EIGRP Route Filtering using Access-Lists

Distribute lists are used to filter routing updates in routing protocols. Actually it is just an access-list or prefix list to capture traffic and bind it with distribute list command to filter exchanging route updates inbound or outbound with a neighboring router or within the process itself. Note that this won't affect locally originated EIGRP routes.

Setup is simple, there are 4 loopback interfaces on R1.

I am going to start a basic EIGRP process in both R1 & R2 which runs on all the interfaces..



R1(config)#router eigrp 10
R1(config-router)#network 0.0.0.0

R2(config)#router eigrp 10
R2(config-router)#network 0.0.0.0

Above commands will result the following output in R2's routing table..

All the loopback interfaces of R1 are advertised to R2 by R1 in EIGRP process.

Filtering using an Standard ACL

If we don't want a specific route (ex:- 192.168.3.0/26) to be advertised to R2, we will simply create an ACL to capture that route and apply in distribute list command..

R1(config)#access-list 10 deny 192.168.3.0 0.0.0.63
R1(config)#access-list 10 permit any

R1(config)#router eigrp 10
R1(config-router)#distribute-list 10 out ethernet 0/0

Now we can see 192.168.3.0/26 is no longer advertised to R2.

Note:-

If we didn't specify the exact interface, it will by default be applied to all the interfaces..
If we wanted to configure the distribute list on R2, we would have used it inbound like the following..


R1(config)#access-list 10 deny 192.168.3.0 0.0.0.63
R1(config)#access-list 10 permit any

R1(config)#router eigrp 10
R1(config-router)#distribute-list 10 in

Either way it will filter the route and will not put in the R2's topology table even.

Filtering using an Extended ACL

In this case it is bit different, now the source field is the sourcing neighbor and the destination field is the route. So that means we can filter routes from a specific neighbor using an extended ACL.

R2(config)#access-list 101 deny ip host 10.1.1.1 192.168.3.0 0.0.0.63
R2(config)#access-list 101 permit ip any any

R2(config)#router eigrp 10
R2(config-router)#distribute-list 101 in

Result will be the same for this example but when we have several neighbors this can be a useful application.