RIP Features 729
This command outputs shows the last update was received 20 seconds ago.
Observing Multiple Paths to Destination
Some routing protocols support multiple paths to the same destination. Unlike single
path algorithms, these multipath algorithms permit traffic over multiple lines, provide
better throughput, and are more reliable.
RIP Features
Routing Information Protocol (RIP) was originally specified in RFC 1058 in 1988.
Its key characteristics include the following:
■ It is a distance vector routing protocol.
■ Hop count is used as the metric for path selection (see Figure 16-11).
■ If the hop count is greater than 15, the packet is discarded.
■ By default, routing updates are broadcast every 30 seconds.
Note in Figure 16-11 that the 19.2-kbps path between the two hosts using the top
routers is 2 hops. The lower alternate path using the three T-1 links is 4 hops. Because
RIP path selection is based solely on the number of hops, in this case, RIP path selection
chooses the 19.2-kbps link instead of the much faster T1 links.
Figure 16-11 RIP Uses Hop Count as Its Metric
200.200.200.0/24 auto-summary
200.200.200.0/24
[1] via 192.168.10.2, 00:00:20, Serial0/0
Example 16-4 show ip rip database Command Output (Continued)
, 00:00:20
T1 T1
T1
19.2 kpbs
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730 Chapter 16: Distance Vector Routing Protocols
RIP has evolved over the years from a classful routing protocol, RIP Version 1 (RIP-1),
to a classless routing Protocol, RIP Version 2 (RIP-2). RIP-2 enhancements include the
following:

■ Capability to carry additional packet routing information
■ Authentication mechanism to secure table updates
■ Capability to support subnet masks
RIP prevents routing loops from continuing indefinitely by implementing a limit on
the number of hops allowed in a path from the source to a destination. The maximum
number of hops allowed in a path is 15. When a router receives a routing update that
contains a new or changed entry, the metric value is increased by one. If this causes the
metric to be incremented beyond 15, it is considered to be infinity, and the network
destination is considered unreachable. RIP includes a number of features that are com-
mon in other routing protocols. For example, RIP implements split horizon and hold-
down mechanisms to prevent incorrect routing information from being propagated.
Enabling RIP on an IP Network
The router rip command enables RIP as the routing protocol. The network command
is then used to tell RIP which networks are directly connected for the router to adver-
tise. The routing process then associates these interfaces with the network addresses
and begins using RIP on interfaces.
RIP sends routing-update messages at regular intervals and triggered updates when
the network topology changes. When a router receives a routing update that includes
changes to an entry, it updates its routing table to reflect the new route. The metric
value for the path is increased by one, and the source interface of the update is indi-
cated as the next hop in the route. RIP routers maintain only the best route to a desti-
nation; however, RIP routers can maintain multiple routes to the same destination if
those routes have the same metric.
After updating its routing table due to a configuration change, the router immediately
begins transmitting routing updates to inform other network routers of the change.
These updates are sent independently of the regularly scheduled updates that RIP routers
forward. RIP advertises classful networks or major class networks only.
To enable RIP, use the commands in Table 16-3, beginning in global configuration
mode.
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RIP Features 731
The following commands show the process of enabling RIP and specifying directly
connected networks.
BHM(config)#router rip
! selects RIP as the routing protocol
BHM(config-router)#network 1.0.0.0
! specifies a directly connected network)
BHM(config-router)#network 2.0.0.0
! specifies a directly connected network
The Cisco router interfaces that are connected to networks 1.0.0.0 and 2.0.0.0 send
and receive RIP updates. These routing updates enable the router to learn the network
topology from a neighboring router that is running RIP.
Using the ip classless Command
Sometimes, a router receives packets destined for an unknown subnet of a network that
has directly connected subnets. To forward these packets to the best supernet route
possible, use the ip classless global configuration command. The ip classless command
is enabled by default in Cisco IOS Software Release 11.3 and later. To disable this fea-
ture, use the no form of this command.
When this feature is disabled and a packet is being sent to a subnet of a network that
has no network default route, the router discards the packet. Figure 16-12 illustrates
this principle. If the host sends a packet to 128.20.4.1 and no network default route
exists, the router discards the packet.
IP classless affects only the operation of the forwarding processes in IOS. IP classless
does not affect the way the routing table is built. This degree of impact is the essence of
classful routing. If part of a major network is known and the destination subnet
toward which the packet is destined is unknown, the packet is dropped.
Table 16-3 Commands to Enable RIP
Command Purpose
Router(config)#router rip Enables a RIP routing process,
which then switches to the

router configuration mode
Router(config-router)#network network-number Associates a network with a RIP
routing process
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732 Chapter 16: Distance Vector Routing Protocols
Figure 16-12 No IP Classless Routing
The most confusing aspect of this rule is that the router uses the default route only if
the major network destination does not exist in the routing table. By default, a router
assumes that all subnets of a directly connected network are present in the routing
table. If a packet is received with an unknown destination address within an unknown
subnet of an attached network, the router assumes that the subnet does not exist.
Therefore, the router drops the packet, even if a default route exists. Configuring ip
classless on the router resolves this problem by instructing the router to ignore the
classful boundaries of the networks in its routing table and simply route to the default
route, as Figure 16-13 demonstrates.
Common RIP Configuration Issues
RIP routers must rely on neighboring routers for network information. RIP uses a dis-
tance vector routing algorithm. All distance vector routing protocols have problems
that result in slow convergence.
Some of these issues include routing loops and counting to infinity. These problems both
result in inconsistencies due to routing update messages with outdated routes being
propagated around the internetwork.
To reduce routing loops and counting to infinity, RIP uses the following methods:
■ Split horizon
■ Poison reverse
■ Hold-down timer
■ Triggered updates
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RIP Features 733
Figure 16-13 IP Classless

Some of these methods might require some configuration, while others do not require
or rarely require configuration.
RIP permits a maximum hop count of 15. Any destination greater than 15 hops away
is tagged as unreachable. The maximum hop count for RIP greatly restricts its use in
large internetworks, but prevents a problem called count to infinity from causing end-
less network routing loops.
The split-horizon rule is based on the fact that it is usually not useful to send informa-
tion about a route back in the direction from which the route came. In some network
configurations, it might be necessary to disable split horizon. It is disabled on a per-
interface basis.
To disable split horizon, use the following command:
Router(config-if)# no ip split-horizon
The hold-down timer is another mechanism that might need some changes. Hold-down
timers help prevent counting to infinity but also increase convergence time. The default
holddown for RIP is 180 seconds. This holddown prevents any inferior route from being
updated, but might also prevent a valid alternative route from being installed. The
hold-down timer can be decreased to speed up convergence, but take such action with
caution. The ideal solution is to set the timer just longer than the longest possible update
time for the internetwork. In Figure 16-14, the loop consists of four routers. With each
router having an update time of 30 seconds, the longest loop possible is 120 seconds.
Therefore, the hold-down timer should be set to slightly more than 120 seconds.
ip classless
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734 Chapter 16: Distance Vector Routing Protocols
Figure 16-14 Hold-Down Timers
To change the hold-down timer, use the following command:
GAD(config-router)# holdown-timer
seconds
One additional item that affects convergence and can be configured is the update inter-
val. By default, Cisco IOS Software runs IP RIP updates every 30 seconds. This time

can be reconfigured for longer intervals to conserve bandwidth or for a shorter interval
to decrease convergence time.
To change the update interval, use the following command:
GAD(config-router)#update-timer
seconds
As discussed earlier in this chapter, another issue with routing protocols is the unwanted
advertisement of routing updates out a particular interface. When a network command
is issued, RIP sends advertisements out all interfaces within that network address
range. To control the set of interfaces that exchanges routing updates, the network
administrator can disable the sending of routing updates on specified interfaces by
configuring the passive-interface command.
Because RIP is a broadcast protocol, the network administrator might have to config-
ure RIP to exchange routing information in a nonbroadcast network such as Frame
Relay. In this type of network, RIP needs to be informed of other neighboring RIP
routers. To define a neighboring router with which to exchange routing information,
use the following command:
GAD(config-router)neighbor
ip address
By default, the software receives RIP-1 and RIP-2 packets, but sends only RIP-1 packets.
The network administrator can configure the router to receive and send only RIP-1
packets or to send only RIP-2 packets. To configure the router to send and receive
packets from only one version, use the commands in the router configuration mode as
specified in Table 16-4.
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RIP Features 735
To control how packets received from an interface are processed, use the following
commands described in Table 16-5.
Verifying RIP Configuration
Several commands can be used to verify that RIP is properly configured. Two of the
most commonly used commands are show ip route and show ip protocols.

The show ip protocols command outputs information about all the IP routing proto-
cols configured on the router, as demonstrated in Example 16-5. This output can be
Table 16-4 Specifying the RIP Version
Command Purpose
(config-router)#version {1 | 2 } Configures the software to receive and send
only RIP-1 or only RIP-2 packets.
(config-if)#ip rip send version 1 Configures an interface to send only RIP-1
packets.
(config-if)#ip rip send version 2 Configures an interface to send only RIP-2
packets.
(config-if)#ip rip send version 1 2 Configures an interface to send only RIP-1 or
RIP-2 packets.
Table 16-5 Controlling Packets
Command Purpose
(config-if)#ip rip receive version 1 Configures an interface to receive only RIP-1
packets.
(config-if)#ip rip receive version 2 Configures an interface to receive only RIP-2
packets.
(config-if)#ip rip receive version 1 2 Configures an interface to receive only RIP-1
or RIP-2 packets.
Lab Activity Configuring RIP
In this lab, you set up an IP addressing scheme using Class C networks and
configure RIP on all routers.
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736 Chapter 16: Distance Vector Routing Protocols
used to verify most, if not all, of the RIP configuration. Some of the most common
configuration items to verify are the following:
■ That RIP is configured
■ That the correct interfaces are sending and receiving RIP updates
■ That the correct RIP version is being sent and received

■ That the router is advertising the correct networks
The show ip route command can be used to verify that the routes received by RIP are
being installed in the routing table, as Example 16-6 shows. Examine the output of the
command and look for RIP routes, signified by “R.” Remember that the network takes
some time to converge so the routes might not appear immediately.
Example 16-5 show ip protocols Command Output
GAD# show ip protocols
Routing Protocol is "rip“
! Line above verifies that RIP is configured
Sending updates every 30 seconds, next due in 5 seconds
Invalid after 180 seconds, hold down 180, flushed after 240
Outgoing update filter list for all interfaces is
Incoming update filter list for all interfaces is
Redistributing: rip
Default version control: send version 1, receive any version
! Line above verifies the RIP version
Interface Send Recv Triggered RIP Key-chain
! Line above verifies RIP interfaces
FastEthernet0/0 1 1 2
Serial0/0 1 1 2
Routing for Networks:
192.168.1.0
192.168.2.0
! Lines above verify networks being advertised
Routing Information Sources:
Gateway Distance Last Update
192.168.2.2 120 00:00:11
Distance: (default is 120)
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RIP Features 737

The last entry, beginning with the letter R, indicates that network 192.168.3.0 was
learned from RIP and it can be reached via adjacent router (next hop) interface
192.168.2.2, which is remotely attached to this router’s Serial 0/0 interface.
Additional commands you can use to check RIP configuration are as follows:
■ show interface interface
■ show ip interface interface
■ show running-config
These commands are useful when it is necessary to find out information about a par-
ticular interface. Using the show interface command displays all information about an
interface, including whether it is up or down and what type of protocol, IP address,
or encapsulation type might be configured on the interface. Basically, these commands
provide an administrator with all the configuration information that is available about
a particular interface. The show running-config command, however, is used to show
the current configuration on the router and all its interfaces. Recall that a Cisco router
runs each protocol separately from other protocols. For this reason including the ip
in the show ip interface command is necessary to list specifically just IP information
regarding the specific interface.
Example 16-6 show ip route Command Output
GAD# show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route

Gateway of last resort is not set

C 192.168.1.0/24 is directly connected, FastEthernet0/0

C 192.168.2.0/24 is directly connected, Serial0/0
R 192.168.3.0/24 [120/1] via 192.168.2.2, 00:00:07, Serial0/0
! line above verifies RIP routes received
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738 Chapter 16: Distance Vector Routing Protocols
Troubleshooting RIP
Most of the RIP configuration errors involve an incorrect network statement, discon-
tinuous subnets, or split horizon. The primary tool for finding RIP update issues is the
debug ip rip command.
The debug ip rip command displays RIP routing updates as they are sent and received.
Figure 16-15 and Example 16-7 demonstrate a router using debug ip rip and receiving
an update.
Figure 16-15 RIP Network for Debugging
Example 16-7 debug ip rip Output
BHM# debug ip rip
RIP event debugging is on
BHM#
00:45:36 RIP:received v1 update from 192.168.13.2 on Serial0/0
00:45:36 192.168.14.0 in 1 hop
00:45:36 172.31.0.0 in 2 hops
00:45:36 172.29.0.0 in 15 hops
00:45:36 RIP sending v1 update to 255.255.255.255 via Serial0/0 (192.168.13.1)
00:45:36 network 10.0.0.0, metric 1
00:45:36 RIP sending v1 update to 255.255.255.255 via FastEthernet0/0 (10.0.0.254)
00:45:36 network 192.168.13.0 metric 1
00:45:33 network 192.168.14.0 metric 2
00:45:33 network 172.31.0.0 metric 3
00:45:36 network 172.29.0.0 metric 16
BHM
GAD BOAZ

S0/0
S0/1
S0/0
S0/1
Fa0/0
192.168.13.1
10.0.0.254
172.31.31.1
192.168.13.2
192.168.14.1
192.168.14.2
S0/0
Ethernet
See debug ip rip results in
Example 25-7.
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