Two switch ports lead to same ethernet MAC address

Discussion in 'Cisco' started by JF Mezei, Oct 21, 2010.

  1. JF Mezei

    JF Mezei Guest

    I will be spending a few hours (or couple of days) with 2 DSL MODEMS
    connected to a switch (2924). With 2 modems connected to the same
    BRAS/BAS router (Bell Canada uses Juniper ERX).

    With the first modem handling an established PPPoE session, the switch
    will initially only find 1 path to the ethernet address of the remote
    BAS router.

    But, when that PPPoE session is restarted, the PPPoE negotiations,
    happening as ethernet broadcasts, will be send to both modems, and the
    BAS may respond to both requests, at which point, the swicth would find
    that both ports 1 and 2 can reach the ethernet address of the BAS.

    Considering that the BAS probably doesn't handle spanning tree, how will
    the switch behave ?

    Will it just send any/all packets destined to the BAS ethernet address
    on both ports ? (causing the BAS to receive 2 copies of each packet)

    Or will it choose just one port for all traffic destined to that
    ethernet address ?

    Are there IOS commands to tell the switch how to handle such a situation ?
    JF Mezei, Oct 21, 2010
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  2. JF Mezei

    bod43 Guest

    I am not familiar with the remote access technology
    after it leaves the customer premises so I can't
    comment on whether your plan will work or not.

    However here is how I think an ethernet switch will handle
    identical source macs on two ports.

    There will at any time be only one entry in the forwarding database
    for the mac.

    The most recently received packet will be used to populate the table.

    Therefore only one copy of each packet will be sent. It
    will be sent to the port that most recently received a frame.

    If there is a high rate of address switching it is possible
    that the switch's performance may be affected. In the bad old
    days I encountered some equipment that could only cope
    with learning about 20 mac address per second. I think it
    unlikely that this will actually be an issue but worth
    watching out for.

    You could use static MAC adddress table entries. I forget what
    it might be called.

    You could consider turning off learning and then the packets would
    be sent on all ports in the VLAN except the receiving port, if that
    suited your purposes better.

    You could also consider configuring etherchannel with no negotiation.
    Then the switch would choose which single port to use to send
    particular traffic and there would only be a single learning
    "virtual" port.
    bod43, Oct 22, 2010
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  3. JF Mezei

    JF Mezei Guest

    Many thanks. This is useful to know. As it turns out, the second DSL
    line connects to a different BRAS router at the telco side, so there are
    different ethernet addresses.

    So when the router here does a PADI request (ethernet broadcast) it
    would get to both BRAS, and the router here woudl get two offers (PADO)
    and choose one.

    Within a couple of days, the old line will be de-activated so that won't
    be an issue anymore.
    JF Mezei, Oct 22, 2010
  4. JF Mezei


    Nov 17, 2010
    Likes Received:
    Dear Friends,

    I guess we are missing out one important point here...
    Mac-addresses do not pass across the routers. When information is learnt about a remote device via two routers simultaneously, then my device will recieve a response with two MAC-Addresses (these MACs will be those of the the two routers and not the far end device), now it is up to my device to choose which mac-address it should accept. Switch will have entry for both the MACs and will switch the traffic depending on the MAC chosen by my device.

    In Short, we do not have to think about how it will work out, the switch and the router will manage this stuff.
    kirandeepmittal, Nov 23, 2010
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