Conditional OSPF Default Route: Tested Configuration

One of my readers asked for a working configuration of the conditional OSPF default route advertisement feature. In my scenario, the OSPF default route would be announced whenever an Internet prefix (172.18.0.0/16) would be present in the IP routing table.

router ospf 1
 log-adjacency-changes
 network 0.0.0.0 255.255.255.255 area 0
 default-information originate always route-map FromInternet
!
router bgp 11
 bgp log-neighbor-changes
 neighbor 172.16.1.2 remote-as 21
!
ip access-list standard FromInternet
 permit 172.18.0.0
!
route-map FromInternet permit 10
 match ip address FromInternet

Caveats:

  • The route map configured in the default-information originate command tests the IP prefixes in the IP routing table. You can thus match only on those attributes that are present in the IP routing table (IP prefix, metric, next-hop), not on additional BGP attributes (like AS-path), which would be really cool
  • Contrary to what Sebastian wrote in his comment, you don’t have to redistribute the BGP route into OSPF to make it work in IOS release 12.4(11)T or 12.2SRC, but it looks the IP prefix you test cannot be a subnet.

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12 comments:

  1. Anonymous 01 August 2007 10:23
    Hi!

    That's strange, it didn't work for me in 12.4(11)T, and this is the response I got from TAC:

    ------------------------
    the problem with originate default information is that it's a special redistribution. In this configuration you 'redistribute' into OSPF routes x.x.x.x/16 and y.y.y.y/20 with the only exception that during redistribution they become default route.

    This said, usual restrictions on redistribution apply and iBGP routes are not redistributed into IGP.
    ------------------------

    Perhaps I have a special configuration which triggered the problem, I don't know.
  2. Ivan Pepelnjak 01 August 2007 16:48
    Did you use default-information originate or default-information originate always? I would sort-of understand why you'd need to redistribute a route into OSPF without the always keyword.
  3. Anonymous 03 August 2007 09:26
    No, I used originate always, because I don't have a default route set on the router... Did you have a (static) default route in your configuration somewhere?
  4. Ivan Pepelnjak 05 August 2007 16:39
    No default route. I might still have the original (complete) router configurations. Send me a message and I'll send them to you.
  5. Unknown 22 February 2008 19:33
    It works for me. I already thought of doing this, then the site confirmed. I do not use the 'always' keywork. I learn the default from BGP, then in my route-map match a prefix-list of 0.0.0.0/0 and a tag of the AS it should be from. No redistribution in my setup.
    FYI: matching on next-hop didn't work.. using tags is cleaner anyway.
  6. Sharath Samanth 01 September 2008 03:08
    Another Caveat is the route-map reference should always match a classful network.

    EIGRP's default-information originate would add a Classful static entry into to the config but OSPF does not.

    Sharath
  7. Kaage 07 September 2011 08:02
    It was a little bit challenging to notice that a route-map, used by default-information-originate, will never match an iBGP route! ;)
  8. Ivan Pepelnjak 08 September 2011 19:39
    Thanks for the tip! It does make sense (IBGP route is supposed to be coming from across the OSPF domain), but it's definitely a bit under-documented.
  9. sulaiman 16 April 2012 19:10
    what will be the configuration for next hop? i tried to do the next hop setting, but it didnt work.

    default-information originate always route-map TEST_1

    route-map TEST_1 permit 10
    match ip next-hop 12
    route-map TEST_1 permit 20
    match ip next-hop 13

    access-list 12 permit 192.10.1.254
    access-list 13 permit 204.12.1.254

    show ip route
    B 118.0.0.0/8 [20/0] via 204.12.1.254, 00:42:37
    B 222.22.2.0/24 [20/0] via 192.10.1.254, 00:27:02
  10. A_Ortiz 08 October 2017 01:06
    Hello. I am curious as to the opposite: ¿is it possible to announce the default route to a OSPF neighbor when a certain prefix is removed from the routing table?
    Replies
    1. Ivan Pepelnjak 08 October 2017 07:54
      You probably need to create a fake route that is tracking an absence of a route, and advertise the default route based on the presence of that fake route. Something along the lines of

      http://blog.ipspace.net/2007/08/install-static-route-when-ip-address-is.html
    2. A_Ortiz 10 October 2017 00:56
      Thanks for your reply. I do admire your work.
      I tested it by means of IP SLA and Boolean Not and it worked, as you pointed out, a bit obscure though. I am going to test what you suggested.
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