No MAC address on port with good traffic

Discussion in 'Cisco' started by professorguy, Apr 20, 2007.

  1. professorguy

    professorguy

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    I have a device plugged into a cisco 3560 switch at port 47. It is working fine, got a good link light, and it's communicating with our network without problems.

    When I do a show mac-address-table I get ~150 addresses--NONE OF THEM ARE ON PORT 47. In fact, when I do a show mac-address-table int fa0/47 it shows an empty table. (Yes, I'm sure it's on port 47.)

    How can the device be talking to my switch without an entry in the table?

    I want to add the MAC to my MAC acl (not yet applied of course) but I can't find it!
     
    professorguy, Apr 20, 2007
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  2. professorguy

    professorguy

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    Found it, but don't get it.

    I remembered that a show interface gives the MAC, so I just did a

    sh int fa0/47

    and there it was. But I still don't understand why this is not in the table.
     
    professorguy, Apr 20, 2007
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  3. professorguy

    professorguy

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    No, wrong side.

    No, the sh int command gives the switch port's MAC. I want to get what that is connected to.

    I can easily get the macs of devices that are connected to several other ports (they have entries in the table), but this one stays stubbornly invisible.
     
    professorguy, Apr 20, 2007
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  4. professorguy

    professorguy

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    Got it, but I'm still not sure I understand how this works.

    When I sent traffic from the switch to the device, it worked fine. But the switch still claimed it had no idea what the mac was for the device. (Then how did the switch talk to it!?!)

    But when I forced traffic from the device to the switch, then the switch suddenly knew the device's mac, and it showed up in the table. Huh!?!

    I guess a simple ping would have worked since that triggers not just response but a 'new' connectionless session back from the device.

    Could someone please explain the switch's behavior here?
     
    professorguy, Apr 20, 2007
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