show mac-address-table

Unanswered Question
Apr 9th, 2009
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Hello Guys,

I am facing a strange behavior;

let's say I have a server connected to a Cisco switch , I issue the command "sh mac-address-table" and I can see the mac address of the server.

however, not all the time, sometimes the servers is still connected and functioning, but the mac address is not appearing, I issue the command multiple times and i get the mac address.

have you faced this before? do you know why this is happening?

thx in advanced.


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lamav Thu, 04/09/2009 - 04:38
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The default flush timer for MAC address entries is 300 seconds. Tt can be reduced to 15 seconds if there is a topology change. STP will flush the MAC address table to hasten convergence by expunging stale entries. If you are running rapid STP, the flush time is even quicker.

See if you are experiencing any changes or instability in your network.

If you just happen to see the entry get flushed and then re-appear, this may be normal if it occurs in a steady and deterministic manner, such as every 300 seconds.



greg.washburn Thu, 04/09/2009 - 04:43
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When the mac-address is not in the table ping the server from the switch if possible. Then immediately check the mac-address-table again. Does the server mac address show up?

wgoulart Thu, 04/09/2009 - 17:44
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the mac will only be in the table when the server is sending traffic. So if the server is not sending any traffic, then the mac will be flushed by default every 300 seconds. So after 300 sec and still do not see the mac, that could mean the server is not sending traffic.


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