Configuring RIP
This
chapter describes how to configure RIP. For a complete description of the RIP
commands that appear in this chapter, refer to the “RIP Commands” chapter of
the Network Protocols Command Reference, Part 1. To locate
documentation of other commands that appear in this chapter, use the command
reference master index or search online.
The
Routing Information Protocol (RIP) is a relatively old, but still commonly
used, interior gateway protocol (IGP) created for use in small, homogeneous
networks. It is a classical distance-vector routing protocol. RIP is documented
in RFC 1058.
RIP uses broadcast User Datagram Protocol (UDP)
data packets to exchange routing information. The Cisco IOS software sends
routing information updates every 30 seconds; this process is termed advertising.
If a router does not receive an update from another router for 180 seconds
or more, it marks the routes served by the nonupdating router as being
unusable. If there is still no update after 240 seconds, the router removes all
routing table entries for the nonupdating router.
The
metric that RIP uses to rate the value of different routes is hop count.
The hop count is the number of routers that can be traversed in a route. A
directly connected network has a metric of zero; an unreachable network has a
metric of 16. This small range of metrics makes RIP an unsuitable routing
protocol 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 network 0.0.0.0 does not exist; RIP treats
0.0.0.0 as a network to implement the default routing feature. The Cisco IOS
software will advertise 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 the specified networks. If an interface’s network
is not specified, it will not be advertised in any RIP update.
Cisco’s
implementation of RIP Version 2 supports plain text and MD5 authentication,
route summarization, classless interdomain routing (CIDR), and variable-length
subnet masks (VLSMs).
For
protocol-independent features, which also apply to RIP, see the chapter
“Configuring IP Routing Protocol-Independent Features” in this document.
RIP Configuration
Task List
To
configure RIP, complete the tasks in the following sections. You must enable
RIP. The remaining tasks are optional.
Enable RIP
For
information about the following topics, see the “Configuring IP Routing
Protocol-Independent Features” chapter:
•
Filtering
RIP information
•
Key management (available in RIP Version 2)
•
VLSM
Enable RIP
To enable RIP, perform the
following tasks, starting in global configuration mode:
Task
|
Command
|
|
Step 1
|
Enable a RIP routing process, which
|
router rip
|
places you in router configuration mode.
|
||
Step 2
|
Associate a network with a RIP routing
|
network network-number
|
process.
|
||
Allow Unicast Updates
for RIP
Because
RIP is normally a broadcast protocol, in order for RIP routing updates to reach
nonbroadcast networks, you must configure the Cisco IOS software to permit this
exchange of routing information. To do so, perform the following task in router
configuration mode:
Task
|
Command
|
Define a neighboring
router with which to
|
neighbor ip-address
|
exchange routing
information.
|
|
To
control the set of interfaces with which you want to exchange routing updates,
you can disable the sending of routing updates on specified interfaces by
configuring the passive-interface command. See the discussion on
filtering in the “Filter Routing Information” section in the “Configuring IP
Routing Protocol-Independent Features” chapter.
Apply Offsets to Routing Metrics
Apply Offsets to
Routing Metrics
An offset
list is the mechanism for increasing incoming and outgoing metrics to routes
learned via RIP. This is done to provide a local mechanism for increasing the
value of routing metrics. Optionally, you can limit the offset list with either
an access list or an interface. To increase the value of routing metrics,
perform the following task in router configuration mode:
Task
|
Command
|
Apply
an offset to routing metrics.
|
offset-list [access-list-number
| name] {in | out}
offset
|
[type number]
|
|
Adjust Timers
Routing
protocols use several timers that determine such variables as the frequency of
routing updates, the length of time before a route becomes invalid, and other
parameters. You can adjust these timers to tune routing protocol performance to
better suit your internetwork needs. You can make the following timer
adjustments:
•
The rate (time in seconds between updates) at
which routing updates are sent
•
The interval of time (in seconds) after which a
route is declared invalid
•
The interval (in seconds) during which routing
information regarding better paths is suppressed
•
The amount of time (in seconds) that must pass
before a route is removed from the routing table
•
The amount of time for which routing updates will
be postponed
It also is possible to tune the
IP routing support in the software to enable faster convergence of the various
IP routing algorithms, and, hence, quicker fallback to redundant routers. The
total effect is to minimize disruptions to end users of the network in
situations where quick recovery is essential.
To adjust the timers, perform the
following task in router configuration mode:
Task
|
Command
|
Adjust routing protocol
timers.
|
timers basic update invalid holddown
flush
[sleeptime]
|
Specify a RIP Version
Cisco’s
implementation of RIP Version 2 supports authentication, key management, route
summarization, classless interdomain routing (CIDR), and variable-length subnet
masks (VLSMs). Key management and VLSM are described in the chapter
“Configuring IP Routing Protocol-Independent Features.”
By
default, the software receives RIP Version 1 and Version 2 packets, but sends
only Version 1 packets. You can configure the software to receive and send only
Version 1 packets. Alternatively, you can configure the software to receive and
send only Version 2 packets. To do so, perform the following task in router
configuration mode:
Task
|
Command
|
Configure the software to
receive and send only
|
version {1 | 2}
|
RIP
Version 1 or only RIP Version 2 packets.
|
|
Enable RIP
Authentication
The
preceding task controls the default behavior of RIP. You can override that
behavior by configuring a particular interface to behave differently. To control
which RIP version an interface sends, perform one of the following tasks in
interface configuration mode:
Task
|
Command
|
Configure
an interface to send only RIP Version 1
|
ip rip send version 1
|
packets.
|
|
Configure
an interface to send only RIP Version 2
|
ip rip send version 2
|
packets.
|
|
Configure
an interface to send RIP Version 1 and
|
ip rip send version 1 2
|
Version
2 packets.
|
|
Similarly,
to control how packets received from an interface are processed, perform one of
the following tasks in interface configuration mode:
Task
|
Command
|
Configure an interface to
accept only RIP
|
ip rip receive version 1
|
Version 1 packets.
|
|
Configure an interface to
accept only RIP
|
ip rip receive version 2
|
Version 2 packets.
|
|
Configure an interface to
accept either RIP
|
ip rip receive version 1
2
|
Version 1 or 2 packets.
|
|
Enable RIP
Authentication
RIP
Version 1 does not support authentication. If you are sending and receiving RIP
Version 2 packets, you can enable RIP authentication on an interface.
The key
chain determines the set of keys that can be used on the interface. If a key
chain is not configured, no authentication is performed on that interface, not
even the default authentication. Therefore, you must also perform the tasks in
the section “Manage Authentication Keys” in the “Configuring IP Routing
Protocol-Independent Features” chapter.
We
support two modes of authentication on an interface for which RIP
authentication is enabled: plain text authentication and MD5 authentication.
The default authentication in every RIP Version 2 packet is plain text
authentication.
Note Do not use
plain text authentication in RIP packets for security purposes, because the unencrypted
authentication key is sent in every RIP Version 2 packet. Use plain text
authentication when security is not an issue, for example, to ensure that
misconfigured hosts do not participate in routing.
To configure RIP authentication,
perform the following tasks in interface configuration mode:
Task
|
Command
|
|
Step 1
|
Enable RIP
authentication.
|
ip rip authentication
key-chain name-of-chain
|
Step 2
|
Configure the interface
to use MD5
|
ip rip authentication
mode {text
| md5}
|
digest authentication (or
let it default to
|
||
plain text
authentication).
|
||
Disable Route Summarization
Task
|
Command
|
Step
3 Perform the authentication key
|
See the section “Manage
Authentication Keys” in the
|
management tasks.
|
“Configuring IP Routing
Protocol-Independent
|
Features” chapter.
|
|
See the
“Key Management Examples” section of the “Configuring IP Routing
Protocol-Independent Features” chapter for key management examples.
Disable Route
Summarization
RIP
Version 2 supports automatic route summarization by default. The software
summarizes subprefixes to the classful network boundary when crossing classful
network boundaries.
If you have disconnected subnets,
disable automatic route summarization to advertise the subnets. When route
summarization is disabled, the software transmits subnet and host routing
information across classful network boundaries. To disable automatic
summarization, perform the following task in router configuration mode:
Task
|
Command
|
Disable automatic
summarization.
|
no auto-summary
|
Run IGRP and RIP
Concurrently
It is
possible to run IGRP and RIP concurrently. The IGRP information will override
the RIP information by default because of IGRP’s administrative distance.
However,
running IGRP and RIP concurrently does not work well when the network topology
changes. Because IGRP and RIP have different update timers, and because they
require different amounts of time to propagate routing updates, one part of the
network will end up believing IGRP routes and another part will end up
believing RIP routes. This will result in routing loops. Even though these
loops do not exist for very long, the time to live (TTL) will quickly reach
zero, and ICMP will send a “TTL exceeded” message. This message will cause most
applications to stop attempting network connections.
Disable the
Validation of Source IP Addresses
By
default, the software validates the source IP address of incoming RIP routing
updates. If that source address is not valid, the software discards the routing
update.
You might
want to disable this feature if you have a router that is “off network” and you
want to receive its updates. However, disabling this feature is not recommended
under normal circumstances. To disable the default function that validates the
source IP addresses of incoming routing updates, perform the following task in
router configuration mode:
Task
|
Command
|
Disable the validation of
the source IP address of
|
no validate-update-source
|
incoming
RIP routing updates.
|
|
Enable or Disable
Split Horizon
Enable or Disable
Split Horizon
Normally,
routers that are connected to broadcast-type IP networks and that use
distance-vector routing protocols employ the split horizon mechanism to
reduce the possibility of routing loops. Split horizon blocks information about
routes from being advertised by a router out of any interface from which that
information originated. This behavior usually optimizes communications among
multiple routers, particularly when links are broken. However, with
nonbroadcast networks (such as Frame Relay and SMDS), situations can arise for
which this behavior is less than ideal. For these situations, you might want to
disable split horizon. This applies to IGRP and RIP.
If an
interface is configured with secondary IP addresses and split horizon is
enabled, updates might not be sourced by every secondary address. One routing
update is sourced per network number unless split horizon is disabled.
To enable or disable split
horizon, perform the following tasks in interface configuration mode:
Task
|
Command
|
Enable split horizon.
|
ip split-horizon
|
Disable split horizon.
|
no ip split-horizon
|
Split horizon for Frame Relay and SMDS
encapsulation is disabled by default. Split horizon is not disabled by default
for interfaces using any of the X.25 encapsulations. For all other
encapsulations, split horizon is enabled by default.
See the “Split Horizon Examples”
section at the end of this chapter for examples of using split horizon.
Note In general, changing the state of
the default is not recommended unless you are certain that your
application requires making a change in order to advertise routes properly.
Remember: If split horizon is disabled on a serial interface (and that
interface is attached to a packet-switched network), you must disable
split horizon for all routers in any relevant multicast groups on that network.
Configure Interpacket
Delay
By default, the software adds no delay between
packets in a multiple-packet RIP update being sent. If you have a high-end
router sending to a low-speed router, you might want to add such interpacket
delay to RIP updates, in the range of 8 to 50 milliseconds. To do so, perform
the following task in router configuration mode:
Task
|
Command
|
Add interpacket delay for
RIP updates sent.
|
output-delay delay
|
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