About the mac address table.

Answered Question
Aug 26th, 2010

Suppose you have a switch and that the mac-address-table has the following entry:

  mac address          port          vlan

1111.1111.1111       fa0/1          10

If the dispositive with mac address 1111.1111.1111 is connected to port fa0/2 in the same vlan, I

wondered if these entries are possible in the same time:

  mac address          port          vlan

1111.1111.1111       fa0/1         10

1111.1111.1111       fa0/2         10

In other words, I wondered if the switch checks the presence of the same mac address on two

different ports in the same vlan and, in this case, delete the first entry as soon as it learns the

second one, or if it is possible to have multiple entries until the older one normally timed out.

If I have understood, it is possible to have multiple entries if they belong to different vlans, because

the switch considers separately the mac address table for different vlans: I have seen a question

where the answer said that it was possible.

I have this problem too.
0 votes
Correct Answer by Nagaraja Thanthry about 6 years 4 months ago

Hello,

It is possible to have a MAC address registered under two ports. But on

Cisco Switches, it is supported when you configure it manually. When you are

dynamically learning, as soon as a port goes down, switch will remove the

MAC associated with that port. If the MAC is still registered on one port

and you see the same MAC on another port, switch will remove the old entry

and put the new entry in there. If this happens too often with the same MAC,

then switch will generate an error log (MAC flap error).

Hope this helps.

Regards,

NT

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Correct Answer
Nagaraja Thanthry Thu, 08/26/2010 - 05:56

Hello,

It is possible to have a MAC address registered under two ports. But on

Cisco Switches, it is supported when you configure it manually. When you are

dynamically learning, as soon as a port goes down, switch will remove the

MAC associated with that port. If the MAC is still registered on one port

and you see the same MAC on another port, switch will remove the old entry

and put the new entry in there. If this happens too often with the same MAC,

then switch will generate an error log (MAC flap error).

Hope this helps.

Regards,

NT

speculor_cisco Thu, 08/26/2010 - 06:15

Thanks for your good answer and for having remembered me the differences with manually entries.

Now I remember that, reading the 2960 configuration guide some time ago, I had understood that

there was more work then I imagined on handling the mac-address-table by the switch.

A simple question about the forum: how can I give a vote to your answer?

Nagaraja Thanthry Thu, 08/26/2010 - 07:14

Hello,

Glad that I was able to help. Just click on the stars below the answer to

rate the answer.

Regards,

NT

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