Many a times, while working on dynamic Routing protocols (especially EIGRP) we often tend to get caught up on functional difference between Distribute List and Offset List. In discussions on various blogs, we get to understand that Distribute list functions to control the Routes which are advertised or received while Offset List function is to modify the advertised/received metric of Routes. Somewhere, one liners or definition of terms may not clearly give a view of the terms. Hence, let’s take a lab scenario to help practically understand the difference between both terms and how to implement them.
In the below scenario, we will take example of EIGRP Routing Protocol and with 2 Routers R1 and R2 connected Back to Back on FastEthernet port Fa0/0 on each end. Both have 3 Loopbacks interfaces of /32 each advertised over EIGRP.
CONFIGURING DISTRIBUTE LIST –
On R2 , we see 3 Prefixes received from R1 (220.127.116.11/32 , 18.104.22.168/32 and 22.214.171.124/32)
We will configure Access List 1 on R2 to deny subnet 126.96.36.199/32 and permit rest of traffic.
Now , in the Routing Protocol (EIGRP 1) , a Distribute List is introduced which calls Access List 1 for incoming traffic over Fa0/0.
Once Distribute List is configured , we see that subnet “188.8.131.52/32” is not received on R2 any more.
Hence , as Rightly said in the definition of “Distribute List” – It controls the Routes which are advertised or received. In the above scenario, distribute calls the Access list matching Subnet 184.108.40.206/32 controls i.e. stops its entry into R2 EIGRP table.
MOVING ON TOWARDS OFFSET LIST –
As shown below, Distribute List configuration is removed and we are back on R2 which is receiving 3 Routes.
Access list 1 is configure which permit 220.127.116.11/32 only.
Now , in EIGRP configuration , we configure Offset List which calls access list 1 to increases the metric for routes matching ACL with value of 100.
The Output of for EIGRP Learned Routes shows that only for network 18.104.22.168/32 the Metric has increased by 100 (Before it was 409600 and post Offset list configuration it became 409700)
Now, Am sure we have more clarity of how Distribute List and Offset List differ and how to configure them.