Route-map & route filtering

Unanswered Question
Feb 21st, 2009

Hi,

I have done route filtering between different routing protocol using route- map and redistribution it is working fine. I have used standard access-list in route-map to filter the necessary routes. But i could not understand how the standard access-list can be used to mark the routes. It just marks the packets with source address as specified in the access-list. But how it filters the route or route-updates.

For example from OSPF to EIGRP. : What I feel is that, if there is IP packet with the soutce IP address as specified in the access-list : then this list will be hit. But happens is in the redistrubuted process those routes are not seen at all ( filterout out with deny keyword). Which works fine ! But still could not understand the logic.

Please share the if it can be explained.

Thanks in advance

Subodh Bapat

I have this problem too.
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Giuseppe Larosa Sat, 02/21/2009 - 12:41

Hello Suboth,

when you use a standard ACL inside a route-map that is used for redistribution that ACL is not used to filter user traffic.

The ACL and the route-map that invokes it are used to decide what routes are to be passed from a protocol to the other protocol.

The routing process that contains the redistribute command is the protocol/instance that receives the routes from the other protocol:

redistribution triggers a complex process:

the database of the source protocol is examined.

Only routes that are installed in the IP routing table are considered.

This list of routes that can be passed to the importing process/instance is then examined using the route filter(s)

.

The permitted routes are actually imported in the database of the receiving protocol using the local parameters for seed metric.

Traffic is not involved here.

The process is somewhat dynamic.

the matches in the ACL increments when the process is executed again.

Hope to help

Giuseppe

bapatsubodh Sat, 02/21/2009 - 19:37

Hi,

Thanks a lot.

Now it has become clear that when routes ( and the corresponding database ) are redistrubited from one protocol process to another protocol process : it resembles to source IP subnet and destination IP subnet. This subnet is matched with the access-list subnets.

Wow that's cool ! But I searched/tried lots on cisco website but could not find as exactly as, what you have expalined.

Thanks once again. Mr. Giuseppe.

Subodh

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