Arp issue with MSCS on different VLAN's

Discussion in 'Cisco' started by Dan Pearson, Jan 2, 2004.

  1. Dan Pearson

    Dan Pearson Guest

    Ok I ve seen this issue posted although can't find a fix for it.

    The basic situation is: I have a Ms cluster server in one vlan and
    clients in another, clients can connect fine until you fail over the
    cluster server and obviously it changes arp mapping to the other nics
    MAC address, clients on the other side of the switch then cannot
    connect until i clear the arp cache on the cisco switch (a 3750)

    Ok so I m guessing this is an ARP broadcast thing and I have to enable
    it on the switch to allow ARP traffic to the other vlan but am a bit
    unsure of how to do this? would it be ARP Proxy'ing?

    Any pointers including the relevent commands and at which level they
    need to be applied would be gratefully received.

    Thanks in advance

    Dan
     
    Dan Pearson, Jan 2, 2004
    #1
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  2. Dan Pearson

    shope Guest

    "Dan Pearson" <> wrote in message
    news:...
    > Ok I ve seen this issue posted although can't find a fix for it.
    >
    > The basic situation is: I have a Ms cluster server in one vlan and
    > clients in another, clients can connect fine until you fail over the
    > cluster server and obviously it changes arp mapping to the other nics
    > MAC address, clients on the other side of the switch then cannot
    > connect until i clear the arp cache on the cisco switch (a 3750)


    i havent done this on 3750, but it sounds more like a cluster setup problem.

    sounds like your cluster is set up to use a unicast MAC for the cluster
    address.

    last time i set this up (on Alcaltel kit), we set the cluster to use a
    multicast MAC - this means every server request gets copied to all machines
    in the cluster (which only makes sense if that isnt a big chunk of
    bandwidth - typical of a web server where most traffic is read from the
    server).

    To support this you need the 3750 to support multicast ARP entries in its
    ARP table.

    microsoft have a white paper on their web site which gives the various
    config options for a cluster and explains how they work and what you need
    from the network for each.
    >
    > Ok so I m guessing this is an ARP broadcast thing and I have to enable
    > it on the switch to allow ARP traffic to the other vlan but am a bit
    > unsure of how to do this? would it be ARP Proxy'ing?


    No - ARP is local to the VLAN where the ip address has to be resolved.

    If the IP is changing to another machine, then that server should send out a
    broadcast ARP to update ARP entries in all the other IP devices on the
    subnet.

    If you are testing by physically pulling the server cable, then maybe
    spanning tree is blocking the gratuitous ARP - try setting the server ports
    on the switch to host mode
    >
    > Any pointers including the relevent commands and at which level they
    > need to be applied would be gratefully received.


    All the config stuff needs exec level access.
    >
    > Thanks in advance
    >
    > Dan

    --
    Regards

    Stephen Hope - remove xx from email to reply
     
    shope, Jan 2, 2004
    #2
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  3. Dan Pearson

    Greg Reaume Guest

    "Dan Pearson" <> wrote in message
    news:...
    >
    > The basic situation is: I have a Ms cluster server in one vlan and
    > clients in another, clients can connect fine until you fail over the
    > cluster server and obviously it changes arp mapping to the other nics
    > MAC address, clients on the other side of the switch then cannot
    > connect until i clear the arp cache on the cisco switch (a 3750)
    >


    I can't speak for how MSCS handles MAC swapping, but from a switch
    perspective, as soon as a frame comes in with a new MAC (one that already
    isn't in the CAM for that interface) it is added to the table. If this
    means bumping it from another interface table then so be it.

    It sounds like your switch is not doing this, as I can't imagine that MSCS
    is staying THAT quiet after a failover. :) You may want to set up a
    sniffer and turn on SPAN to see what is REALLY going on. Once you
    understand that, watch what your switch is doing with its CAM as the
    failover events happen and the MAC swap takes place. One workaround may be
    to lower the caching time.

    Let us know what you turn up. HTH, good luck.

    Greg
     
    Greg Reaume, Jan 3, 2004
    #3
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