Did some OSPF labbing yesterday. Ran into some interesting stuff. Imagine that you are running a frame-relay network which is hub and spoke. All routers are using their main interface for communication. The hub router has the static frame relay mapping with the broadcast keyword but the spokes don’t. They are all running OSPF network type point-to-multipoint.

What will happen if we only have broadcast capability in one direction? The hub router sends multicast out its main interface reaching all the spokes. The surprising part is that spokes reply with unicast packet using immediate hello. The adjacency forms and everything is fine and dandy. However after a while the hub declares the spokes as dead, why?

Since the spokes don’t have multicast capability the hub never receives their hello packets. Remember the hello packets are used as keepalive mechanism so after 120 seconds they are declared dead. Quite an interesting scenario and would be interesting to put as trouble ticket at the CCIE lab 🙂 If you as I never heard of immediate hellos you can read this URL. Basically its a way of forming adjacencies and converging faster. Instead of only sending hello packets at HelloInterval the router will immediately respond to incoming hello packets. The draft was written by 3 Huawei engineers.

Fun with OSPF
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2 thoughts on “Fun with OSPF

  • October 31, 2011 at 11:54 pm

    Wow, nice finding. The nasty thing about it is that you won`t notice the issue right away due to the high dead timer value.

    I had to lab it up right away after reading this. The debug logs can be found here: http://inetpro.org/pastebin/11150

    line 76: hello from spoke fails due to missing “broadcast” keyword on DLCI 405
    line 82: immediate hello

    Now back to INE`s BGP videos 😉

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