 20-10  IP EIGRP Clear and Show Commands  

Command
Purpose

clear ip eigrp neighbors [if-address | interface]

Delete neighbors from the neighbor table.

show ip eigrp interface [interface] [as number]

Display information about interfaces configured for EIGRP.

show ip eigrp neighbors [type-number]

Display EIGRP discovered neighbors.

show ip eigrp topology [autonomous-system-number] | [[ip-address] mask]]

Display the EIGRP topology table for a given process.

show ip eigrp traffic [autonomous-system-number]

Display the number of packets sent and received for all or a specified EIGRP process.


This is sample output from the show ip eigrp interfaces command:

Switch# show ip eigrp interfaces

IP EIGRP interfaces for process 109
Xmit Queue    Mean   Pacing Time   Multicast   Pending
Interface   Peers   Un/Reliable   SRTT   Un/Reliable   Flow Timer  Routes
Gi0/1         0         0/0          0      11/434          0          0
Gi0/3         1         0/0        337       0/10           0          0
Gi0/4         1         0/0         10       1/63         103          0
Gi0/5         1         0/0        330       0/16           0          0

This is sample output from the show ip eigrp neighbors command:

Switch# show ip eigrp neighbors

IP-EIGRP Neighbors for process 77
Address            Interface           Hold Uptime         Q        Seq    Type 
                                        (secs)   (h:m:s)  Count      Num   
160.89.81.28       GigabitEthernet 0/1    13      0:00:41   0         11     
160.89.80.28       GigabitEthernet 0/3    14      0:02:01   0         10     
160.89.80.31       GigabitEthernet 0/4    12      0:02:02   0          4     

This is sample output from the show ip eigrp topology command:

Switch# show ip eigrp topology
 
IP-EIGRP Topology Table for process 77
 
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - Reply status
 
P 160.89.90.0 255.255.255.0, 2 successors, FD is 0
          via 160.89.80.28 (46251776/46226176), GigabitEthernet0/1
          via 160.89.81.28 (46251776/46226176), GigabitEthernet0/3
          via 160.89.80.31 (46277376/46251776), GigabitEthernet0/1
P 160.89.81.0 255.255.255.0, 1 successors, FD is 307200
          via Connected, GigabitEthernet0/3
          via 160.89.81.28 (307200/281600), GigabitEthernet0/3
          via 160.89.80.28 (307200/281600), GigabitEthernet0/1
          via 160.89.80.31 (332800/307200), GigabitEthernet0/1

This is sample output from the show ip eigrp traffic command:

Switch# show ip eigrp traffic

IP-EIGRP Traffic Statistics for process 1
  Hellos sent/received: 19812/19821
  Updates sent/received: 1/1
  Queries sent/received: 0/0
  Replies sent/received: 0/0
  Acks sent/received: 1/0
  Input queue high water mark 1, 0 drops
  SIA-Queries sent/received: 0/0
  SIA-Replies sent/received: 0/0 

Configuring Protocol-Independent Features

This section describes how to configure IP routing protocol-independent features. For a complete description of the IP routing protocol-independent commands in this chapter, refer to the "IP Routing Protocol-Independent Commands" chapter of the Cisco IOS IP and IP Routing Command Reference for Release 12.1.

This section includes these procedures:

Configuring Cisco Express Forwarding

Configuring the Number of Equal-Cost Routing Paths

Configuring Static Routes

Specifying Default Routes

Redistributing Routing Information

Filtering Routing Information

Managing Authentication Keys

Configuring Cisco Express Forwarding

Cisco Express Forwarding (CEF) is a Layer 3 IP switching technology used to optimizes network performance. CEF implements an advanced IP look-up and forwarding algorithm to deliver maximum Layer 3 switching performance. CEF is less CPU-intensive than fast switching route caching, allowing more CPU processing power to be dedicated to packet forwarding. In the Catalyst 3550 switch, the hardware uses CEF to achieve Gigabit speed line rate IP traffic. In dynamic networks, fast switching cache entries are frequently invalidated due to routing changes, which can cause traffic to be process switched using the routing table, instead of fast switched using the route cache. CEF uses the Forwarding Information Base (FIB) lookup table to perform destination-based switching of IP packets.

The two main components in CEF are the FIB and adjacency tables.

The FIB is similar to a routing table or information base and maintains a mirror image of the forwarding information in the IP routing table. When routing or topology changes occur in the network, the IP routing table is updated, and those changes are reflected in the FIB. The FIB maintains next-hop address information based on the information in the IP routing table. Because the FIB contains all known routes that exist in the routing table, CEF eliminates route cache maintenance, is more efficient for switching traffic, and is not affected by traffic patterns.

Nodes in the network are said to be adjacent if they can reach each other with a single hop across a link layer. CEF uses adjacency tables to prepend Layer 2 addressing information. The adjacency table maintains Layer 2 next-hop addresses for all FIB entries.

CEF is enabled globally by default. If for some reason it is disabled, you can re-enable it by using the ip cef global configuration command.

The default configuration, which is recommended, is CEF enabled on all Layer 3 interfaces. You can disable CEF on an interface by using the no ip route-cache cef interface configuration command; you can enable CEF on an interface by using the ip route-cache cef interface configuration command.

Beginning in privileged EXEC mode, follow these steps to enable CEF on an interface after it has been disabled:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

interface interface-id

Enter interface configuration mode, and specify the Layer 3 interface to configure.

Step 3 

ip route cache cef

Enable CEF on the interface.

Step 4 

end

Return to privileged EXEC mode.

Step 5 

show ip cef

(Optional) Display the CEF status on all interfaces.

Step 6 

show adjacency

(Optional) Display CEF adjacency table information.

Step 7 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

This is an example of the show ip cef privileged EXEC command:

Switch# show ip cef
Prefix              Next Hop             Interface
0.0.0.0/32          receive
1.0.0.0/24          attached             GigabitEthernet0/2
1.0.0.0/32          receive
1.0.0.1/32          receive
1.0.0.55/32         1.0.0.55             GigabitEthernet0/2
1.0.0.255/32        receive
2.0.0.0/24          attached             GigabitEthernet0/3
2.0.0.0/32          receive
2.0.0.1/32          receive
2.0.0.55/32         2.0.0.55             GigabitEthernet0/3
2.0.0.255/32        receive
224.0.0.0/4         drop
224.0.0.0/24        receive
255.255.255.255/32  receive

This is an example of the show adjacency privileged EXEC command:

Switch# show adjacency
Protocol Interface                 Address
IP       GigabitEthernet0/3        2.0.0.55(5)
IP       GigabitEthernet0/2        1.0.0.55(5)

Configuring the Number of Equal-Cost Routing Paths

When a router has two or more routes to the same network with the same metrics, these routes can be thought of as having an equal cost. The term parallel path is another way to refer to occurrences of equal-cost routes in a routing table. If a router has two or more equal-cost paths to a network, it can use them concurrently. In addition to providing redundancy in case of a circuit failure, the availability of parallel paths enables a router to load balance packets over the available paths for more efficient use of available bandwidth.

Although the router automatically learns about and configures equal-cost routes, you can control the maximum number of parallel paths supported by an IP routing protocol in its routing table.

Beginning in privileged EXEC mode, follow these steps to change the maximum number of parallel paths installed in a routing table from the default:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

router protocol

Enter a router configuration mode.

Step 3 

maximum-paths maximum

Set the maximum number of parallel paths for the protocol routing table. The range is from 1 to 6; the default is 4.

Step 4 

end

Return to privileged EXEC mode.

Step 5 

show ip protocols

(Optional) Verify the setting in the Maximum path field.

Step 6 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

Use the no maximum-paths command to restore the default value.

Configuring Static Routes

Static routes are user-defined routes that cause packets moving between a source and a destination to take a specified path. Static routes can be important if the router cannot build a route to a particular destination. They are also useful for specifying a gateway of last resort to which all unroutable packets are sent.

Beginning in privileged EXEC mode, follow these steps to configure a static route:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

ip route prefix mask {address | interface} [distance]

Establish a static route.

Step 3 

end

Return to privileged EXEC mode.

Step 4 

show ip route

(Optional) Display the current state of the routing table to verify the configuration.

Step 5 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

This is an example of the show ip route privileged EXEC command with a static route configured:

Switch# show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
       * - candidate default, U - per-user static route, o - ODR
       P - periodic downloaded static route

Gateway of last resort is 172.20.135.193 to network 0.0.0.0

S*   0.0.0.0/0 [1/0] via 172.20.135.193
C    172.20.0.0/14 is directly connected, Vlan1 

The switch retains static routes until you remove them (by using the no form of the ip route global configuration command). However, you can override static routes with dynamic routing information by assigning administrative distance values. Each dynamic routing protocol has a default administrative distance, as listed in Table 20-11. If you want a static route to be overridden by information from a dynamic routing protocol, set the administrative distance of the static route higher than that of the dynamic protocol.

Table 20-11  Dynamic Routing Protocol Default Administrative Distances  

Route Source
Default Distance

Connected interface

0

Static route

1

Enhanced IRGP summary route

5

Internal Enhanced IGRP

90

IGRP

100

OSPF

110

RIP

120

Unknown

225


Static routes that point to an interface are advertised through RIP, IGRP, and other dynamic routing protocols, regardless of whether redistribute static commands were specified for those routing protocols. These static routes are advertised because static routes that point to an interface are considered in the routing table to be connected and hence lose their static nature. However, if you define a static route to an interface that is not one of the networks defined in a network command, no dynamic routing protocols advertise the route unless a redistribute static command is specified for these protocols.

When an interface goes down, all static routes through that interface are removed from the IP routing table. When the software can no longer find a valid next hop for the address specified as the forwarding router's address in a static route, the static route is also removed from the IP routing table.

Specifying Default Routes

A router might not be able to determine the routes to all other networks. To provide complete routing capability, you can use some routers as smart routers and give the remaining routers default routes to the smart router. (Smart routers have routing table information for the entire internetwork.) These default routes can be learned dynamically or can be configured in the individual routers. Most dynamic interior routing protocols include a mechanism for causing a smart router to generate dynamic default information that is then forwarded to other routers.

Specifying a Default Network

If a router has a directly connected interface to the specified default network, the dynamic routing protocols running on that device generate or source a default route. In the case of RIP, it advertises the pseudonetwork 0.0.0.0. In the case of IGRP, the network itself is advertised and flagged as an exterior route.

A router that is generating the default for a network also might need a default of its own. One way a router can generate its own default is to specify a static route to the network 0.0.0.0 through the appropriate device.

Beginning in privileged EXEC mode, follow these steps to define a static route to a network as the static default route:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

ip default-network network number

Specify a default network.

Step 3 

end

Return to privileged EXEC mode.

Step 4 

show ip route

(Optional) Display the selected default route in the gateway of last resort display.

Step 5 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

Use the no ip default-network network number command to remove the route.

When default information is passed through a dynamic routing protocol, no further configuration is required. The system periodically scans its routing table to choose the optimal default network as its default route. In IGRP networks, there might be several candidate networks for the system default. Cisco routers use administrative distance and metric information to determine the default route or the gateway of last resort.

If dynamic default information is not being passed to the system, candidates for the default route are specified with the ip default-network command. If this network appears in the routing table from any source, it is flagged as a possible choice for the default route. If the router has no interface on the default network, but does have a path to it, the network is considered as a possible candidate, and the gateway to the best default path becomes the gateway of last resort.

Redistributing Routing Information

In addition to running multiple routing protocols simultaneously, the switch can redistribute information from one routing protocol to another. For example, you can instruct the switch to readvertise IGRP-derived routes by using RIP or to readvertise static routes by using IGRP. Redistributing information from one routing protocol to another applies to all supported IP-based routing protocols.

You also can conditionally control the redistribution of routes between routing domains by defining a method known as route maps between the two domains. Although redistribution is a protocol-independent feature, some of the match and set commands are specific to a particular protocol.

One or more match commands and one or more set commands follow a route-map command. If there are no match commands, everything matches. If there are no set commands, nothing is done, other than the match. Therefore, you need at least one match or set command.


Note Although each of Steps 3 through 14 in the following section is optional, you must enter at least one match command and one set command.


Beginning in privileged EXEC mode, follow these steps to a route map for redistribution:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

route-map map-tag {permit | deny} [sequence number]

Define any route maps needed to control redistribution and enter route-map configuration mode.

Step 3 

match ip address {access-list-number | access-list-name} [...access-list-number |...access-list-name]

Match a standard access list.

Step 4 

match metric metric value

Match the specified metric.

Step 5 

match ip next-hop {access-list-number | access-list-name} [...access-list-number |...access-list-name]

Match a next-hop router address passed by one of the access lists specified.

Step 6 

match tag tag value [...tag-value]

Match the specified tag value.

Step 7 

match interface type number [...type number]

Match the specified next hop route out one of the specified interfaces.

Step 8 

match ip route-source {access-list-number | access-list-name} [...access-list-number |...access-list-name]

Match the address specified by the specified advertised access lists.

Step 9 

match route-type {local | internal | external [type-1 | type-2] | level-1 | level-2}

Match the specified route type.

Step 10 

set next-hop next-hop

Specify the address of the next hop.

Step 11 

set level {level-1 | level-2 | level-1-2 | stub-area | backbone}

Set the level for routes that are advertised into the specified area of the routing domain.

Step 12 

set metric metric value

Set the metric value to give the redistributed routes (for any protocol except IGRP or EIGRP).

Step 13 

set metric bandwidth delay reliability loading mtu

Set the metric value to give the redistributed routes (for IGRP or EIGRP only)

Step 14 

set metric-type {internal | external | type-1 | type-2}

Set the metric type to give redistributed routes.

Step 15 

end

Return to privileged EXEC mode.

Step 16 

show route-map

(Optional) Display all route maps configured or only the one specified to verify configuration.

Step 17 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

To delete an entry, use the no route-map map tag command or the no form of any of the match or set commands.

You can distribute routes from one routing domain into another and control route distribution.

Beginning in privileged EXEC mode, follow these steps to control route redistribution:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

router {rip | ospf | igrp | eigrp}

Enter a router configuration mode.

Step 3 

redistribute protocol [process-id] {level-1 | level-1-2 | level-2} [metric metric-value] [metric-type type-value] [match internal | external type-value] [tag tag-value] [route-map map-tag] [weight weight] [subnets]

Redistribute routes from one routing protocol to another routing protocol.

Step 4 

default-metric number

Cause the current routing protocol to use the same metric value for all redistributed routes (RIP and OSPF).

Step 5 

default-metric bandwidth delay reliability loading mtu

Cause the IGRP or EIGRP routing protocol to use the same metric value for all non-IGRP redistributed routes.

Step 6 

no default-information {in | out}

Disable the redistribution of default information between IGRP processes, which is enabled by default.

Step 7 

show route-map

(Optional) Display all route maps configured or only the one specified to verify configuration.

Step 8 

end

Return to privileged EXEC mode.

Step 9 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

To disable redistribution, use the no form of the commands.

The metrics of one routing protocol do not necessarily translate into the metrics of another. For example, the RIP metric is a hop count, and the IGRP metric is a combination of five qualities. In these situations, an artificial metric is assigned to the redistributed route. Uncontrolled exchanging of routing information between different routing protocols can create routing loops and seriously degrade network operation.

If you have not defined a default redistribution metric that replaces metric conversion, some automatic metric translations occur between routing protocols:

RIP can automatically redistribute static routes. It assigns static routes a metric of 1 (directly connected).

IGRP can automatically redistribute static routes and information from other IGRP-routed autonomous systems. IGRP assigns static routes a metric that identifies them as directly connected. It does not change the metrics of routes derived from IGRP updates from other autonomous systems.

Any protocol can redistribute other routing protocols if a default mode is in effect.

Filtering Routing Information

You can filter routing protocol information by performing the tasks described in this section.


Note When routes are redistributed between OSPF processes, no OSPF metrics are preserved.


Setting Passive Interfaces

To prevent other routers on a local network from learning about routes dynamically, you can use the router configuration passive-interface command to keep routing update messages from being sent through a router interface. When you use this command in the OSPF protocol, the interface address you specify as passive appears as a stub network in the OSPF domain. OSPF routing information is neither sent nor received through the specified router interface.

In networks with many interfaces, to avoid having to manually set them as passive, you can set all interfaces to be passive by default by using the passive-interface default command and actively set interfaces where adjacencies are desired.

Beginning in privileged EXEC mode, follow these steps to configure passive interfaces:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

router protocol

Enter a router configuration mode.

Step 3 

passive-interface interface-id

Suppress sending routing updates through the specified Layer 3 interface.

Step 4 

passive-interface default

(Optional) Set all interfaces as passive by default.

Step 5 

no passive-interface interface type

(Optional) Activate only those interfaces that need to have adjacencies sent.

Step 6 

network network-address

(Optional) Specify the list of networks for the routing process. The network-address is an IP address.

Step 7 

end

Return to privileged EXEC mode.

Step 8 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

Use a network monitoring user EXEC command such as show ip ospf interface to verify the interfaces that you enabled as passive, or use the show ip interface user EXEC command to verify the interfaces that you enabled as active.

To re-enable the sending of routing updates, use the no passive-interface interface-id command. The default keyword sets all interfaces as passive by default. You can then configure individual interfaces where you want adjacencies by using the no passive-interface command. The default keyword is useful in Internet service provider and large enterprise networks where many of the distribution routers have more than 200 interfaces.

Controlling Advertising and Processing in Routing Updates

You can use the distribute-list command with access control lists to suppress routes from being advertised in routing updates and to prevent other routers from learning one or more routes. When used in OSPF, this feature applies only to external routes, and you cannot specify an interface name.

You can also use a distribute-list command to avoid processing certain routes listed in incoming updates. (This feature does not apply to OSPF.)

Beginning in privileged EXEC mode, follow these steps to control the advertising or processing of routing updates:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

router protocol

Enter a router configuration mode.

Step 3 

distribute-list {access-list-number | access-list-name} out [interface-name | routing process | autonomous-system-number]

Permit or deny routes from being advertised in routing updates, depending upon the action listed in the access list.

Step 4 

distribute-list {access-list-number | access-list-name} in [type-number]

Suppress processing in routes listed in updates.

Step 5 

end

Return to privileged EXEC mode.

Step 6 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

Use the no distribute-list in router configuration command to change or cancel a filter. To cancel suppression of network advertisements in updates, use the no distribute-list out router configuration command.

Filtering Sources of Routing Information

Because some routing information might be more accurate than others, you can use filtering to prioritize information coming from different sources. An administrative distance is a rating of the trustworthiness of a routing information source, such as a router or group of routers. In a large network, some routing protocols can be more reliable than others. By specifying administrative distance values, you enable the router to intelligently discriminate between sources of routing information. The router always picks the route whose routing protocol has the lowest administrative distance. Table 20-11 shows the default administrative distances for various routing information sources.

Because each network has its own requirements, there are no general guidelines for assigning administrative distances.

Beginning in privileged EXEC mode, follow these steps to filter sources of routing information:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

router protocol

Enter router configuration mode.

Step 3 

distance weight {ip-address {ip-address mask}}
[ip access list]

Define an administrative distance.

weight is the administrative distance as an integer from 10 to 255. Used alone, weight specifies a default administrative distance that is used when no other specification exists for a routing information source. Routes with a distance of 255 are not installed in the routing table.

(Optional) ip access list is an IP standard or extended access list to be applied to incoming routing updates.

Step 4 

end

Return to privileged EXEC mode.

Step 5 

show ip protocols

(Optional) Display the default administrative distance for a specified routing process.

Step 6 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

To remove a distance definition, use the no distance command.

Managing Authentication Keys

Key management is a method of controlling authentication keys used by routing protocols. Not all protocols can use key management. Authentication keys are available for EIGRP and RIP Version 2.

Before you manage authentication keys, you must enable authentication. See the appropriate protocol section to see how to enable authentication for that protocol. To manage authentication keys, define a key chain, identify the keys that belong to the key chain, and specify how long each key is valid. Each key has its own key identifier (specified with the key number command), which is stored locally. The combination of the key identifier and the interface associated with the message uniquely identifies the authentication algorithm and Message Digest 5 (MD5) authentication key in use.

You can configure multiple keys with life times. Only one authentication packet is sent, regardless of how many valid keys exist. The software examines the key numbers in order from lowest to highest, and uses the first valid key it encounters. The lifetimes allow for overlap during key changes. Note that the router must know these lifetimes.

Beginning in privileged EXEC mode, follow these steps to manage authentication keys:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

key chain name-of-chain

Identify a key chain, and enter key chain configuration mode.

Step 3 

key number

Identify the key number.

Step 4 

key-string text

Identify the key string.

Step 5 

accept-lifetime start-time {infinite | end-time | duration seconds}

(Optional) Specify the time period during which the key can be received.

The start-time and end-time syntax can be either hh:mm:ss Month date year or hh:mm:ss date Month year. The default is forever with the default start-time and the earliest acceptable date is January 1, 1993. The default end-time and duration is infinite.

Step 6 

send-lifetime start-time {infinite | end-time | duration seconds}

(Optional) Specify the time period during which the key can be sent.

The start-time and end-time syntax can be either hh:mm:ss Month date year or hh:mm:ss date Month year. The default is forever with the default start-time and the earliest acceptable date as January 1, 1993. The default end-time and duration is infinite.

Step 7 

end

Return to privileged EXEC mode.

Step 8 

show key chain

(Optional) Display authentication key information.

Step 9 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

To remove the key chain, use the no key chain name-of-chain command.

Monitoring and Maintaining the IP Network

You can remove all contents of a particular cache, table, or database. You can also display specific statistics.

Beginning in privileged EXEC mode, use these commands to clear routes or display status:

Command
Purpose

clear ip route {network [mask | *]}

Clear one or more routes from the IP routing table.

show ip protocols

Display the parameters and state of the active routing protocol process.

show ip route [address [mask] [longer-prefixes]] | [protocol [process-id]]

Display the current state of the routing table.

show ip route summary

Display the current state of the routing table in summary form.

show ip route supernets-only

Display supernets.

show ip cache

Display the routing table used to switch IP traffic.

show route-map [map-name]

Display all route maps configured or only the one specified.


This is a sample output from the show ip route command when entered without an address:

Switch# show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
       * - candidate default, U - per-user static route, o - ODR
       P - periodic downloaded static route

Gateway of last resort is 172.20.135.193 to network 0.0.0.0

S*   0.0.0.0/0 [1/0] via 172.20.135.193
C    172.20.0.0/14 is directly connected, Vlan1

This is sample output from the show ip protocols command, showing IGRP processes:

Switch# show ip protocols

Routing Protocol is "igrp 1"
  Sending updates every 90 seconds, next due in 65 seconds
  Invalid after 270 seconds, hold down 280, flushed after 630
  Outgoing update filter list for all interfaces is
  Incoming update filter list for all interfaces is
  Default networks flagged in outgoing updates
  Default networks accepted from incoming updates
  IGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
  IGRP maximum hopcount 100
  IGRP maximum metric variance 1
  Redistributing: eigrp 1, igrp 1
  Maximum path: 4
  Routing for Networks:
    172.20.0.0
  Routing Information Sources:
    Gateway         Distance      Last Update
  Distance: (default is 100) 

This is a sample output from the show ip route summary command:

Switch# show ip route summary
IP routing table name is Default-IP-Routing-Table(0)
Route Source    Networks    Subnets     Overhead    Memory (bytes)
connected       1           0           64          144
static          1           0           64          144
rip             0           0           0           0
eigrp 1         0           0           0           0
igrp 1          0           0           0           0
ospf 1          0           0           0           0
  Intra-area: 0 Inter-area: 0 External-1: 0 External-2: 0
  NSSA External-1: 0 NSSA External-2: 0
Total           2           0           128         288 

This is a sample output from the show ip route supernets-only command. This display shows supernets only; it does not show subnets.

Switch# show ip route supernets-only

Codes: I - IGRP derived, R - RIP derived, O - OSPF derived
       C - connected, S - static, E - EGP derived, B - BGP derived
       i - IS-IS derived, D - EIGRP derived
       * - candidate default route, IA - OSPF inter area route
       E1 - OSPF external type 1 route, E2 - OSPF external type 2 route
       L1 - IS-IS level-1 route, L2 - IS-IS level-2 route
       EX - EIGRP external route
 
Gateway of last resort is not set
 
B    198.92.0.0 (mask is 255.255.0.0) [20/0] via 198.92.72.30, 0:00:50
B    192.0.0.0 (mask is 255.0.0.0) [20/0] via 198.92.72.24, 0:02:50

This is a sample output from the show route-map command:

Switch# show route-map

route-map abc, permit, sequence 10
  Match clauses:
    tag 1 2
  Set clauses:
    metric 5
route-map xyz, permit, sequence 20
  Match clauses:
    tag 3 4
  Set clauses:
    metric 6