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I have three routers in area 0, two of which are connected to separate WAN circuits. I have one router that is sending the default route but when I try to add a second router with the configuration statement: default-information originate metric 1000 I do not see that populate anywhere via OSPF. There is a static default route in the second site I want to advertise from. I'm assuming this isn't supported but I thought I would ask in case I am missing something.

Output of OSPF database external:

#sh ip ospf database external 0.0.0.0

            OSPF Router with ID (172.16.1.204) (Process ID 1)

                Type-5 AS External Link States

  LS age: 118
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 0.0.0.0 (External Network Number )
  Advertising Router: 172.16.1.30
  LS Seq Number: 80000006
  Checksum: 0x10F
  Length: 36
  Network Mask: /0
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 1000
        Forward Address: 172.17.1.1
        External Route Tag: 1

  LS age: 1838
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 0.0.0.0 (External Network Number )
  Advertising Router: 172.16.1.255
  LS Seq Number: 800000CE
  Checksum: 0xEA51
  Length: 36
  Network Mask: /0
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 1
        Forward Address: 172.16.1.1
        External Route Tag: 1

You can see the default route the second site is sending (as noted by the metric).

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    You seem to get both default routes. The problem is that the router will only install a single route in its routing table. If one of the routers advertising the route goes down, the other route will automatically be installed in the routing table.
    – Ron Maupin ♦
    Dec 8, 2020 at 19:57
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could post and accept your own answer.
    – Ron Maupin ♦
    Dec 31, 2020 at 5:06

1 Answer 1

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You can have multiple ASBRs redistributing their (set(s) of) external routes into the OSPF domain. The routes the various ASBRs generate/inject might be identical, partially overlapping or even completely disjoint, and yes, there can be multiple ASBRs generating/injecting a default route.

The other routers will ingest these LSAs, but of course will apply their given set of rules before they accept the LSA into their topology database or the routing table. In the latter, you'll see multiple external routes for the same (external or default) destination only if a few conditions are met.

You might want to show some config snippets of the given ASBRs and their redistribution config, and some excerpt of the OSPF topology from the non-ASBRs, so we might be able to help finding what is missing and why.

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  • Thanks for the detailed feedback. Did more research on this and looked at the ospf database topology and noticed a type-5 link state with the router-id of the second router in question, which is a good sign. I guess the question is, should the default route learned from another router, show up in the routing table if it is less preferred (with higher metric)? I can't do it now, but I will set a interface at my main site to passive and see if the default route learned from the second site.
    – Ryan F
    Dec 8, 2020 at 16:10
  • If there is more than one type 5 link state for network 0.0.0.0 in your topolgy database, that's a good starting point, indeed. However, That alone does not make a router promote both to the routing table. You might want to show us the equivalent of the cicso-style show ip ospf database external 0.0.0.0 from your router. Assuming that both ASBRs are successfuly generating the LSA, it should yield two entries.
    – Marc 'netztier' Luethi
    Dec 8, 2020 at 17:46
  • FYI, set primary interfaces as passive and it did failover as expected. Thanks for everyone's feedback!
    – Ryan F
    Dec 17, 2020 at 13:03

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