How To Refresh MAC Table?

Discussion in 'Cisco' started by JohnD, Feb 21, 2008.

  1. JohnD

    JohnD Guest

    I've got a stack of 3750s running 12.1.14.

    I've got CAT5 ethernet cables plugged into ports Gi3/0/12 and Gi3/0/18. The
    LEDs are green. Show counters indicates that traffic is going through these
    ports. But for some reason I can't get the MAC address of the devices
    plugged into this port (which means I'm going to have to trace a LONG cable
    run through a very tight wiring cabinet).

    CoreSwitch#sh mac-add int gi3/0/18
    Mac Address Table

    Vlan Mac Address Type Ports
    ---- ----------- -------- -----

    I've run IP scans on all our subnets, I've done clear arp -- nothing seems
    to work. I'm fairly certain there's an ethernet NIC on the other end and
    that it's got a MAC address -- it has to, right?

    Is there any way to force a refresh of the MAC table? Is there any way to
    trigger a broadcast? (Would pinging work?)

    JohnD, Feb 21, 2008
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  2. JohnD

    Merv Guest

    if you have a spare port on the switch then you could set up a span/
    monitor port to span the traffic on each port ( one at a time) to th
    emonitor port where you could connect a PC runing Ethereal sniiffer
    software *free) to see if you can see anything useful to identify the
    devices in question
    Merv, Feb 22, 2008
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  3. Sometimes when interface configured with a static IP address, which is not
    correct, the host does not communicate to the port, and switch has no MAC
    address. I've seen it just two days ago - had an Linux appliance with a
    static IP address configured on the interface. Plugged interface to the
    switch, port says "Connected", but no MAC address in the CAM table. After I
    changed an IP address to whatever it should be for the VLAN - NIC started to
    communicate, and MAC address appears on the port.

    Good luck,

    CCNP, CCDP, CCSP, Cisco Voice, MCSE W2K, MCSE+I, Security+, etc.
    CCIE R&S (in progress), CCIE Voice (in progress)
, Feb 22, 2008
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