Multihoming question / BGP (default route)

Discussion in 'Cisco' started by Christian Lox, Jul 4, 2004.

  1. Hi!

    I am trying to set up multihoming and need some help.
    Our topology:

    | |
    | |
    E1 ADSL
    | |
    | |
    -------- --------------- -------
    | 2610 |-----------| Cat2924XL-EN |-----------| 2610 |
    -------- --------------- -------
    | Linux Box |

    The Linux box is routing to 4 subnets and has at the moment a static
    default route to the 2610 with the E1 interface. This cisco has a
    default route to the provider (transfer net with,
    btw: both internet connections are going to the same provider). We
    do have a /25 + the according transfer nets.

    Our goal:
    Automatic switch-over to the ADSL line in case the E1 goes down.
    We cannot use full-routing table, since the memory limitations of
    the 2610.

    I am trying to set this up in a lab environment before.
    I am using a 7507 with 8 port Fast Serial card to connect to the
    two 2610 (with WIC-1T).
    The 2610 announce our subnet, one with metric 10. So far, so good!?
    On the 2610 with the E1 I set up two default routes:
    the first one goes to the E1
    the second one, higher metric) goes to the other 2610.
    This should work, but we still have a SPOF (the E1 router)...
    How can I learn the default route ti either the E1 or the ADSL an
    tell this the Linux Box?

    Thanks for any help,
    Christian Lox, Jul 4, 2004
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  2. Configure RIP routing on the 2610's LAN interfaces, to advertise the
    default route. On the backup router, use the "default-metric" router
    configuration command to bump up the metric of this route.

    Run "routed" on the Linux box to learn the default route.
    Barry Margolin, Jul 4, 2004
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  3. Hi Christian,


    there will be probably more solitions to your problem as I will
    only mention two possibles way:

    a) you should run a routing protocol on both of your Cisco boxes
    on your LAN segment and let them exchange your default routes
    with the E1-link preferred and use a kind of a "floating static"
    on the ADSL router. Additionally, you should configure HSRP on the LAN
    side of the routers as the HSRP address will be the gateway for your Linux

    b) Instead of using HSRP on your routers you could run a routing
    process on the Linux box (eg. zebra or quagga) and import the routes from
    the Cisco routers. Therefore you could set your preferences there.


    Klaus Thielking-Riechert, Jul 4, 2004
  4. Hi Klaus (and also Barry),
    thanks for your answers.
    I already thougt of running a routing protocol (quagga/OSPF?) on the
    linux box.
    What makes me stumble is that floating static routes won't work with
    linux, or am i wrong?
    I thought it might be best to learn (but how?) the default route
    from the uplink and transfer it to, say, quagga. Thus I asked in
    this group :)

    Christian Lox, Jul 4, 2004
  5. No, not in this setup.

    Consider the following situation: your E1-router has configured a
    static default route pinned to the E1-interface. Your ADSL-router
    has also configured such a route but pinned to your ADSL-link and
    with a high administrative distance, eg. 230.

    Therefore if both routers exchange their routing table with RIP, EIGRP or
    another protocol, the static route of the ADSL-router won't appear in
    the routing table of the ADSL-router as the administrative distance
    of the default-route learned by the routing protocol will alway be
    better. So, if the E1 link has a failure, the default-router of the
    ADSL-router will be "floating" into its routing table and therefore
    will be announced to the other router (which is not a question of
    Linux or Cisco -- just a question of the design).

    So in this case, it is not really necessary to run a routing daemon
    on the Linux box -- except you just want to do it ;-). You just need
    to configure HSRP on both routers and the Linux box has alway the right

    Yes, that is just one way to do it. You can set your preferences
    to the E1-router using quagga on your Linux box and as soon as the
    E1 has a failure this routes will be withdrawn and you can use your
    other neighbour as the next-hop.


    Klaus Thielking-Riechert, Jul 4, 2004
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