Your software release
may not support all the features documented in this module. For the latest
caveats and feature information, see Bug Search Tool and the release notes for
your platform and software release. To find information about the features
documented in this module, and to see a list of the releases in which each
feature is supported, see the feature information table at the end of this
module.
Use Cisco Feature
Navigator to find information about platform support and Cisco software image
support. To access Cisco Feature Navigator, go to
http://www.cisco.com/go/cfn.
An account on Cisco.com is not required.
Information About Configuring IP Unicast Routing
This module describes how to
configure IP Version 4 (IPv4) unicast routing on the switch.
A switch stack operates and appears as a single router to the
rest of the routers in the network. Basic
routing functions like static routing
are available with
.
Note
In addition to IPv4 traffic,
you can also enable IP Version 6 (IPv6) unicast routing and configure
interfaces to forward IPv6 traffic.
Information About 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 1. Routing Topology
Example. This figure 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.
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.
Types of Routing
Routers and Layer 3 switches
can route packets in these 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.
Beginning with Cisco IOS
release 12.2(58)SE, switches running the LAN base feature set support 16
user-configured static routes, in addition to any default routes used for the
management interface. The LAN base image supports static routing only on SVIs
and only when the switch is running the default SDM template.
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 active
switch. If the switch is
running the LAN base feature set, you can configure 16 static routes on SVIs.
All other routing protocols require the IP services feature set.
IP Routing and Switch Stacks
A
Switch
stack appears to the network as a single
Switch,
regardless of which
Switch
in the stack is connected to a routing peer.
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
Switch
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
Switch,
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.
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
Switch
over, 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.
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.
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 run all supported protocols,
including Open Shortest Path First (OSPF), and Enhanced IGRP (EIGRP)
and Border
Gateway Protocol (BGP). If the stack master fails and the new elected
stack master is running the IP base or LAN 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.
How to Configure IP 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.
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.
Note
If the
Switch
is running the LAN base feature set, static routes are supported only on SVIs.
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” chapter in the Layer 2
Configuration Guide.
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.
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 when running the IP base or IP services feature
set.
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 the "Configuring VLANs” chapter in the VLAN Configuration
Guide.
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).
How to Configure 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. The following sections describe how to configure
various IP addressing features. Assigning IP addresses to the interface is
required; the other procedures are optional.
Note
On switches running the LAN
base feature set, you can only assign an IP address to an SVI and configure a
static unicast route on the interface. Other configurations are not supported.
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 IP Addressing Configuration
Table 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.
Procedure
Command or Action
Purpose
Step 1
enable
Example:
Switch> enable
Enables
privileged EXEC mode. Enter your password if prompted.
Step 2
configure terminal
Example:
Switch# configure terminal
Enters the global
configuration mode.
Step 3
interface interface-id
Example:
Switch(config)# interface gigabitethernet 1/0/1
Enters interface
configuration mode, and specifies the Layer 3 interface to configure.
Note
Only SVI
interfaces are supported on switches running the LAN base feature set.
Step 4
no
switchport
Example:
Switch(config-if)# no switchport
Removes the
interface from Layer 2 configuration mode (if it is a physical interface).
Step 5
ip address ip-address
subnet-mask
Example:
Switch(config-if)# ip address 10.1.5.1 255.255.255.0
Configures the IP
address and IP subnet mask.
Step 6
no
shutdown
Example:
Switch(config-if)# no shutdown
Enables the
physical interface.
Step 7
end
Example:
Switch(config)# end
Returns to
privileged EXEC mode.
Step 8
show ip
route
Example:
Switch# show ip route
Verifies your
entries.
Step 9
show ip interface [interface-id]
Example:
Switch# show ip interface gigabitethernet 1/0/1
Verifies your
entries.
Step 10
show running-config
Example:
Switch# show running-config
Verifies your entries.
Step 11
copy running-config
startup-config
Example:
Switch# copy running-config startup-config
(Optional) Saves your entries
in the configuration file.
Step 12
On switches running the LAN base image, you can only assign an IP
address to an SVI and then configure a static route for the SVI. The switch
supports 16 user-configured static routes. No other routing configurations are
supported.
Using 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.
Use the
no ip subnet-zero global configuration
command to restore the default and disable the use of subnet zero.
Procedure
Command or Action
Purpose
Step 1
enable
Example:
Switch> enable
Enables
privileged EXEC mode. Enter your password if prompted.
Step 2
configure terminal
Example:
Switch# configure terminal
Enters the global
configuration mode.
Step 3
ip
subnet-zero
Example:
Switch(config)# ip subnet-zero
Enables the use of
subnet zero for interface addresses and routing updates.
Step 4
end
Example:
Switch(config)# end
Returns to
privileged EXEC mode.
Step 5
show running-config
Example:
Switch# show running-config
Verifies your entries.
Step 6
copy running-config
startup-config
Example:
Switch# copy running-config startup-config
(Optional) Saves your entries
in the configuration file.
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 the figure, 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 2. IP Classless
Routing
In the figure , 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 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.
Disabling 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.
Procedure
Command or Action
Purpose
Step 1
enable
Example:
Switch> enable
Enables
privileged EXEC mode. Enter your password if prompted.
Step 2
configure terminal
Example:
Switch# configure terminal
Enters the global
configuration mode.
Step 3
no ip
classless
Example:
Switch(config)#no ip classless
Disables classless
routing behavior.
Step 4
end
Example:
Switch(config)# end
Returns to
privileged EXEC mode.
Step 5
show running-config
Example:
Switch# show running-config
Verifies your entries.
Step 6
copy running-config
startup-config
Example:
Switch# copy running-config startup-config
(Optional) Saves your entries
in the configuration file.
Step 7
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 perform the following tasks to configure address resolution.
Address Resolution
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 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
Defining 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.
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.
Procedure
Command or Action
Purpose
Step 1
enable
Example:
Switch> enable
Enables
privileged EXEC mode. Enter your password if prompted.
Step 2
configure terminal
Example:
Switch# configure terminal
Enters the global
configuration mode.
Step 3
arp ip-address hardware-address
type
Example:
Switch(config)# ip 10.1.5.1 c2f3.220a.12f4 arpa
Associates an IP
address with a MAC (hardware) address in the ARP cache, and specifies
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 4
arp ip-address hardware-address
type [alias ]
Example:
Switch(config)# ip 10.1.5.3 d7f3.220d.12f5 arpa alias
(Optional)
Specifies that the switch respond to ARP requests as if it were the owner of
the specified IP address.
Step 5
interface interface-id
Example:
Switch(config)# interface gigabitethernet 1/0/1
Enters interface
configuration mode, and specifies the interface to configure.
Step 6
arp timeout seconds
Example:
Switch(config-if)# arp 20000
(Optional) Sets
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 7
end
Example:
Switch(config)# end
Returns to
privileged EXEC mode.
Step 8
show interfaces [interface-id]
Example:
Switch# show interfaces gigabitethernet 1/0/1
Verifies the type
of ARP and the timeout value used on all interfaces or a specific interface.
Step 9
show
arp
Example:
Switch# show arp
Views the contents
of the ARP cache.
Step 10
show ip
arp
Example:
Switch# show ip arp
Views the contents
of the ARP cache.
Step 11
copy running-config
startup-config
Example:
Switch# copy running-config startup-config
(Optional) Saves your entries
in the configuration file.
Setting 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.
To disable an encapsulation type, use the
no arp arpa or
no arp snap interface configuration command.
Procedure
Command or Action
Purpose
Step 1
enable
Example:
Switch> enable
Enables
privileged EXEC mode. Enter your password if prompted.
Step 2
configure terminal
Example:
Switch# configure terminal
Enters the global
configuration mode.
Step 3
interface interface-id
Example:
Switch(config)# interface gigabitethernet 1/0/2
Enters interface
configuration mode, and specifies the Layer 3 interface to configure.
Step 4
arp {arpa |
snap }
Example:
Switch(config-if)# arp arpa
Specifies the ARP
encapsulation method:
arpa —Address Resolution Protocol
snap —Subnetwork Address Protocol
Step 5
end
Example:
Switch(config)# end
Returns to
privileged EXEC mode.
Step 6
show interfaces [interface-id]
Example:
Switch# show interfaces
Verifies ARP
encapsulation configuration on all interfaces or the specified interface.
Step 7
copy running-config
startup-config
Example:
Switch# copy running-config startup-config
(Optional) Saves your entries
in the configuration file.
Enabling Proxy ARP
By default, the
Switch
uses proxy ARP to help hosts learn MAC addresses of hosts on other networks or
subnets.
To disable proxy ARP on the
interface, use the
no ip proxy-arp interface configuration
command.
Procedure
Command or Action
Purpose
Step 1
enable
Example:
Switch> enable
Enables
privileged EXEC mode. Enter your password if prompted.
Step 2
configure terminal
Example:
Switch# configure terminal
Enters the global
configuration mode.
Step 3
interface interface-id
Example:
Switch(config)# interface gigabitethernet 1/0/2
Enters interface
configuration mode, and specifies the Layer 3 interface to configure.
Step 4
ip
proxy-arp
Example:
Switch(config-if)# ip proxy-arp
Enables proxy ARP
on the interface.
Step 5
end
Example:
Switch(config)# end
Returns to
privileged EXEC mode.
Step 6
show ip interface [interface-id]
Example:
Switch# show ip interface gigabitethernet 1/0/2
Verifies the
configuration on the interface or all interfaces.
Step 7
copy running-config
startup-config
Example:
Switch# copy running-config startup-config
(Optional) Saves your entries
in the configuration file.
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
Proxy ARP is enabled by default. To enable it after it has been disabled, see the “Enabling 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 non-local
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.
Use the
no ip default-gateway global configuration
command to disable this function.
Procedure
Command or Action
Purpose
Step 1
enable
Example:
Switch> enable
Enables
privileged EXEC mode. Enter your password if prompted.
Step 2
configure terminal
Example:
Switch# configure terminal
Enters the global
configuration mode.
Step 3
ip
default-gateway ip-address
Example:
Switch(config)# ip default gateway 10.1.5.1
Sets up a default
gateway (router).
Step 4
end
Example:
Switch(config)# end
Returns to
privileged EXEC mode.
Step 5
show ip
redirects
Example:
Switch# show ip redirects
Displays the
address of the default gateway router to verify the setting.
Step 6
copy running-config
startup-config
Example:
Switch# copy running-config startup-config
(Optional) Saves your entries
in the configuration file.
ICMP Router Discovery Protocol
Router discovery allows the
Switch
to dynamically learn about routes to other networks using ICMP router discovery
protocol (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.
ICMP Router Discovery Protocol (IRDP)
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. 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.
Procedure
Command or Action
Purpose
Step 1
enable
Example:
Switch> enable
Enables
privileged EXEC mode. Enter your password if prompted.
Step 2
configure terminal
Example:
Switch# configure terminal
Enters the global
configuration mode.
Step 3
interface interface-id
Example:
Switch(config)# interface gigabitethernet 1/0/1
Enters interface
configuration mode, and specifies the Layer 3 interface to configure.
Step 4
ip
irdp
Example:
Switch(config-if)# ip irdp
Enables IRDP
processing on the interface.
Step 5
ip irdp
multicast
Example:
Switch(config-if)# ip irdp multicast
(Optional) Sends
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 6
ip irdp
holdtime seconds
Example:
Switch(config-if)# ip irdp holdtime 1000
(Optional) Sets
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 7
ip irdp
maxadvertinterval seconds
Example:
Switch(config-if)# ip irdp maxadvertinterval 650
(Optional) Sets
the IRDP maximum interval between advertisements. The default is 600 seconds.
Step 8
ip irdp
minadvertinterval seconds
Example:
Switch(config-if)# ip irdp minadvertinterval 500
(Optional) Sets
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 9
ip irdp
preference number
Example:
Switch(config-if)# ip irdp preference 2
(Optional) Sets 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 10
ip irdp address address [number]
Example:
Switch(config-if)# ip irdp address 10.1.10.10
(Optional)
Specifies an IRDP address and preference to proxy-advertise.
Step 11
end
Example:
Switch(config)# end
Returns to
privileged EXEC mode.
Step 12
show ip
irdp
Example:
Switch# show ip irdp
Verifies
settings by displaying IRDP values.
Step 13
copy running-config
startup-config
Example:
Switch# copy running-config startup-config
(Optional) Saves your entries
in the configuration file.
Configuring Broadcast Packet Handling
Perform the tasks in these
sections to enable these schemes:
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.
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.
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 the “Information about Network Security with
ACLs" section in the Security Configuration Guide.
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.
Procedure
Command or Action
Purpose
Step 1
enable
Example:
Switch> enable
Enables
privileged EXEC mode. Enter your password if prompted.
Step 2
configure terminal
Example:
Switch# configure terminal
Enters the global
configuration mode.
Step 3
interface interface-id
Example:
Switch(config)# interface gigabitethernet 1/0/2
Enters interface
configuration mode, and specifies the interface to configure.
Step 4
ip directed-broadcast
[access-list-number]
Example:
Switch(config-if)# ip directed-broadcast 103
Enables 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.
Step 5
exit
Example:
Switch(config-if)# exit
Returns to global
configuration mode.
Step 6
ip forward-protocol
{udp [port] |
nd |
sdns }
Example:
Switch(config)# ip forward-protocol nd
Specifies 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 7
end
Example:
Switch(config)# end
Returns to
privileged EXEC mode.
Step 8
show ip interface [interface-id]
Example:
Switch# show ip interface
Verifies the
configuration on the interface or all interfaces
Step 9
show running-config
Example:
Switch# show running-config
Verifies your entries.
Step 10
copy running-config
startup-config
Example:
Switch# copy running-config startup-config
(Optional) Saves your entries
in the configuration file.
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 lists the ports that are
forwarded by default if you do not specify any UDP ports.
Forwarding UDP Broadcast Packets and Protocols
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.
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.
Procedure
Command or Action
Purpose
Step 1
enable
Example:
Switch> enable
Enables
privileged EXEC mode. Enter your password if prompted.
Step 2
configure terminal
Example:
Switch# configure terminal
Enters the global
configuration mode.
Step 3
interface interface-id
Example:
Switch(config)# interface gigabitethernet 1/0/1
Enters interface
configuration mode, and specifies the Layer 3 interface to configure.
Step 4
ip
helper-address address
Example:
Switch(config-if)# ip helper address 10.1.10.1
Enables
forwarding and specifies the destination address for forwarding UDP broadcast
packets, including BOOTP.
Step 5
exit
Example:
Switch(config-if)# exit
Returns to global
configuration mode.
Step 6
ip forward-protocol {udp [port] |
nd |
sdns }
Example:
Switch(config)# ip forward-protocol sdns
Specifies which
protocols the router forwards when forwarding broadcast packets.
Step 7
end
Example:
Switch(config)# end
Returns to
privileged EXEC mode.
Step 8
show ip interface [interface-id]
Example:
Switch# show ip interface gigabitethernet 1/0/1
Verifies the
configuration on the interface or all interfaces.
Step 9
show running-config
Example:
Switch# show running-config
Verifies your entries.
Step 10
copy running-config
startup-config
Example:
Switch# copy running-config startup-config
(Optional) Saves your entries
in the configuration file.
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.
To restore the default IP
broadcast address, use the
no ip broadcast-address interface
configuration command.
Procedure
Command or Action
Purpose
Step 1
enable
Example:
Switch> enable
Enables
privileged EXEC mode. Enter your password if prompted.
Step 2
configure terminal
Example:
Switch# configure terminal
Enters the global
configuration mode.
Step 3
interface interface-id
Example:
Switch(config)# interface gigabitethernet 1/0/1
Enters interface
configuration mode, and specifies the interface to configure.
Step 4
ip
broadcast-address ip-address
Example:
Switch(config-if)# ip broadcast-address 128.1.255.255
Enters a
broadcast address different from the default, for example 128.1.255.255.
Step 5
end
Example:
Switch(config)# end
Returns to
privileged EXEC mode.
Step 6
show ip interface [interface-id]
Example:
Switch# show ip interface
Verifies the
broadcast address on the interface or all interfaces.
Step 7
copy running-config
startup-config
Example:
Switch# copy running-config startup-config
(Optional) Saves your entries
in the configuration file.
IP Broadcast Flooding
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.
Use the
no ip forward-protocol spanning-tree global
configuration command to disable the flooding of IP broadcasts.
In the
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.
To disable this feature, use
the
no ip forward-protocol turbo-flood global
configuration command
Flooding IP Broadcasts
Procedure
Command or Action
Purpose
Step 1
enable
Example:
Switch> enable
Enables
privileged EXEC mode. Enter your password if prompted.
Step 2
configure terminal
Example:
Switch# configure terminal
Enters the global
configuration mode.
Step 3
ip
forward-protocol spanning-tree
Example:
Switch(config)# ip forward-protocol spanning-tree
Uses the bridging
spanning-tree database to flood UDP datagrams.
Step 4
end
Example:
Switch(config)# end
Returns to
privileged EXEC mode.
Step 5
show running-config
Example:
Switch# show running-config
Verifies your entries.
Step 6
copy running-config
startup-config
Example:
Switch# copy running-config startup-config
(Optional) Saves your entries
in the configuration file.
Step 7
configure terminal
Example:
Switch# configure terminal
Enters the global
configuration mode.
Step 8
ip
forward-protocol turbo-flood
Example:
Switch(config)# ip forward-protocol turbo-flood
Uses the
spanning-tree database to speed up flooding of UDP datagrams.
Step 9
end
Example:
Switch(config)# end
Returns to
privileged EXEC mode.
Step 10
show running-config
Example:
Switch# show running-config
Verifies your entries.
Step 11
copy running-config
startup-config
Example:
Switch# copy running-config startup-config
(Optional) Saves your entries
in the configuration file.
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. The Table lists
the commands for clearing contents.
Table 2. Commands to Clear Caches,
Tables, and Databases
clear
arp-cache
Clears the IP ARP cache and
the fast-switching cache.
clear host {name |
* }
Removes one or all entries
from the hostname and the address cache.
clear ip route {network [mask] |
* }
Removes 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. The Table lists the privileged EXEC commands for displaying IP
statistics.
Table 3. Commands to Display Caches,
Tables, and Databases
show
arp
Displays the entries in the
ARP table.
show
hosts
Displays the default domain
name, style of lookup service, name server hosts, and the cached list of
hostnames and addresses.
show ip
aliases
Displays IP addresses mapped
to TCP ports (aliases).
show ip
arp
Displays the IP ARP cache.
show ip interface
[interface-id]
Displays the IP status of
interfaces.
show ip
irdp
Displays IRDP values.
show ip
masks address
Displays the masks used for
network addresses and the number of subnets using each mask.
show ip
redirects
Displays the address of a
default gateway.
show ip route [address [mask]] | [protocol]
Displays the current state of
the routing table.
show ip route
summary
Displays the current state of
the routing table in summary form.
How to Configure IP Unicast Routing
Enabling IP Unicast 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.
Use the
no ip routing
global configuration command to disable routing.
Procedure
Command or Action
Purpose
Step 1
enable
Example:
Switch> enable
Enables
privileged EXEC mode. Enter your password if prompted.
Step 2
configure terminal
Example:
Switch# configure terminal
Enters the global
configuration mode.
Step 3
ip
routing
Example:
Switch(config)# ip routing
Enables IP
routing.
Step 4
end
Example:
Switch(config)# end
Returns to
privileged EXEC mode.
Step 5
show running-config
Example:
Switch# show running-config
Verifies your entries.
Step 6
copy running-config
startup-config
Example:
Switch# copy running-config startup-config
(Optional) Saves your entries
in the configuration file.
Example of Enabling IP
Routing
This example shows how to
enable IP routingusing 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)# end
Information About 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.
Note
RIP is supported in the
.
Note
RIP is the only routing protocol supported by the IP base feature set;
other routing protocols require the switch or stack master to be running the IP
services feature set.
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.
How to Configure RIP
Default RIP Configuration
Table 4. 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 triggered
Disabled
IP RIP receive version
According to the
version router configuration command.
IP RIP send version
According to the
version router configuration command.
IP RIP triggered
According to the
version router configuration 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 2
packets; sends Version 1 packets.
Configuring Basic RIP Parameters
To configure RIP, you enable
RIP routing for a network and optionally configure other parameters. On the
Switch,
RIP configuration commands are ignored until you configure the network number.
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.
Use the
show ip rip database privileged EXEC command
to display summary address entries in the RIP database.
Procedure
Command or Action
Purpose
Step 1
enable
Example:
Switch> enable
Enables
privileged EXEC mode. Enter your password if prompted.
Step 2
configure terminal
Example:
Switch# configure terminal
Enters the global
configuration mode.
Step 3
ip
routing
Example:
Switch(config)# ip routing
Enables IP
routing. (Required only if IP routing is disabled.)
Step 4
router
rip
Example:
Switch(config)# router rip
Enables a RIP
routing process, and enter router configuration mode.
Step 5
network network number
Example:
Switch(config)# network 12
Associates a
network with a RIP routing process. You can specify multiple
network commands. RIP routing updates are sent and
received through interfaces only on these networks.
Note
You must
configure a network number for the RIP commands to take effect.
Step 6
neighbor ip-address
Example:
Switch(config)# neighbor 10.2.5.1
(Optional)
Defines 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 7
offset-list [access-list
number |
name] {in |
out }
offset [type number]
Example:
Switch(config)# offset-list 103 in 10
(Optional)
Applies 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 8
timers
basic update invalid holddown
flush
Example:
Switch(config)# timers basic 45 360 400 300
(Optional)
Adjusts routing protocol timers. Valid ranges for all timers are 0 to
4294967295 seconds.
update—The time between sending routing updates.
The default is 30 seconds.
invalid—The timer after which a route is declared
invalid. The default is 180 seconds.
holddown—The time before a route is removed from
the routing table. The default is 180 seconds.
flush—The amount of time for which routing updates
are postponed. The default is 240 seconds.
Step 9
version {1 |
2 }
Example:
Switch(config)# version 2
(Optional)
Configures 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 10
no auto
summary
Example:
Switch(config)# no auto summary
(Optional)
Disables 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 11
no
validate-update-source
Example:
Switch(config)# no validdate-update-source
(Optional)
Disables 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 12
output-delay delay
Example:
Switch(config)# output-delay 8
(Optional) Adds
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 13
end
Example:
Switch(config)# end
Returns to
privileged EXEC mode.
Step 14
show ip
protocols
Example:
Switch# show ip protocols
Verifies your
entries.
Step 15
copy running-config
startup-config
Example:
Switch# copy running-config startup-config
(Optional) Saves your entries
in the configuration file.
Configuring 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 specifies 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.
The
Switch
supports two modes of authentication on interfaces for which RIP authentication
is enabled: plain text and MD5. The default is plain text.
To restore clear text
authentication, use the
no ip rip authentication mode interface
configuration command. To prevent authentication, use the
no ip rip authentication key-chain interface
configuration command.
Procedure
Command or Action
Purpose
Step 1
enable
Example:
Switch> enable
Enables
privileged EXEC mode. Enter your password if prompted.
Step 2
configure terminal
Example:
Switch# configure terminal
Enters the global
configuration mode.
Step 3
interface interface-id
Example:
Switch(config)# interface gigabitethernet 1/0/1
Enters interface
configuration mode, and specifies the interface to configure.
Step 4
ip rip
authentication key-chain name-of-chain
Example:
Switch(config-if)# ip rip authentication key-chain trees
Enables RIP
authentication.
Step 5
ip rip authentication mode {text |
md5 }
Example:
Switch(config-if)# ip rip authentication mode md5
Configures the
interface to use plain text authentication (the default) or MD5 digest
authentication.
Step 6
end
Example:
Switch(config)# end
Returns to
privileged EXEC mode.
Step 7
show running-config
Example:
Switch# show running-config
Verifies your entries.
Step 8
copy running-config
startup-config
Example:
Switch# copy running-config startup-config
(Optional) Saves your entries
in the configuration file.
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.
Configuring Summary Addresses and Split Horizon
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.
To disable IP summarization,
use the
no ip summary-address rip router
configuration command.
Procedure
Command or Action
Purpose
Step 1
enable
Example:
Switch> enable
Enables
privileged EXEC mode. Enter your password if prompted.
Step 2
configure terminal
Example:
Switch# configure terminal
Enters the global
configuration mode.
Step 3
interface interface-id
Example:
Switch(config)# interface gigabitethernet 1/0/1
Enters interface
configuration mode, and specifies the Layer 3 interface to configure.
Step 4
ip address ip-address
subnet-mask
Example:
Switch(config-if)# ip address 10.1.1.10 255.255.255.0
Configures the IP
address and IP subnet.
Step 5
ip
summary-address rip ip address ip-network mask
Example:
Switch(config-if)# ip summary-address rip ip address 10.1.1.30 255.255.255.0
Configures the IP
address to be summarized and the IP network mask.
Step 6
no ip split
horizon
Example:
Switch(config-if)# no ip split horizon
Disables split
horizon on the interface.
Step 7
end
Example:
Switch(config)# end
Returns to
privileged EXEC mode.
Step 8
show ip
interface interface-id
Example:
Switch# show ip interface gigabitethernet 1/0/1
Verifies your
entries.
Step 9
copy running-config
startup-config
Example:
Switch# copy running-config startup-config
(Optional) Saves your entries
in the configuration file.
Configuring 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.
To enable the split horizon
mechanism, use the
ip split-horizon interface configuration
command.
Procedure
Command or Action
Purpose
Step 1
enable
Example:
Switch> enable
Enables
privileged EXEC mode. Enter your password if prompted.
Step 2
configure terminal
Example:
Switch# configure terminal
Enters the global
configuration mode.
Step 3
interface interface-id
Example:
Switch(config)# interface gigabitethernet 1/0/1
Enters interface
configuration mode, and specifies the interface to configure.
Step 4
ip address ip-address
subnet-mask
Example:
Switch(config-if)# ip address 10.1.1.10 255.255.255.0
Configures the IP
address and IP subnet.
Step 5
no ip
split-horizon
Example:
Switch(config-if)# no ip split-horizon
Disables split
horizon on the interface.
Step 6
end
Example:
Switch(config)# end
Returns to
privileged EXEC mode.
Step 7
show ip
interface interface-id
Example:
Switch# show ip interface gigabitethernet 1/0/1
Verifies your
entries.
Step 8
copy running-config
startup-config
Example:
Switch# copy running-config startup-config
(Optional) Saves your entries
in the configuration file.
Configuration Example for Summary Addresses and Split Horizon
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 port 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 interface configuration command.
Note
If split horizon is enabled, neither autosummary nor interface summary addresses (those configured with the ip summary-address rip router configuration command) are advertised.
Switch(config)# router rip
Switch(config-router)# interface gigabitethernet1/0/2
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-if)# exit
Switch(config)# router rip
Switch(config-router)# network 10.0.0.0
Switch(config-router)# neighbor 2.2.2.2 peer-group mygroup
Switch(config-router)# end
Information About OSPF
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, OSPF management information base (MIB).
Note
OSPF classifies different
media into broadcast, nonbroadcast, and point-to-point networks. The switch
supports broadcast (Ethernet, Token Ring, and FDDI) and point-to-point networks
(Ethernet interfaces configured as point-to-point links).
The Cisco implementation
conforms to the OSPF Version 2 specifications with these key features:
Definition of stub areas is
supported.
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 EIGRP
and RIP. OSPF routes can also be exported into RIP.
Plain text and MD5
authentication among neighboring routers within an area is supported.
Configurable routing
interface parameters include interface output cost, retransmission interval,
interface transmit delay, router priority, router dead and hello intervals, and
authentication key.
Virtual links are supported.
Not-so-stubby-areas (NSSAs) per RFC 1587are
supported.
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.
How to Configure OSPF
Default OSPF Configuration
Table 5. 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 Mb/s.
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.
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.
Nonstop Forwarding
(NSF) awareness
Enabled. Allows
Layer 3
Switch
to continue forwarding packets from a neighboring NSF-capable router during
hardware or software changes.
NSF capability
Disabled.
Note
The
Switch
stack supports OSPF NSF-capable routing for IPv4.
Router ID
No OSPF routing process defined.
Summary address
Disabled.
Timers LSA group
pacing
240 seconds.
Timers shortest path
first (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.
OSPF for Routed
Access
With Cisco IOS Release
12.2(55)SE, the IP Base image supports OSPF for routed access. The IP services
image is required if you need multiple OSPFv2 and OSPFv3 instances without
route restrictions. Additionally, the IP services image is required to enable
the multi-VRF-CE feature.
OSPF for Routed Access
is specifically designed so that you can extend Layer 3 routing capabilities to
the wiring closet.
Note
OSPF for Routed
Access supports only one OSPFv2 and one OSPFv3 instance with a combined total
of 200 dynamically learned routes. The IP Base image provides OSPF for routed
access.
However, these
restrictions are not enforced in this release.
With the typical
topology (hub and spoke) in a campus environment, where the wiring closets
(spokes) are connected to the distribution switch (hub) that forwards all
nonlocal traffic to the distribution layer, the wiring closet
Switch
need not hold a complete routing table. A best practice design, where the
distribution
Switch
sends a default route to the wiring closet
Switch
to reach interarea and external routes (OSPF stub or totally stub area
configuration) should be used when OSPF for Routed Access is used in the wiring
closet.
For more details, see
the “High Availability Campus Network Design—Routed Access Layer using EIGRP or
OSPF” document.
OSPF Nonstop Forwarding
The
Switch
or switch stack supports two levels of nonstop forwarding (NSF):
The IP-services feature set
supports OSPF NSF Awareness supported for IPv4. When the neighboring router is
NSF-capable, the Layer 3
Switch
continues to forward packets from the neighboring router during the interval
between the primary Route Processor (RP) in a router crashing and the backup RP
taking over, or while the primary RP is manually reloaded for a non-disruptive
software upgrade.
This feature cannot be
disabled.
OSPF NSF Capability
The IP services feature set
supports the OSPFv2 NSF IETF format in addition to the OSPFv2 NSF Cisco format
that is supported in earlier releases. For information about this feature, see
: NSF—OSPF (RFC 3623 OSPF
Graceful Restart).
The IP-services feature set
also supports OSPF NSF-capable routing for IPv4 for better convergence and
lower traffic loss following a stack master change. When a stack master change
occurs in an OSPF NSF-capable stack, the new stack master must do two things to
resynchronize its link-state database with its OSPF neighbors:
Release the available OSPF
neighbors on the network without resetting the neighbor relationship.
Reacquire the contents of the
link-state database for the network.
After a stack master change,
the new master sends an OSPF NSF signal to neighboring NSF-aware devices. A
device recognizes this signal to mean that it should not reset the neighbor
relationship with the stack. As the NSF-capable stack master receives signals
from other routes on the network, it begins to rebuild its neighbor list.
When the neighbor
relationships are reestablished, the NSF-capable stack master resynchronizes
its database with its NSF-aware neighbors, and routing information is exchanged
between the OSPF neighbors. The new stack master uses this routing information
to remove stale routes, to update the routing information database (RIB), and
to update the forwarding information base (FIB) with the new information. The
OSPF protocols then fully converge.
Note
OSPF NSF requires that all
neighbor networking devices be NSF-aware. If an NSF-capable router discovers
non-NSF aware neighbors on a network segment, it disables NSF capabilities for
that segment. Other network segments where all devices are NSF-aware or
NSF-capable continue to provide NSF capabilities.
Use the
nsf OSPF routing configuration command to enable
OSPF NSF routing. Use the
show ip ospf privileged EXEC command to verify
that it is enabled.
NSF is not supported
on interfaces configured for Hot Standby Router Protocol (HSRP).
Configuring Basic OSPF Parameters
To enable OSPF, create an
OSPF routing process, specify the range of IP addresses to associate with the
routing process, and assign area IDs to be associated with that range.
For switches running the IP
services image, you can configure either the Cisco OSPFv2 NSF format or the
IETF OSPFv2 NSF format.
Procedure
Command or Action
Purpose
Step 1
configure terminal
Example:
Switch# configure terminal
Enters the global
configuration mode.
Step 2
router
ospf process-id
Example:
Switch(config)# router ospf 15
Enables OSPF
routing, and enter 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.
Note
OSPF for Routed
Access supports only one OSPFv2 and one OSPFv3 instance with a maximum number
of 200 dynamically learned routes.
Step 3
nsf cisco [enforce global ]
Example:
Switch(config)# nsf cisco enforce global
(Optional)
Enables Cisco NSF operations for OSPF. The
enforce global keyword cancels NSF restart when
non-NSF-aware neighboring networking devices are detected.
Note
Enter the command
in Step 3 or Step 4, and go to Step 5.
Step 4
nsf ietf [restart-interval seconds]
Example:
Switch(config)# nsf ietf restart-interval 60
(Optional)
Enables IETF NSF operations for OSPF. The
restart-interval keyword specifies the length of
the graceful restart interval, in seconds. The range is from 1 to 1800. The
default is 120.
Note
Enter the command
in Step 3 or Step 4, and go to Step 5.
Step 5
network address wildcard-mask
area area-id
Example:
Switch(config)# network 10.1.1.1 255.240.0.0 area 20
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 6
end
Example:
Switch(config)# end
Returns to
privileged EXEC mode.
Step 7
show ip
protocols
Example:
Switch# show ip protocols
Verifies your
entries.
Step 8
copy running-config
startup-config
Example:
Switch# copy running-config startup-config
(Optional) Saves your entries
in the configuration file.
Example: Configuring Basic OSPF Parameters
This example shows how to configure an OSPF routing process and assign it a process number of 109:
Switch(config)# router ospf 109
Switch(config-router)# network 131.108.0.0 255.255.255.0 area 24
Configuring OSPF Interfaces
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 interface
configuration commands are all optional.
Procedure
Command or Action
Purpose
Step 1
configure terminal
Example:
Switch# configure terminal
Enters the global
configuration mode.
Step 2
interface interface-id
Example:
Switch(config)# interface gigabitethernet 1/0/1
Enters interface
configuration mode, and specifies the Layer 3 interface to configure.
Step 3
ip
ospf cost
Example:
Switch(config-if)# ip ospf 8
(Optional)
Explicitly specifies the cost of sending a packet on the interface.
Step 4
ip ospf
retransmit-interval seconds
Example:
Switch(config-if)# ip ospf transmit-interval 10
(Optional)
Specifies 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
Example:
Switch(config-if)# ip ospf transmit-delay 2
(Optional) Sets
the estimated number of seconds to wait before sending a link state update
packet. The range is 1 to 65535 seconds. The default is 1 second.
Step 6
ip ospf
priority number
Example:
Switch(config-if)# ip ospf priority 5
(Optional) Sets
priority to help find 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
Example:
Switch(config-if)# ip ospf hello-interval 12
(Optional) Sets
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
Example:
Switch(config-if)# ip ospf dead-interval 8
(Optional) Sets
the number of seconds after the last 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
Example:
Switch(config-if)# ip ospf authentication-key password
(Optional) 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
Example:
Switch(config-if)# ip ospf message digest-key 16 md5 your1pass
(Optional)
Enables 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
Example:
Switch(config-if)# ip ospf database-filter all out
(Optional) 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
Example:
Switch(config)# end
Returns to
privileged EXEC mode.
Step 13
show ip ospf interface [interface-name]
Example:
Switch# show ip ospf interface
Displays
OSPF-related interface information.
Step 14
show ip ospf
neighbor detail
Example:
Switch# show ip ospf neighbor detail
Displays NSF
awareness status of neighbor switch. The output matches one of these examples:
Options is
0x52
LLS Options is
0x1 (LR)
When both of
these lines appear, the neighbor switch is NSF aware.
Options is
0x42—This means the neighbor switch is not NSF aware.
Step 15
copy running-config
startup-config
Example:
Switch# copy running-config startup-config
(Optional) Saves your entries
in the configuration file.
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 router configuration command to configure the ABR to advertise a summary route that covers all networks in the range.
Configuring OSPF Area Parameters
Before you begin
Note
The OSPF
area router configuration commands are all
optional.
Procedure
Command or Action
Purpose
Step 1
configure terminal
Example:
Switch# configure terminal
Enters the global
configuration mode.
Step 2
router
ospf process-id
Example:
Switch(config)# router ospf 109
Enables OSPF
routing, and enter router configuration mode.
Step 3
area area-idauthentication
Example:
Switch(config-router)# area 1 authentication
(Optional) 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-idauthentication
message-digest
Example:
Switch(config-router)# area 1 authentication message-digest
(Optional)
Enables MD5 authentication on the area.
Step 5
area area-idstub [no-summary ]
Example:
Switch(config-router)# area 1 stub
(Optional) 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-idnssa [no-redistribution ] [default-information-originate ] [no-summary ]
Example:
Switch(config-router)# area 1 nssa default-information-originate
(Optional)
Defines an area as a not-so-stubby-area. Every router within the same area must
agree that the area is NSSA. Select one of these keywords:
no-redistribution —Select when the router is an
NSSA ABR and you want the
redistribute command to import routes into normal
areas, but not into the NSSA.
default-information-originate —Select on an ABR to
allow importing type 7 LSAs into the NSSA.
no-redistribution —Select to not send summary LSAs
into the NSSA.
Step 7
area area-idrange address mask
Example:
Switch(config-router)# area 1 range 255.240.0.0
(Optional)
Specifies an address range for which a single route is advertised. Use this
command only with area border routers.
Step 8
end
Example:
Switch(config)# end
Returns to
privileged EXEC mode.
Step 9
show ip ospf
[process-id]
Example:
Switch# show ip ospf
Displays
information about the OSPF routing process in general or for a specific process
ID to verify configuration.
Step 10
show ip ospf [process-id
[area-id]]
database
Example:
Switch# show ip osfp database
Displays lists of
information related to the OSPF database for a specific router.
Step 11
copy running-config
startup-config
Example:
Switch# copy running-config startup-config
(Optional) Saves your entries
in the configuration file.
Other OSPF Parameters
You can optionally configure
other OSPF parameters in router configuration mode.
Route summarization: When redistributing routes from other
protocols. 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 router configuration 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 privileged EXEC 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 specified by the
bandwidth interface configuration 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 sending interface,
you must configure the sending 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.
Configuring Other OSPF Parameters
Procedure
Command or Action
Purpose
Step 1
configure terminal
Example:
Switch# configure terminal
Enters the global
configuration mode.
Step 2
router ospf process-id
Example:
Switch(config)# router ospf 10
Enables OSPF
routing, and enter router configuration mode.
spf-delay—Delay between receiving a change to SPF
calculation. The range is from 1 to 600000 miliseconds.
spf-holdtime—Delay between first and second SPF
calculation. The range is form 1 to 600000 in milliseconds.
spf-wait—Maximum wait time in milliseconds for SPF
calculations. The range is from 1 to 600000 in milliseconds.
Step 11
ospf
log-adj-changes
Example:
Switch(config)# ospf log-adj-changes
(Optional)
Sends syslog message when a neighbor state changes.
Step 12
end
Example:
Switch(config)# end
Returns to
privileged EXEC mode.
Step 13
show ip ospf [process-id [area-id]]
database
Example:
Switch# show ip ospf database
Displays lists
of information related to the OSPF database for a specific router.
Step 14
copy running-config
startup-config
Example:
Switch# copy running-config startup-config
(Optional) Saves your entries
in the configuration file.
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.
Changing LSA Group Pacing
Procedure
Command or Action
Purpose
Step 1
configure terminal
Example:
Switch# configure terminal
Enters the global
configuration mode.
Step 2
router ospf process-id
Example:
Switch(config)# router ospf 25
Enables OSPF
routing, and enter router configuration mode.
Step 3
timers
lsa-group-pacing seconds
Example:
Switch(config-router)# timers lsa-group-pacing 15
Changes the group
pacing of LSAs.
Step 4
end
Example:
Switch(config)# end
Returns to
privileged EXEC mode.
Step 5
show running-config
Example:
Switch# show running-config
Verifies your entries.
Step 6
copy running-config
startup-config
Example:
Switch# copy running-config startup-config
(Optional) Saves your entries
in the configuration file.
Loopback Interfaces
OSPF uses the highest 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 higher 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.
Configuring a Loopback Interface
Procedure
Command or Action
Purpose
Step 1
configure terminal
Example:
Switch# configure terminal
Enters the global
configuration mode.
Step 2
interface
loopback 0
Example:
Switch(config)# interface loopback 0
Creates a
loopback interface, and enter interface configuration mode.
Step 3
ip
address address mask
Example:
Switch(config-if)# ip address 10.1.1.5 255.255.240.0
Assign an IP
address to this interface.
Step 4
end
Example:
Switch(config)# end
Returns to
privileged EXEC mode.
Step 5
show ip
interface
Example:
Switch# show ip interface
Verifies your
entries.
Step 6
copy running-config
startup-config
Example:
Switch# copy running-config startup-config
(Optional) Saves your entries
in the configuration file.
Monitoring OSPF
You can display specific statistics
such as the contents of IP routing tables, caches, and databases.
Table 6. Show IP OSPF Statistics
Commands
show ip ospf [process-id]
Displays 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 ]
Displays lists of information
related to the OSPF database.
show ip ospf
border-routes
Displays the internal OSPF
routing ABR and ASBR table entries.
show ip ospf interface
[interface-name]
Displays OSPF-related
interface information.
show ip ospf neighbor
[interface-name] [neighbor-id]
detail
Displays OSPF interface
neighbor information.
show ip ospf
virtual-links
Displays OSPF-related virtual
links information.
Information About EIGRP
Enhanced IGRP (EIGRP) is a Cisco proprietary enhanced version of the IGRP. EIGRP uses the same distance vector algorithm and
distance information as IGRP; however, the convergence properties and the operating efficiency of EIGRP 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. 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 used as the next hop to the destination, the transport control
field is incremented as usual.
EIGRP Features
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 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.
EIGRP Components
EIGRP 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 learn that a neighbor is alive and
functioning. When 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.
Note
To enable EIGRP, the
Switch
or stack master must be running the IP services feature set.
How to Configure EIGRP
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 see
the “Configuring Split Horizon” section. You must use the same AS number for routes to be automatically redistributed.
Default EIGRP Configuration
Table 7. Default EIGRP
Configuration
Feature
Default Setting
Auto summary
Disabled.
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 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 kb/s.
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.
Nonstop Forwarding (NSF)
Awareness
Enabled for IPv4 on switches
running the IP services feature set. Allows Layer 3 switches to continue
forwarding packets from a neighboring NSF-capable router during hardware or
software changes.
NSF capability
Disabled.
Note
The
Switch
supports EIGRP NSF-capable routing for IPv4.
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).
EIGRP Nonstop Forwarding
The
Switch
stack supports two levels of EIGRP nonstop forwarding:
EIGRP NSF Awareness
EIGRP NSF Capability
EIGRP NSF Awareness
The IP-services feature set
supports EIGRP NSF Awareness for IPv4. When the neighboring router is
NSF-capable, the Layer 3
Switch
continues to forward packets from the neighboring router during the interval
between the primary Route Processor (RP) in a router failing and the backup RP
taking over, or while the primary RP is manually reloaded for a nondisruptive
software upgrade.
This feature cannot be
disabled. For more information on this feature, see the “EIGRP Nonstop
Forwarding (NSF) Awareness” section of the
Cisco IOS IP Routing
Protocols Configuration Guide, Release 12.4.
EIGRP NSF Capability
The IP services feature set
supports EIGRP Cisco NSF routing to speed up convergence and to eliminate
traffic loss after a stack master change. For details about this NSF
capability, see the “Configuring Nonstop Forwarding” chapter in the
High Availability
Configuration Guide, Cisco IOS XE Release 3S.
The IP-services feature set
also supports EIGRP NSF-capable routing for IPv4 for better convergence and
lower traffic loss following a stack master change. When an EIGRP NSF-capable
stack master restarts or a new stack master starts up and NSF restarts, the
Switch
has no neighbors, and the topology table is empty. The
Switch
must bring up the interfaces, reacquire neighbors, and rebuild the topology and
routing tables without interrupting the traffic directed toward the
Switch
stack. EIGRP peer routers maintain the routes learned from the new stack master
and continue forwarding traffic through the NSF restart process.
To prevent an adjacency reset
by the neighbors, the new stack master uses a new Restart (RS) bit in the EIGRP
packet header to show the restart. When the neighbor receives this, it
synchronizes the stack in its peer list and maintains the adjacency with the
stack. The neighbor then sends its topology table to the stack master with the
RS bit set to show that it is NSF-aware and is aiding the new stack master.
If at least one of the stack
peer neighbors is NSF-aware, the stack master receives updates and rebuilds its
database. Each NSF-aware neighbor sends an end of table (EOT) marker in the
last update packet to mark the end of the table content. The stack master
recognizes the convergence when it receives the EOT marker, and it then begins
sending updates. When the stack master has received all EOT markers from its
neighbors or when the NSF converge timer expires, EIGRP notifies the routing
information database (RIB) of convergence and floods its topology table to all
NSF-aware peers.
Note
NSF is not supported
on interfaces configured for Hot Standby Router Protocol (HSRP).
Use the
nsf EIGRP
routing configuration command to enable EIGRP NSF routing. Use the show ip protocols privileged EXEC command to
verify that NSF is enabled on the device. See the command reference for this
release for information about the
nsf command.
Configuring Basic EIGRP Parameters
Procedure
Command or Action
Purpose
Step 1
configure terminal
Example:
Switch# configure terminal
Enters the global
configuration mode.
Step 2
router
eigrp autonomous-system
Example:
Switch(config)# router eigrp 10
Enables an EIGRP
routing process, and enter router configuration mode. The AS number identifies
the routes to other EIGRP routers and is used to tag routing information.
Step 3
nsf
Example:
Switch(config)# nsf
(Optional)
Enables EIGRP NSF. Enter this command on the stack master and on all of its
peers.
Step 4
network network-number
Example:
Switch(config)# network 192.168.0.0
Associate
networks with an EIGRP routing process. EIGRP sends updates to the interfaces
in the specified networks.
Step 5
eigrp
log-neighbor-changes
Example:
Switch(config)# eigrp log-neighbor-changes
(Optional)
Enables logging of EIGRP neighbor changes to monitor routing system stability.
Step 6
metric
weights tos k1 k2 k3 k4
k5
Example:
Switch(config)# metric weights 0 2 0 2 0 0
(Optional) Adjust
the EIGRP metric. Although the defaults have been carefully set to provide
excellent operation in most networks, you can adjust them.
Caution
Setting metrics is complex
and is not recommended without guidance from an experienced network designer.
Step 7
offset-list [access-list
number |
name] {in |
out }
offset [type number]
Example:
Switch(config)# offset-list 21 out 10
(Optional)
Applies 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 8
auto-summary
Example:
Switch(config)# auto-summary
(Optional)
Enables automatic summarization of subnet routes into network-level routes.
Step 9
ip
summary-address eigrp autonomous-system-number
address mask
Example:
Switch(config)# ip summary-address eigrp 1 192.168.0.0 255.255.0.0
(Optional)
Configures a summary aggregate.
Step 10
end
Example:
Switch(config)# end
Returns to
privileged EXEC mode.
Step 11
show ip
protocols
Example:
Switch# show ip protocols
Verifies your
entries.
For NSF
awareness, the output shows:
*** IP Routing is NSF aware
*** EIGRP NSF enabled
Step 12
copy running-config
startup-config
Example:
Switch# copy running-config startup-config
(Optional) Saves your entries
in the configuration file.
Configuring EIGRP Interfaces
Other optional EIGRP parameters can be
configured on an interface basis.
Procedure
Command or Action
Purpose
Step 1
configure terminal
Example:
Switch# configure terminal
Enters the global
configuration mode.
Step 2
interface interface-id
Example:
Switch(config)# interface gigabitethernet 1/0/1
Enters interface
configuration mode, and specifies the Layer 3 interface to configure.
Step 3
ip
bandwidth-percent eigrp percent
Example:
Switch(config-if)# ip bandwidth-percent eigrp 60
(Optional)
Configures 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
Example:
Switch(config-if)# ip summary-address eigrp 109 192.161.0.0 255.255.0.0
(Optional)
Configures 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
Example:
Switch(config-if)# ip hello-interval eigrp 109 10
(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
Example:
Switch(config-if)# ip hold-time eigrp 109 40
(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
Example:
Switch(config-if)# no ip split-horizon eigrp 109
(Optional)
Disables split horizon to allow route information to be advertised by a router
out any interface from which that information originated.
Step 8
end
Example:
Switch(config)# end
Returns to
privileged EXEC mode.
Step 9
show ip eigrp
interface
Example:
Switch# show ip eigrp interface
Displays which
interfaces EIGRP is active on and information about EIGRP relating to those
interfaces.
Step 10
copy running-config
startup-config
Example:
Switch# copy running-config startup-config
(Optional) Saves your entries
in the configuration file.
Configuring 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.
Procedure
Command or Action
Purpose
Step 1
configure terminal
Example:
Switch# configure terminal
Enters the global
configuration mode.
Step 2
interface interface-id
Example:
Switch(config)# interface gigabitethernet 1/0/1
Enters interface
configuration mode, and specifies the Layer 3 interface to configure.
Step 3
ip authentication
mode eigrp autonomous-system
md5
Example:
Switch(config-if)# ip authentication mode eigrp 104 md5
Enables MD5
authentication in IP EIGRP packets.
Step 4
ip authentication
key-chain eigrp autonomous-system
key-chain
Example:
Switch(config-if)# ip authentication key-chain eigrp 105 chain1
Enables
authentication of IP EIGRP packets.
Step 5
exit
Example:
Switch(config-if)# exit
Returns to global
configuration mode.
Step 6
key
chain name-of-chain
Example:
Switch(config)# key chain chain1
Identify a key
chain and enter key-chain configuration mode. Match the name configured in Step
4.
Step 7
key number
Example:
Switch(config-keychain)# key 1
In key-chain
configuration mode, identify the key number.
Step 8
key-string text
Example:
Switch(config-keychain-key)# key-string key1
In key-chain key
configuration mode, identify the key string.
Switch(config-keychain-key)# accept-lifetime 13:30:00 Jan 25 2011 duration 7200
(Optional)
Specifies 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 as
January 1, 1993. The default
end-time and
duration is
infinite .
Switch(config-keychain-key)# send-lifetime 14:00:00 Jan 25 2011 duration 3600
(Optional)
Specifies 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 11
end
Example:
Switch(config)# end
Returns to
privileged EXEC mode.
Step 12
show key
chain
Example:
Switch# show key chain
Displays
authentication key information.
Step 13
copy running-config
startup-config
Example:
Switch# copy running-config startup-config
(Optional) Saves your entries
in the configuration file.
EIGRP Stub Routing
The EIGRP stub routing
feature reduces resource
utilization by moving routed traffic closer to the end user.
Note
The
feature set contains EIGRP stub routing capability,
which only advertises connected or summary routes from the routing tables to
other
Switches
in the network. The
Switch
uses EIGRP stub routing at the access layer to eliminate the need for other
types of routing advertisements.
In a network using EIGRP stub
routing, the only allowable route for IP traffic to the user is through a
Switch
that is configured with EIGRP stub routing. The
Switch
sends the routed traffic to interfaces that are configured as user interfaces
or are connected to other devices.
When using EIGRP stub
routing, you need to configure the distribution and remote routers to use EIGRP
and to configure only the
Switch
as a stub. Only specified routes are propagated from the
Switch.
The
Switch
responds to all queries for summaries, connected routes, and routing updates.
Any neighbor that receives a
packet informing it of the stub status does not query the stub router for any
routes, and a router that has a stub peer does not query that peer. The stub
router depends on the distribution router to send the proper updates to all
peers.
In the figure given below,
Switch
B is configured as an EIGRP stub router.
Switches
A and C are connected to the rest of the WAN.
Switch
B advertises connected, static, redistribution, and summary routes to
Switch
A and C.
Switch
B does not advertise any routes learned from
Switch
A (and the reverse).
Figure 4. EIGRP Stub Router
Configuration
For more information about
EIGRP stub routing, see “Configuring EIGRP Stub Routing” section of the
Cisco IOS IP Configuration
Guide, Volume 2 of 3: Routing Protocols.
Monitoring and Maintaining EIGRP
You can delete neighbors from
the neighbor table. You can also display various EIGRP routing statistics. The
table given below lists the privileged EXEC commands for deleting neighbors and
displaying statistics. For explanations of fields in the resulting display, see
the
Cisco IOS IP Command
Reference, Volume 2 of 3: Routing Protocols, Release 12.4.
Table 8. IP EIGRP Clear and Show
Commands
clear ip eigrp
neighbors [if-address |
interface]
Deletes neighbors from the
neighbor table.
show ip eigrp interface
[interface] [as number]
Displays information about
interfaces configured for EIGRP.
show ip eigrp neighbors
[type-number]
Displays EIGRP discovered
neighbors.
show ip eigrp topology [autonomous-system-number] | [[ip-address]
mask]]
Displays the EIGRP topology
table for a given process.
show ip eigrp traffic
[autonomous-system-number]
Displays the number of
packets sent and received for all or a specified EIGRP process.
Information About BGP
The Border Gateway Protocol (BGP) is an exterior
gateway protocol used to set up an interdomain routing system that guarantees
the loop-free exchange of routing information between autonomous systems.
Autonomous systems are made up of routers that operate under the same
administration and that run Interior Gateway Protocols (IGPs), such as RIP or
OSPF, within their boundaries and that interconnect by using an Exterior
Gateway Protocol (EGP). BGP Version 4 is the standard EGP for interdomain
routing in the Internet. The protocol is defined in RFCs 1163, 1267, and 1771.
You can find detailed information about BGP in
Internet Routing
Architectures,
published by Cisco Press, and in the “Configuring BGP” chapter in the
Cisco IP and IP Routing
Configuration Guide.
For details about BGP
commands and keywords, see the “IP Routing Protocols” part of the
Cisco IOS IP Command
Reference, Volume 2 of 3: Routing Protocols
.
BGP Network Topology
Routers that belong to the same autonomous system
(AS) and that exchange BGP updates run internal BGP (IBGP), and routers that
belong to different autonomous systems and that exchange BGP updates run
external BGP (EBGP). Most configuration commands are the same for configuring
EBGP and IBGP. The difference is that the routing updates are exchanged either
between autonomous systems (EBGP) or within an AS (IBGP). The figure given
below shows a network that is running both EBGP and IBGP.
Figure 5. EBGP, IBGP, and Multiple
Autonomous Systems
Before exchanging information
with an external AS, BGP ensures that networks within the AS can be reached by
defining internal BGP peering among routers within the AS and by redistributing
BGP routing information to IGPs that run within the AS, such as IGRP and OSPF.
Routers that run a BGP
routing process are often referred to as BGP speakers. BGP uses the
Transmission Control Protocol (TCP) as its transport protocol (specifically
port 179). Two BGP speakers that have a TCP connection to each other for
exchanging routing information are known as peers or neighbors. In the above
figure, Routers A and B are BGP peers, as are Routers B and C and Routers C and
D. The routing information is a series of AS numbers that describe the full
path to the destination network. BGP uses this information to construct a
loop-free map of autonomous systems.
The network has these
characteristics:
Routers A and B are running
EBGP, and Routers B and C are running IBGP. Note that the EBGP peers are
directly connected and that the IBGP peers are not. As long as there is an IGP
running that allows the two neighbors to reach one another, IBGP peers do not
have to be directly connected.
All BGP speakers within an AS
must establish a peer relationship with each other. That is, the BGP speakers
within an AS must be fully meshed logically. BGP4 provides two techniques that
reduce the requirement for a logical full mesh: confederations and route
reflectors.
AS 200 is a transit AS for AS
100 and AS 300—that is, AS 200 is used to transfer packets between AS 100 and
AS 300.
BGP peers initially exchange
their full BGP routing tables and then send only incremental updates. BGP peers
also exchange keepalive messages (to ensure that the connection is up) and
notification messages (in response to errors or special conditions).
In BGP, each route consists of
a network number, a list of autonomous systems that information has passed
through (the autonomous system path), and a list of other path attributes. The
primary function of a BGP system is to exchange network reachability
information, including information about the list of AS paths, with other BGP
systems. This information can be used to determine AS connectivity, to prune
routing loops, and to enforce AS-level policy decisions.
A router or
Switch
running Cisco IOS does not select or use an IBGP route unless it has a route
available to the next-hop router and it has received synchronization from an
IGP (unless IGP synchronization is disabled). When multiple routes are
available, BGP bases its path selection on attribute values. See the
“Configuring BGP Decision Attributes” section for information about BGP
attributes.
BGP Version 4 supports classless interdomain routing
(CIDR) so you can reduce the size of your routing tables by creating aggregate
routes, resulting in supernets. CIDR eliminates the concept of network classes
within BGP and supports the advertising of IP prefixes.
How to Configure BGP
Default BGP Configuration
The table given below shows the basic
default BGP configuration. For the defaults for all characteristics, see the
specific commands in the
Cisco IOS IP Command
Reference, Volume 2 of 3: Routing Protocols, Release 12.4.
