An Offset List is routinely used to control path selection by influencing routing metrics of learned routes of either EIGRP or RIP. It is important to note that Offset List are only used with Distance Vector Protocol which means that we can not use it with OSPF as it is a Link State protocol. It is a very simple and efficient method to quickly alter routing information especially with EIGRP depending on what you are looking for. For the sake of this topic, we will use the following topology…

Here, we have 2 routing domains which are the RIP domain as well as the EIGRP domain. We also have a couple of Loopback addresses on R3 which are to be redistributed into the RIP domain. We will leverage Offset List to alter the receiving metric of those prefixes on R1.

First, let’s perform basic redistribution on R2 to send our loopback prefixes across with a metric of 1…

All right ! Let’s check R1 routing table and make sure our prefixes are received and installed in the routing table…

Fantastic ! As you can see here, our prefixes are received and installed in the routing table. Notice how the metrics are equal to 1. Let’s use an Offset List statement to change the metric of Loopback 1 to 6…

Here, we created an access-list to match on the prefix in question. Then, under the RIP process we specified our Offset List statement basically calling access-list 1 and increasing the metric by 5. We then defined the outgoing interface (Note here that we did not have to specify an outgoing interface due to the simplicity of our topology. The solution would still work without the “Ethernet0/0” command). Let’s now check R1 routing table and make sure the metric has changed to 6…

Yes it did ! Here we can clearly see that the metric has been altered and changed to 6. Note that the initial metric of Loopback 1 prefix was 1 and we added 5 on R2 which resulted to 6.

Let’s now say that Loopack 2 prefix is not to be installed in the routing table of R1. Let’s first create an access-list matching Loopback 2 prefix on R2…

All right ! our access-list is in ! So the idea is not to see 10.1.2.0/25 in R1 routing table. The best way to accomplish that is to poison the route by increasing the metric to 16 since the maximum hop count for RIP is 15. So, let’s turn on debugging on R1 to better visualize what happens in the background…

Let’s now make a road trip to R2 and create our Offset List statement…

Good ! Let’s now check R1 console messages…

We can clearly see here that the route has been poisoned and marked as “inaccessible”. Let’s now check the routing table as well as the database…

Fantastic ! We can clearly see here that the route has not been installed in the routing table and is displayed in the RIP database as “possibly down”.

This is the basics of Offset List. Please do not hesitate to let me know if you have any questions.