How long does the switch retain the MAC address?

Discussion in 'Cisco' started by William J King, Oct 11, 2006.

  1. Anyone know how long a cisco switch 29xx retains the
    MAC address in the table after the PC disconnects?

    Any reference ?

    Thanx in advance

    William J. King
    ( e-mail )
    Actually
    21 degrees 17 minutes 56.2653 seconds North
    157 degrees 48 minutes 57.8565 seconds West
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    William J King, Oct 11, 2006
    #1
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  2. Walter Roberson, Oct 12, 2006
    #2
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  3. William J King

    Bod43 Guest

    This is true but is not quite the whole story.

    When a spanning tree (let's call it) detects a topology
    change all participating bridges are notified and
    switch to using the fast aging timer. This timer defaults to
    15 seconds.

    A topology change occurs when a port changes state. An exception
    is that ports with portfast configured do not cause a topology
    change notification to be sent and do not themselves do fast aging
    when they change state.

    The scope of a topology change will be one vlan with
    per vlan spanning tree and may bigger if some sort of single
    spanning tree is beng used.

    802.1d is available free from the IEEE web site. Well it was the
    last time I looked. I recall that it was not necessarily that easy
    to find on the site and that it was free because someone was
    paying the IEEE to publish it in that way.
     
    Bod43, Oct 12, 2006
    #3
  4. William J King

    Peter Guest

    Hi William,
    On a Cisco Etherswitch the default MAC address AGING time is 4 hours,
    but it can be changed on a global switch basis.

    An Etherswitch works on the principle that as long as the Ethernet
    interface on the Switch is UP, then the MAC address that is LEARNT for
    any connected device on that port is retained for as long as the MAC
    AGING time.

    HOWEVER... If the Interface into the Switch goes DOWN, then the MAC
    table for ALL devices seen on that port is cleared INSTANTLY.
    The one catch that many people run into is that if the Etherswitch
    port connects to say a HUB, and the PC is removed from the HUB, but
    the Switch link to the HUB remains UP, then the Switch port has NOT
    gone down so the MAC is still present until it times out. This is the
    prime reason why it is never a good idea to chain an Etherswitch to a
    HUB.

    Cheers..............pk.
     
    Peter, Oct 12, 2006
    #4
  5. IEEE standards that have -become- standards (not just in progress),
    and have been standards for at least 6 months, are available free
    for download.... but not all the approaches tell you that!

    802.1D-2004 can be downloaded free from

    http://standards.ieee.org/getieee802/802.1.html
     
    Walter Roberson, Oct 13, 2006
    #5
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