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Forum Replies

  1. Hi Lokesh,

    Let’s say you have these routes: - cost 10 - cost 20 - cost 5 - cost 30

    When you create a summary route that matches these 4 routes then we will have to pick a cost. By default, it will use the lowest cost for the summary route…5 in this case.


  2. Hi Jason, is an exact match for these four networks:


    You could have used the wildcard but then you would require a network statement for each 172.16.x.0 network. Another option could be but that means any interface in the 172.16.x.x range would be advertised.


  3. Rota,
    I modified my network to behave more closely to yours now that I can see your diagram. I can confirm I get the same behavior which was very puzzling to me. I noticed that as soon as I tore down the Virtual Link, the summary appeared for Router 05. I believe you have stumbled upon an obscure corner case where the OSPF RFC (2328) states that any area through which a virtual link is formed is not allowed to summarize. The RFC is huge, but if you search for the term “TransitCapability” you will find this:

                If an area is capable of carrying transit traffic (i.e.,
                its TransitCapability is set to TRUE), <strong>routing
                information concerning backbone networks should not be
                condensed before being summarized into the area.  Nor
                should the advertisement of backbone networks into
                transit areas be suppressed</strong>.  In other words, the
                backbone's configured ranges should be ignored when
                originating summary-LSAs into transit areas.

    A transit area is defined as an area that can carry traffic that is neither originated from nor destined to routers within that area. Once you enable a virtual link, you transform Area 10 into a transit area. Therefore, summarization is not allowed per the RFC.

    I believe the intent here is to prevent the possibility of a routing loop being introduced by the endpoint of the Virtual Link. Any router that is forming a Virtual Link is technically also an ABR for area 0.

    I found a discussion about this exact issue here, if you want to read about it further.

  4. Hello Sims,

    I tried a different example from the one which are shown in the lesson, just to make sure that I understand OSPF summarization well enough. But unfortunately, I couldn’t make it work. Can you please review and let me know what’s done wrong? Thank you for your support.

    R1-R3-R5 (Area 1)
    R1-R2 (Area 0 - Gbps link* and reference bandwidth has been adapted to all routers to 1000)
    R2-R4-R6 (Area 2 - but it’s not configured / used in this example)

    R1 f1/0 and g0/0 interfaces have “ip ospf priority 2” to make the R1 the DR for all the segments.
    R3 router-id: and lo if IP addresses are advertised in ospf with “ip ospf 1 area 1” command
    R5 router-id: and lo if IP addresses are advertised in ospf with “ip ospf 1 area 1” command.

    R1 ABR learns the routes via OSPF , marked as “O” in its routing table and same routes are LSA Type3 routes on R2 and shown as “O IA”. So at this stage, I return back to R1 to run the OSPF command "area 0 range " but still after this command the routes are not summarized in R2 and are shown as before.

    Summary -routes for lo networks (R5 loopbacks) (R3 loopbacks)

  5. Hi @tsirgogik

    Which area range command did you use? On R1, this should work:

    router ospf 1
    area 0 range

    You can make your summary a bit more specific btw, will cover -


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