Interpreting MAC Table Data

[devasia1000]

Hello,

I was parsing ARP and then MAC table i.e. taking an entry from ARP table and looking for that mac address in the MAC table but for the same mac address I see several mac entries in the table. Any idea which one will be selected :

sh mac address-table

Legend: * - primary entry

        age - seconds since last seen

        n/a - not available

        S - secure entry

        R - router's gateway mac address entry

        D - Duplicate mac address entry

Displaying entries from active supervisor:

             vlan        mac address              type      learn      age         ports

---------+----------+----------------------------+--------------+-----------+------------+------------+-------------------------------------

       *       50       d44c.12e5.1f80       dynamic     Yes          0        Po1

               502      d44c.12e5.1f80       dynamic    Yes         130      Po1

               510      d44c.12e5.1f80      dynamic     Yes         130      Po1

        *      23        d44c.12e5.1f80      dynamic     Yes          5        Po1

        *      22        d44c.12e5.1f80      dynamic     Yes           0       Po1

Though the ports is the same for all values but not sure why so many entries for same mac address. The Legend says * is primary entry but again there are 3 such * entries

Thanks,

[Marvin Rhoads]

They're entries in different vlans and all across a port channel interface. Likely they are a downstream switch interface and thus the multiple entries.

Sent from Cisco Technical Support iPad App

[jitenshah1989]

I thought mac addresses were globally unique.

So do you mean these are host in different vlan with same mac addresses.

[chandra_rc16]

Hi Shah,

He doesn't mean that they are repesenting the different host with the same mac. You need to understand port-channels to understand this. Take a look at the below link.

http://www.cisco.com/en/US/docs/switches/lan/catalyst4500/12.2/20ew/configuration/guide/channel.html

Don't forget to rate the helpful posts.

Regards,
Chandu

[devils_advocate]

The MAC address itself will be globally unique, you are seeing multiple entries because the downstream switch is connected via a Port Channel.

If you were to logon to the actual switch where the host with that MAC was connected, you will find there is only one entry in the table for it.

Trace the MAC address to find out what device/host it belongs to and ensure on THAT switch, there is only one entry in the CAM table for it.

Po1 will be a trunk link to another switch yes? If the switch recieves a frame destined for that MAC address, it will forward it out of the Port Channel as that is the only outgoing port.

If you saw the same MAC appear in the table with multiple outgoing ports, then you would have an issue.