Distrbute-List Vs Filter-List in OSPF

Guys,

I understand that with the help of "distribute-list in" command under OSPF process, we can not filter the prefixes as the same filters the prefixes from entering in to the Routing Table and not inside LSDB table.

Since OSPF prepares LSDB table first, so the routes though are filtered using Distribute-list will go into OSPF database table and thus could further be advertised to other OSPF running routers in other Area through LSDB exchange,

However, if we use Filter-list in OSPF, we can prevent prefixes from entering in to OSPF database,

Please correct if I am wrong.

Comments


  • Hi ashse,

    Correct, in OSPF you always have to filter between areas, all routers inside an area has to have the same view of the network (in other words the same LSDB). 

    So the filter-list cames handy here, as it allows you to filter the LSAs leaving or entering a particular area and these LSAs don't even make their way to the LSDB.

    HTH

    Good luck!

  • Hi Ashse,

    you are correct! this is summary from my side:

    -distribute-list prevents prefix from entering into routing table but still present on Link State Database.
    - fliter-list is inter-area filtering mechanism, which prevents from entering into Link State Database.

  • Nice summarization! Just added it to FAVs!

    Regards!

  • Yes which means that OSPF behaves like link state intra-area and like distance vector for inter-area since we don't have the full information about the other areas.

  • you are correct! this is summary from my side:

    -distribute-list prevents prefix from entering into routing table but still present on Link State Database.
    - fliter-list is inter-area filtering mechanism, which prevents from entering into Link State Database.

    Just a side note, there can be two other ways besides filter-list by which we can filter inter-area prefixes. Apply at ABR like filter-list

    1. area <area-id> range <summary> no-advertise 

    2. distribute-list <prefix> in

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