Can you have failover to ISDN in a bridging environment with a standalone ISDN box?

Discussion in 'Cisco' started by BobCov, Nov 20, 2003.

  1. BobCov

    BobCov Guest

    Consider this a question from a non-expert. Consider too that it may
    be nearly idiotic. I am bridging from one office to another over 256K
    synch into a multiplexer which then handles the comms up to the other
    office. There is currently no backup. Changing from bridging to
    routing would be a Big Deal because there are devices around the world
    talking to my device with its present IP, so we don't want to upset

    I want to add ISDN backup, but I don't want to use ISDN in the bridge
    (Cisco in bridging mode) because if the bridge dies completely, so
    would the ISDN module and thus so would the backup. So, I would like
    to have a standalone Cisco 801 ethernet ISDN unit to do this.

    A single pc on my end uses ftp over the bridge to a specific address
    on the destination end. In the back of my mind, I'm thinking there is
    some kind of way to do this by manipulating the routing table on the
    PC and messing with the metric value, but on the otherhand, I'm
    thinking this may not be possible.

    What I need is for the PC to realize that when it cannot get to the
    ftp destination via the normal bridge, it will then go to the ISDN
    bridge. Could this be done by messing with the bridge learning table
    in the ISDN bridge and telling it there's a higher cost "route"
    available? As I said, I'm not an expert, and this really does strike
    me as not doable. Let me know, please what you think.

    BobCov, Nov 20, 2003
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  2. I think it's a bad idea, but with enough effort, you should be able to
    get it to work. Whether you could get it to work well enough to meet
    your needs is another story. One approach would be to pick a routable
    protocol which you do not need to bridge (such as IPX), and use it to
    control up/down of the backup link. Spanning tree would then ensure
    that only one or the other link is used at any one time for bridging.

    Aside from the ugliness of the "solution" it could easily take one to
    two minutes for a failure to be detected and the alternate link to
    start carrying traffic. Plus what kind of performance to you expect
    to get from a single B channel backup? Bridging is typically only
    marginally useful with a full 256K.

    Good luck and have fun!
    Vincent C Jones, Nov 21, 2003
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  3. BobCov

    BobCov Guest

    Thanks, Vincent. I had my suspicions about my idea... but I still
    wonder if there isn't some way to do this. If you're right about the
    delay to come up and the other points, then I'll just have to go back
    to the idea of having isdn in the router (bridge) do the the backup
    and use a cold standby in case of hardware failure. Although I didn't
    say it, I did mean for it to be 128k on the backup.

    BobCov, Nov 21, 2003
  4. BobCov

    Andre Beck Guest

    Theoretically, there should not be any problem. You just place the
    new bridges there, make sure the connection works, and STP is sorting
    out the rest.

    Practically, this is only useful in certain environments, as the ISDN
    line would stay up all the time (STP hellos every two seconds, full
    traffic flooding in one direction). So if the ISDN comes from an internal
    PBX and doesn't cost you money, use that concept. Else we would need some
    mechanism that keeps the remote bridge down as long as it is not needed.

    Again in theory, this could be easily achieved: The 801s would see the
    current topology as established by STP and could just freeze the dial
    connection, spoofing STP. If they sense any change to the topology through
    their ethernets, they would again dial into the remote side and run STP
    until it has converged again. This is exactly the same thing as
    "ip ospf demand-circuit", just on a spanning tree one layer down.
    The problem is, I don't know whether you can do this with Ciscos, and
    to be honest, I doubt it.
    Bridges sort that out themselves using STP. That's how they work, and
    you probably don't want to mess with that.
    STP actually does establish path costs and this way, it decides
    what port has to go blocking in order to prevent a loop. But such
    port is not actually inactive, it just *ignores* packets it receives,
    and it transmits nothing but BPDUs. STP doesn't disable a link, it
    disables just the port at one side of the link. Now the remote bridge
    consists of two half bridges and I'm not entirely sure how that makes
    things different - but I expect it to be a challenge to get that ISDN
    connection down during normal operation. Except if Cisco would have
    something like the "STP spoofing" described above...
    Andre Beck, Nov 21, 2003
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