Routing Information Protocol : along with commands

The Routing Information Protocol (RIP) is one of the oldest distance-vector routing protocols, which employs the hop count as a routing metric. RIP prevents routing loops 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 for RIP is 15. This hop limit, however, also limits the size of networks that RIP can support.

RIP uses the User Datagram Protocol (UDP) as its transport protocol, and is assigned the reserved port number 520.

A hop count of 16 is considered an infinite distance, in other words the route is considered unreachable.

RIP uses some methods to prevent routing loops and count to infinity problem

SPLIT HORIZON:

A router never sends information about a route back in same direction which is original information came, routers keep track of where the information about a route came from. Means when router A sends update to router B about any failure network, router B does not send any update for same network to router A in same direction.

ROUTE POISONING:

Router consider route advertised with an infinitive metric to have failed ( metric=16) instead of marking it down. For example, when network  goes down, router  starts route poisoning by advertising the metric (hop count) of this network as 16, which indicates an unreachable network.

POISON REVERSE:

The poison reverse rule overwrites split horizon rule. For example, if router B receives a route poisoning of network B from router C  then router B will send an update back to router C (which breaks the split horizon rule) with the same poisoned hop count of 16. This ensures all the routers in the domain receive the poisoned route update.

Notice that every router performs poison reverse when learning about a downed network. In the above example, router A also performs poison reverse when learning about the downed network from B.

HOLD DOWN TIMERS:

After hearing a route poisoning, router starts a hold-down timer for that route. If it gets an update with a better metric than the originally recorded metric within the hold-down timer period, the hold-down timer is removed and data can be sent to that network. Also within the hold-down timer, if an update is received from a different router than the one who performed route poisoning with an equal or poorer metric, that update is ignored. During the hold-down timer, the “downed” route appears as “possibly down” in the routing table.

For example, in the above example, when B receives a route poisoning update from C, it marks network 4 as “possibly down” in its routing table and starts the hold-down timer for network 4. In this period if it receives an update from C informing that the network 4 is recovered then B will accept that information, remove the hold-down timer and allow data to go to that network. But if B receives an update from A informing that it can reach network by 1 (or more) hop, that update will be ignored and the hold-down timer keeps counting.

Note: The default hold-down timer value = 180 second.

TRIGGERED UPDATE :

When any route failed in network ,do not wait for the next periodic update instead send an immediate update listing the poison route.

RIP Operation:

RIP defines two types of messages.

Request Message

Response Message

When a RIP router comes up, it sends a broadcast Request Message on all of its RIP enabled interfaces. All the neighboring routers which receive the Request message respond back with the Response Message containing their Routing table. The Response Message is also gratuitously sent when the Update timer expires. On receiving the Routing table, the router processes each entry of the routing table as per the following rules:

If there are no route entry matching the one received then the route entry is added to the routing table automatically, along with the information about the router from which it received the routing table

If there are matching entry but the hop count metric is lower than the one already in its routing table, then the routing table is updated with the new route.

If there are matching entry but the hop count metric is higher than the one already in its routing table, then the routing entry is updated with hop count of 16 (infinite hop). The packets are still forwarded to the old route. A Holddown timer is started and all the updates for that from other routers are ignored. If after the Holddown timer expires and still the router is advertising with the same higher hop count then the value is updated into its routing table. Only after the timer expires, the updates from other routers are accepted for that route.

Configuring RIP

Router(config)#router rip

Enter router RIP configuration mode

Router(config-router)#network

Identify networks that will participate in the router protocol. Notice that you identify networks, and not interfaces.

NOTE: You need to advertise only the classful network number, not a subnet:

R2(config)#router rip
R2(config-router)#version 2

Router(config-router)#network 172.16.0.0

not

Router(config-router)#network 172.16.10.0
Router(config-router)#no auto-summary

If you advertise a subnet, you will not receive an error message, because the router will automatically convert the subnet to the classful network address.
By default, auto-summary is enabled in rip. If this is not disabled there will be differences in the number of routes that appear in show ip route. However, it will not affect the routing process.

RIP Timers:

Update Timer

This timer controls the interval between two gratuitous Response Message. By default the value is 30 seconds. The response message is broadcast to all its RIP enabled interface.

Invalid Timer

This timer specifies how long a routing entry can be in the routing table without being updated. This is also called as expiration Timer. By default, the value is 180 seconds. After the timer expires the hop count of the routing entry will be set to 16, marking the destination as unreachable.

Flush Timer

This timer controls the time between the route is invalidated or marked as unreachable and removal of entry from the routing table. By default the value is 240 seconds. This is 60 seconds longer than Invalid timer. So for 60 seconds the router will be advertising about this unreachable route to all its neighbours. This timer has to be longer than Invalid Timer.

Holddown Timer

This timer is started per route entry, when the hop count is changing from lower value to higher value. This allows the route to get stabilized. During this time no update can be done to that routing entry.The default value of this timer is 180 seconds.