Catalyst 3750-E and 3560-E Switch Software Configuration Guide, 12.2(37)SE - Configuring IP Unicast Routing [Cisco Catalyst 3750-E Series Switches]

Table Of Contents

Configuring IP Unicast Routing

Understanding IP Routing

Types of Routing

IP Routing and Switch Stacks

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

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 Unicast Routing

Configuring RIP

Default RIP Configuration

Configuring Basic RIP Parameters

Configuring RIP Authentication

Configuring Summary Addresses and Split Horizon

Configuring Split Horizon

Configuring OSPF

Default OSPF Configuration

OSPF Nonstop Forwarding

Configuring Basic OSPF Parameters

Configuring OSPF Interfaces

Configuring OSPF Area Parameters

Configuring Other OSPF Parameters

Changing LSA Group Pacing

Configuring a Loopback Interface

Monitoring OSPF

Configuring EIGRP

Default EIGRP Configuration

EIGRP Nonstop Forwarding

Configuring Basic EIGRP Parameters

Configuring EIGRP Interfaces

Configuring EIGRP Route Authentication

EIGRP Stub Routing

Monitoring and Maintaining EIGRP

Configuring BGP

Default BGP Configuration

Nonstop Forwarding Awareness

Enabling BGP Routing

Managing Routing Policy Changes

Configuring BGP Decision Attributes

Configuring BGP Filtering with Route Maps

Configuring BGP Filtering by Neighbor

Configuring Prefix Lists for BGP Filtering

Configuring BGP Community Filtering

Configuring BGP Neighbors and Peer Groups

Configuring Aggregate Addresses

Configuring Routing Domain Confederations

Configuring BGP Route Reflectors

Configuring Route Dampening

Monitoring and Maintaining BGP

Configuring Multi-VRF CE

Understanding Multi-VRF CE

Default Multi-VRF CE Configuration

Multi-VRF CE Configuration Guidelines

Configuring VRFs

Configuring a VPN Routing Session

Configuring BGP PE to CE Routing Sessions

Multi-VRF CE Configuration Example

Displaying Multi-VRF CE Status

Configuring Unicast Reverse Path Forwarding

Configuring Protocol-Independent Features

Configuring Distributed Cisco Express Forwarding

Configuring the Number of Equal-Cost Routing Paths

Configuring Static Unicast Routes

Specifying Default Routes and Networks

Using Route Maps to Redistribute Routing Information

Configuring Policy-Based Routing

PBR Configuration Guidelines

Enabling PBR

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 Version 4 (IPv4) unicast routing on the Catalyst 3750-E or 3560-E switch. Unless otherwise noted, the term switch refers to a Catalyst 3750-E or 3560-E standalone switch and to a Catalyst 3750-E switch stack. A switch stack operates and appears as a single router to the rest of the routers in the network. Basic routing functions, including static routing and the Routing Information Protocol (RIP), are available with both the IP base feature set and the IP services feature set. To use advanced routing features and other routing protocols, you must have the IP services feature set enabled on the standalone switch or on the stack master.

Note If the switch or switch stack is running the advanced IP services feature set, you can also enable IP Version 6 (IPv6) unicast routing and configure interfaces to forward IPv6 traffic in addition to IPv4 traffic. For information about configuring IPv6 on the switch, see Chapter 39, "Configuring IPv6 Unicast Routing."

For more detailed IP unicast configuration information, see the Cisco IOS IP Configuration Guide, Release 12.2. For complete syntax and usage information for the commands used in this chapter, see these command references:

•Cisco IOS IP Command Reference, Volume 1 of 3: Addressing and Services, Release 12.2

•Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.2

•Cisco IOS IP Command Reference, Volume 3 of 3: Multicast, Release 12.2

This chapter consists of these sections:

•Understanding IP Routing

•Steps for Configuring Routing

•Configuring IP Addressing

•Enabling IP Unicast Routing

•Configuring RIP

•Configuring OSPF

•Configuring EIGRP

•Configuring BGP

•Configuring Multi-VRF CE

•Configuring Protocol-Independent Features

•Monitoring and Maintaining the IP Network

Note When configuring routing parameters on the switch and 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 (sdm) feature to the routing template. For more information on the SDM templates, see Chapter 8, "Configuring SDM Templates" or see the sdm prefer command in the command reference for this release.

Understanding IP Routing

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, network devices in different VLANs cannot communicate with one another without a Layer 3 device (router) to route traffic between the VLAN, referred to as inter-VLAN routing. You configure one or more routers to route traffic to the appropriate destination VLAN.

Figure 38-1 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 38-1 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, finds 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.

This section contains information on these routing topics:

•Types of Routing

•IP Routing and Switch Stacks

Types of Routing

Routers and Layer 3 switches can route packets 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 unicast 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 failures, 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:

•Routers using distance-vector protocols maintain routing tables with distance values of networked resources, and 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 switch are Routing Information Protocol (RIP), which uses a single distance metric (cost) to determine the best path and Border Gateway Protocol (BGP), which adds a path vector mechanism. 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 Interior Gateway Routing Protocol (IGRP) to improve efficiency.

Note On a switch or switch stack, the supported protocols are determined by the software running on the switch or stack master. If the switch or stack master is running the IP base feature set, only default routing, static routing and RIP are supported. All other routing protocols require the IP services feature set.

IP Routing and Switch Stacks

A Catalyst 3750-E switch stack appears to the network as a single router, regardless of which switch in the stack is connected to a routing peer. For additional information about switch stack operation, see Chapter 5, "Managing Switch Stacks."

The stack master performs these functions:

•It initializes and configures the routing protocols.

•It sends routing protocol messages and updates to other routers.

•It processes routing protocol messages and updates received from peer routers.

•It generates, maintains, and distributes the distributed Cisco Express Forwarding (dCEF) database to all stack members. The routes are programmed on all switches in the stack bases on this database.

•The MAC address of the stack master is used as the router MAC address for the whole stack, and all outside devices use this address to send IP packets to the stack.

•All IP packets that require software forwarding or processing go through the CPU of the stack master.

Stack members perform these functions:

•They act as routing standby switches, ready to take over in case they are elected as the new stack master if the stack master fails.

•They program the routes into hardware. The routes programmed by the stack members are the same that are downloaded by the stack master as part of the dCEF database.

If a stack master fails, the stack detects that the stack master is down and elects one of the stack members to be the new stack master. During this period, except for a momentary interruption, the hardware continues to forward packets with no active protocols.

However, even though the switch stack maintains the hardware identification after a failure, the routing protocols on the router neighbors might flap during the brief interruption before the stack master restarts. Routing protocols such as OSPF and EIGRP need to recognize neighbor transitions. The router uses two levels of nonstop forwarding (NSF) to detect a switchover, to continue forwarding network traffic, and to recover route information from peer devices:

•NSF-aware routers tolerate neighboring router failures. After the neighbor router restarts, an NSF-aware router supplies information about its state and route adjacencies on request.

•NSF-capable routers support NSF. When they detect a stack master change, they rebuild routing information from NSF-aware or NSF-capable neighbors and do not wait for a restart.

The switch stack supports NSF-capable routing for OSPF and EIGRP. For more information, see the "OSPF NSF Capability" section and the "EIGRP NSF Capability" section.

Upon election, the new stack master performs these functions:

•It starts generating, receiving, and processing routing updates.

•It builds routing tables, generates the CEF database, and distributes it to stack members.

•It uses its MAC address as the router MAC address. To notify its network peers of the new MAC address, it periodically (every few seconds for 5 minutes) sends a gratuitous ARP reply with the new router MAC address.

Note If you configure the persistent MAC address feature on the stack and the stack master changes, the stack MAC address does not change for the configured time period. If the previous stack master rejoins the stack as a member switch during that time period, the stack MAC address remains the MAC address of the previous stack master. See the "Enabling Persistent MAC Address" section on page 5-20.

•It attempts to determine the reachability of every proxy ARP entry by sending an ARP request to the proxy ARP IP address and receiving an ARP reply. For each reachable proxy ARP IP address, it generates a gratuitous ARP reply with the new router MAC address. This process is repeated for 5 minutes after a new stack master election.

Note When a stack master is running the IP services feature set, the stack can to run all supported protocols, including Open Shortest Path First (OSPF), Enhanced IGRP (EIGRP), and Border Gateway Protocol (BGP). If the stack master fails and the new elected stack master is running the IP base feature set, these protocols will no longer run in the stack.

Caution Partitioning of the switch stack into two or more stacks might lead to undesirable behavior in the network.

Steps for Configuring Routing

By default, IP routing is disabled on the switch, and you must enable it before routing can take place. For detailed IP routing configuration information, see the Cisco IOS IP Configuration Guide, Release 12.2

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 on page 37-15.

Note The switch does not support tunnel interfaces for unicast routed traffic.

All Layer 3 interfaces on which routing will occur must have IP addresses assigned to them. See the "Assigning IP Addresses to Network Interfaces" section.

Note A Layer 3 switch can have an IP address assigned to each routed port and SVI. The number of routed ports and SVIs that you can configure is not limited by software. However, the interrelationship between this number and the number and volume of features being implemented might have an impact on CPU utilization because of hardware limitations. To optimize system memory for routing, use the sdm prefer routing global configuration command.

Configuring routing consists of several main procedures:

•To support VLAN interfaces, create and configure VLANs on the switch or switch stack, and assign VLAN membership to Layer 2 interfaces. For more information, see Chapter 13, "Configuring 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 38-1 shows the default addressing configuration.

Table 38-1 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 IRDP advertisements.

•Maximum interval between advertisements: 600 seconds.

•Minimum interval between advertisements: 0.75 times max interval

•Preference: 0.

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. RFC 1166, "Internet Numbers," contains the official description of IP addresses.

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

Return to privileged EXEC mode.

Step 7

show interfaces [interface-id]
show ip interface [interface-id]
show running-config interface [interface-id]

Verify your entries.

Step 8

copy running-config startup-config

(Optional) Save your entries in the configuration file.

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

Return to privileged EXEC mode.

Step 4

show running-config

Verify your entry.

Step 5

copy running-config startup-config

(Optional) Save your entry in the configuration file.

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

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 38-2, 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 38-2 IP Classless Routing

In Figure 38-3, 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 38-3 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

Return to privileged EXEC mode.

Step 4

show running-config

Verify your entry.

Step 5

copy running-config startup-config

(Optional) Save your entry 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.

Note In a Catalyst 3750-E switch stack, network communication uses a single MAC address and the IP address of the stack.

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 learn the MAC address of the device. The process of learning the MAC address from an IP address is called address resolution. The process of learning 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 learns 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 learn 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.

The switch also uses the Reverse Address Resolution Protocol (RARP), which functions the same as ARP does, except that the RARP packets request 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 ip rarp-server address interface configuration command to identify the server.

For more information on RARP, see the Cisco IOS Configuration Fundamentals Configuration Guide, Release 12.2.

You can perform these tasks to configure address resolution:

•Define a Static ARP Cache

•Set ARP Encapsulation

•Enable Proxy ARP

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, and specify encapsulation type as one of these:

•arpa—ARP encapsulation for Ethernet interfaces

•snap—Subnetwork Address Protocol encapsulation for Token Ring and FDDI interfaces

•sap—HP's ARP type

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. The default is 14400 seconds (4 hours). The range is 0 to 2147483 seconds.

Step 6

end

Return to privileged EXEC mode.

Step 7

show interfaces [interface-id]

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

Step 8

show arp

or

show ip arp

View the contents of the ARP cache.

Step 9

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.

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 if required by your network.

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 | snap}

Specify the ARP encapsulation method:

•arpa—Address Resolution Protocol

•snap—Subnetwork Address Protocol

Step 4

end

Return to privileged EXEC mode.

Step 5

show interfaces [interface-id]

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 arpa or no arp snap interface configuration command.

Enable Proxy ARP

By default, the switch uses proxy ARP to help hosts learn 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]

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 interface 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 learn 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

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.

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) 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. The 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 -2³¹to 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

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.

Configuring Broadcast Packet Handling

After configuring an IP interface address, you can 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 storm-control interface configuration command to set traffic suppression levels. For more information, see Chapter 26, "Configuring Port-Based 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 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 Chapter 34, "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, only IP packets permitted by the access list can be translated

Note The ip directed-broadcast interface configuration command can be configured on a VPN routing/forwarding(VRF) interface and is VRF aware. Directed broadcast traffic is routed only within the VRF.

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

end

Return to privileged EXEC mode.

Step 7

show ip interface [interface-id]

or

show running-config

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 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 find 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 Command Reference, Volume 1 of 3: Addressing and Services, Release 12.2 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 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.

Step 6

end

Return to privileged EXEC mode.

Step 7

show ip interface [interface-id]

or

show running-config

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]

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 interface configuration 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.

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