Anyone know about default ip routes?

Posted: October 29, 2010 in Infrastructure Management and Disaster Recovery

I have a little problem. I can’t work out how to make my default route resilient to various connections going down.

Take the simple example below of a full mesh WAN. Routers 0, 1 and 2 are all private network ips managed via OSPF. Router 0 is connected to Router 3 which is the internet (via NAT/PAT). This is the only route to the internet  for all devices.

Private network communications are kept working in the event of a failure. I.e. if the connection between R0 and R2 fails, R2 can still communicate with R0 via R1 for private traffic. However, what if R1 or R2 needs to get out to the internet?

I have static default routes on R1 and R2 which point to R0. In the event of a failure that static route might not be available, so I’d want it to switch instantly to the other router, which will then forward on the traffic. However, I can’t manager this with OSPF (I think) because it’s unknown, public traffic.

If I set two static default routes then 50% of the traffic will succeed (round robin), but this isn’t what I want. I want my default route to switch when unavailability is detected.

What am I missing?

  1. Jeremy Curtis says:

    Can you not add the default ( route as a static on Router3 (or 2 for that matter) and have that re-distrubite the static route into OSPF? Job done?

    • rymixxx says:

      Thanks Jeremy. Did you have a good holiday? Try any of my biscuits?

      So if I understand you correctly, OSPF can manage the network too? If I add a OSPF entry to each of my routers they will automatically know when one default route becomes unavailable and push the traffic out of the other interface?

      If so that’s remarkably simple, and I hang my head in shame for not having thought of it.

      • Jeremy Curtis says:

        Holiday & Biscuits great, thanks.

        You should only need to add a static route, such as “ip route” on R3 then have it re-distribute static routes into OSPF. The default should then appear on all the others, in effect R3 is then an ASBR – easy on HP ProCurve just use “router ospf redistribution static”. Cisco I think is a bit different in commands used, but again just on R3 add the default route and in the OSPF router config on that I think it is “default-information originate”. Handy really, as you can also then use ECMP to balance traffic.

        I’ll see if I have any old configs I can mail you…

  2. Jeremy Curtis says:

    Great Holiday & Biscuits, thanks.

    You should only need to add a static route to on R3, then have R3 re-distribute static routes into OSPF, everything else should then pick it up. I’ll dig around for an old config and mail some stuff to you – may give a pointer or two.

    • rymixxx says:

      Ah, got it! I guess this is a cornerstone of the internet and networking in general, but this concept kind of passed me by somehow. Cheers.

      • Jeremy Curtis says:

        Good, I’ve just run up a Windows VM to get into the old mail files with the configs in – but the 1Gb file isn’t indexed to search was a pain – I’ll stop now. Yes, it is simple when you think about it, handy too as the likes of ECMP come into play. Multiple outgoing links can be “managed” for preference with artificial costs etc.

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