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Forum Replies

  1. Within an area, the LSDB has to be same on all OSPF routers. We can't filter LSAs within an area, the only thing we can do is to prevent a route from being installed in the routing table. That's what we are doing here with the distribute-list.

  2. Hello Rene,
    One quick question. I am trying to use distribute-list in OSPF in outbound direction, but the command is being rejected and the below error message is showing up. Would you please describe why? Thank you so much.

    R1(config-router)#distribute-list prefix cisco out gigabitEthernet 1/0
    **% Interface not allowed with OUT in case of OSPF**

    Best Regards,

  3. Hi Azm,

    The LSDB has to be the same in the area so you can't filter LSAs like that. The only thing you can do is an inbound filter that prevents a LSA from being installed in your routing table, that's it.


  4. Hello Rene,
    Thanks for your reply. However, I still have a confusion. How does distribute-list work? does it filter LSAs or it resists routes from being installed in the routing table? Let's say we are looking at topology like below and they are running OSPF:

    Router A------------Router B-(inbound distribute-list)---------------Router C

    Let's say, Router C has a loopback and it is advertised in OSPF. So if I apply an inbound distribute-list to block on Router B, it does not install route in its routing table. However, it passes the route out to Router A. That means it is not filtering LSAs. It's just not installing that route in its own routing table. So when a distribute-list is applied on Router C to the outbound direction, why would it filter LSAs? Please shed some light on it. Thank you so much in advance.

    Best Regards,

  5. Hello Azm

    Yes, you are correct. The filtering that takes place is filtering of routes from entering the routing table. The filtering does not occur at the interface, but at the mechanism of adding routes to the routing table. Rene mentions the following in the lesson:

    Since the LSDB within the area has to be the same we can’t filter LSAs within the area, we can however filter routes from entering the routing table. Filtering LSAs between areas on an ABR or ASBR is no problem.

    So it is a matter of the definition of the word filter. Filtering in this case does not occur at the interface, but at the entering of routes in the routing table.

    Filtering at the interface however, can occur between OSPF areas.

    I hope this has been helpful!


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