Difference between redistribute and distribute-list command

[steven.crutchley]

Can someone please explain the difference between the follow 2 scenarios

SCENARIO 1

On an ASBR, I do the following command:

Router1(config)#router ospf 1

Router1(config-router)#redistribute eigrp 2 route-map ROUTE_MAP_NAME subnets

Where my route map allows only routes from 192.168.4.0/24.

SCENARIO 2

On an ASBR, I do the following command:

Router1(config)#router ospf 1

Router1(config-router)#redistribute eigrp 2 subnets

Router1(config-router)#distribute-list 11 out eigrp 2

Where ACL 11 allows only routes from 192.168.4.0/24 

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I am having trouble understanding the purpose of the distribute-list command within the context of refering to routing processes. I understand how it filters routes from being added to the routing table in OSPF (without removing the LSA for that subnet).

However in the context of a routing process, I am told that the ACL, prefix-list or route-map will be applied with respect to the referenced routing process. But isn't that just what the route-map keyword in the my SCENARIO 1 does anyway? Why use a distribute-list command?

[Peter Paluch · Accepted solution]

Hello Steven,

Referencing a routing protocol in the distribute-list out command allows you to apply the filtering only to routes redistributed from the specified routing protocol. However, you are very correct that there seems to be no advantage to perform the filtering directly in the appropriate redistribute command, and the results will be identical.

There is really no difference between the two scenarios. The results will be the same each time. It should be stressed that the whole concept of distribute lists is foreign and unnatural to link-state routing protocols. In IS-IS, there is no support for distribute lists at all, so no confusion there In OSPF, Cisco tried to implement distribute-list functionality despite its problematic applicability to link-state routing, and the results are - as you see yourself - confusing and questionable.

Don't try to look for some deep meaning here. There is none.

Best regards,

Peter