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mgjk
Asked: 2014-09-10 12:49:48 +0800 CST

Cisco CGS2520 switch to reveal MAC address when port goes active?

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Is there a way to have a Cisco switch reveal the MAC address of a device via Syslog when it is plugged in?

I get messages that a device is connected, but there isn't a lot of detail:

Sep  9 12:50:51 10.9.8.7 6531: *Sep  9 12:47:50: %LINK-3-UPDOWN: Interface GigabitEthernet0/1, changed state to up
Sep  9 12:50:51 10.9.8.7 6532: *Sep  9 12:47:51: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/1, changed state to up

The hope is that if unrecognized devices are plugged into the network, we can be notified at the switch level.

cisco

1 Answers

  1. Best Answer

    This can't work, because the switch will learn which MAC address is on the other end of a connection only when some traffic starts flowing through it; plugging in the cable just isn't enough. This will very likely happen in a few seconds, but it doesn't happen at the same time, thus the switch doesn't have any way to know and report this information when a link goes up.

    Besides, there could be many more than a single address on the other end, if the cable goes to another switch; there could even not be any address at all, if there's a switch on the other end but there is nothing else plugged into it.

    The only way to gather this information is to have a look at the MAC address table, and only after the switch has had enough time to learn what is connected to it.

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