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Forum Replies

  1. Hi Lokesh,

    Let’s say you have these routes:

    192.168.0.0/24 - cost 10
    192.168.1.0/24 - cost 20
    192.168.2.0/24 - cost 5
    192.168.3.0/24 - 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.

    Rene

  2. Hi Jason,

    172.16.0.0 0.0.3.255 is an exact match for these four networks:

    • 172.16.0.0
    • 172.16.1.0
    • 172.16.2.0
    • 172.16.3.0

    You could have used the 0.0.0.255 wildcard but then you would require a network statement for each 172.16.x.0 network. Another option could be 172.16.0.0 0.0.255.255 but that means any interface in the 172.16.x.x range would be advertised.

    Rene

  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), 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
    . 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: 3.3.3.3 and lo if IP addresses are advertised in ospf with “ip ospf 1 area 1” command
    R5 router-id: 5.5.5.5 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
    10.10.8.0 255.255.248.0 (R5 loopbacks)
    172.16.16.0 255.255.248.0 (R3 loopbacks)

  5. Hi @tsirgogik

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

    router ospf 1
    area 0 range 10.10.8.0 255.255.248.0
    

    You can make your summary a bit more specific btw, 255.255.252.0 will cover 10.10.8.0/24 - 10.10.11.0/24

    Rene

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