More than one default router on MPLS network

Discussion in 'Cisco' started by MC, Jan 24, 2007.

  1. MC

    MC Guest

    WE have several remote sites on an MPLS WAN.

    We would like some remote sites to have their default route back to one
    location and the other remote sites back to a second location.

    This was working with our frame WAN but need to see if can make this
    work with the MPLS that we are replacing the frame WAN with,

    Is an ATT MPLS WAN, we have to run BGP, familiar with the simple setup
    for BGP, was hoping that could tag the BGP routes so specific remote
    sites would use a specifc default route ?

    Can this be done and anyone have any ideas what I need to look at to do
    this ?

    Thanks ,
    MC, Jan 24, 2007
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  2. MC

    briggs Guest

    I assume that your AT&T MPLS is like ours. You aren't "really"
    participating in the MPLS network. You just have an IP connection
    to AT&T and a bgp peering arrangement over that IP connection.
    The fact that AT&T is using MPLS internally is transparent to you.

    In that case, I don't believe that it can be done, short of some
    approach such as tunnelling or segregating your network into multiple
    VRFs each with its own connection to AT&T. (We use both of those
    approaches on our AT&T MPLS network, though not for the purposes of
    Internet load sharing which you seem to be after).

    In the frame relay world, you were handing off frame-relay frames to
    the carrier. They were routing those frames based on DLCI. So at
    each of your source sites where you were injecting traffic into
    the carrier's cloud you were in a position to choose the egress
    site where that traffic would emerge from the cloud.

    In the AT&T MPLS world, you are handing off IP frames to AT&T.
    They are routing those frames based on destination IP address. You
    are not in a position to do source-specific routing. AT&T will
    route toward each destination IP address based on its internal
    routing policies.

    AT&T's internal routing tables are constructed based on the routes
    you advertise to AT&T. However, unless I'm missing something,
    if you advertise three default routes into AT&T and tag them, AT&T
    will ignore those tags when constructing its own internal routing
    tables for your traffic. Perhaps you could work directly with AT&T
    on this.

    At the far end, your bgp process will listen to AT&T and see the
    three default routes. And it can select one of the three based on
    your administrative policy. But all three routes have the same
    next-hop gateway -- that site's AT&T peer.

    No clever bgp tactics are going to do you any good.

    At the destination site, you need to advertise the route to AT&T
    and they need to see it -- else they can't route the traffic.

    At the source site you have no choices. You need to route the traffic
    to AT&T.

    What you _can_ do is to create tunnels over the MPLS WAN. And you could
    choose to default-route your traffic over the tunnel of your choice. That
    puts you back in control of the egress site at the far side of the cloud.
    Of course, you'd need to be prepared to deal with the resulting
    MTU issues if you choose to take this approach.
    briggs, Jan 24, 2007
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  3. www.BradReese.Com, Jan 24, 2007
  4. MC

    MC Guest

    We are just connecting to ATT by IP only.

    I was hoping that even though using BGP to route to ATT MPLS network
    could deploy eBGP multi-hop between the far end BGP peers with tagged
    traffic and some route map and/or ACL to only route by that advertised
    route for the default route and hoping since that tagged BGP routes are
    between the far end eBGP AS peering ATT would not even do anything with
    that traffic and would just be using iBGP bettwen our and ATT routers.

    I am going to get in touch with some of the ATT engineers to discuss
    soon, will find out then what can be done.

    Thanks much for the help.

    MC, Jan 25, 2007
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