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Changing RIP Metric

Changing RIP Metric

We can change the metric learned by default (hop count) using the Offset-lists in RIP.

Changing RIP Metric

In above topology, on router R1, we see the route for prefix learned via 3 paths, i.e. via R2, R3, and R4 with equal metrics (hop count 2).

Changing RIP Metric

Important Note:  In RIP, Hop count or the Metric gets increased to 1 when the update enters the interface (inwards) but not when it exits the interface. On R1, we can see the route of prefix is learned with metric/hop count 2 from all three paths as per topology shown above. While R5 advertises the prefix, hop count will be zero because the update is exiting the router. As the update is received by router R3 on interface f1/1, metric/hop count gets increased from zero to 1. When R3 advertises the prefix to R1, metric/hop count remains 1 as the update is exiting the router. When R1 receives this update on f1/1, metric/hop count again gets increased to 1 making the metric/hop count total to 2.

Let's try changing the metric/hop count on R3 by configuring an offset-list where R3 will advertise the prefix to R1 with additional metric offset of 2. When R1 receives the update on f1/1 from R3, the total metric we will see for the prefix learned via R2 is 4 (Kindly note that the original metric was 2 and then we will add an offset of 2 making the total metric/hop count to 4).

Step1: Create an access-list.

R3(config)#ip access-list standard rip_offset
R3(config-std-nacl)#permit any

Step2: Under RIP config mode, configure an offset-list to change the metric. Metric can be changed while advertising or while an update is received.

To change metric while the update is advertised by R3 through f1/0  to R1. In our topology we have use this method.

R3(config-std-nacl)#router rip
R3(config-router)#offset-list rip_offset out 2 f1/0  

To change metric while the update is received by R3 through f1/1 from R5.

R3(config-std-nacl)#router rip
R3(config-router)#offset-list rip_offset in 2 F1/1 

After we configure the offset list to change RIP metric on R3 while it advertises the prefix to R1, below are debug outputs.

R1# debug ip rip
*Apr 16 18:58:36.831: RIP: received v2 update from on FastEthernet1/1
*Apr 16 18:58:36.839: via in 4 hops


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