Redundant links between subnets

Discussion in 'Cisco' started by Ghazan Haider, Jun 20, 2006.

  1. We have two 1841s. So the connection is between two cisco routers over
    two non-cisco routers. I havent tried it but I was under the impression
    static routes could be entered for the other cisco router on each
    router for the sake of hello packets, and the neighbor command can be
    used to specify the neighbor for hello packets. If this doesnt work,
    you're right I'll have to resort to GRE tunnels.

    I wonder if I can do GRE tunnels just for the hello packets. Another
    thing I just thought of was packets in each side will go over to the
    other side's cisco router before entering the other side's subnet,
    since the destination subnet should be behind the cisco router running
    the IGP. I wonder if the hello packet triggers can be used to bring
    up/down different routes.
    Ghazan Haider, Jun 28, 2006
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  2. Ghazan  Haider

    Merv Guest

    Its not quite that simple, because each routing protocol can have
    additional conditions that must be met.

    EIGRP certainly does and that condition being that neighbours MUST be
    addressed from the same subnet which you will not be true in your
    particular circumstance.

    OSPF will require testing. OSPF may form an adjacency and allow LSA
    exchange. The real test is whether it will insert the learned LSA's
    into the main routing table given that the neighbours are on two
    different subnets. If it does you are homefree. You can then tune the
    hello timers as required.

    As your stated objective is to make your connectivity more robust, keep
    the setup as simple.

    And as Vince indicate what out for the MTU hit with GRE tunnels.

    Also for GRE tunnels remebber to configure a static route for the
    tunnel destination address to prevent recursive lookups.
    Merv, Jun 29, 2006
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  3. You can do GRE tunnels just for the hello packets BUT...
    1- It is ugly and error prone.
    2- Even if you get it right, last time I checked Cisco only updates static
    routes once per minute unless a link has failed hard, so your average
    recovery time will be over 30 seconds and be nowhere close to meeting your
    availability requirements.

    Been there, done that, been burnt...
    Vincent C Jones, Jun 29, 2006
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