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MAC address tables in Cisco switches

Posted on 2013-01-10
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Last Modified: 2013-01-16
A VMware server is attached to a Cisco switch. The VM has serveral VM guests and they have assigned the virtual MAC address by the server.

Under the MAC address table, will those virtual MAC addresses be displayed on that port.

Can a physical port be associated with different MAC addrese, or there is on one-to-one mapping ?

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Question by:AXISHK
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LVL 57

Assisted Solution

by:giltjr
giltjr earned 350 total points
ID: 38765731
If you do a show mac-address-table (or show mac address-table on some switches) it will show which physical port that last ARP response it saw the port come in on.

A port can have many mac addresses associated with it.  Think about it, if you have two switches connected to each other.  Switch 1 will show all MAC addresses for devices on switch 2 on the port that switch 2 is connected to.
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Author Comment

by:AXISHK
ID: 38765774
If Switch 1 connect to Switch 2 and on Switch 1, it will shows all MAC addresses learned on Switch 2, not the MAC address port on Switch 2. Correct ?

Is there any screen dump to show how it actually look on a Cisco switch ?

Tks
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by:Akinsd
Akinsd earned 150 total points
ID: 38766075
http://www.cisco.com/en/US/docs/switches/datacenter/nexus5000/sw/configuration/guide/cli/MACAddress.html

This example shows how to display the MAC address table:

 switch# show mac-address-table


 VLAN      MAC Address       Type    Age       Port


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


 1         0018.b967.3cd0    dynamic 10        Eth1/3


 1         001c.b05a.5380    dynamic 200       Eth1/3


 Total MAC Addresses: 2


 

This example shows how to display the current aging time:

 switch# show mac-address-table aging-time


 Vlan  Aging Time


 ----- ----------


 1     300


 13    300


 42    300
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LVL 9

Expert Comment

by:Sandeep Gupta
ID: 38766313
yes you can do one to one mapping of physical port and mac address
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Author Comment

by:AXISHK
ID: 38766571
To clarify my understanding, if Switch 1 connect to Switch 2 and on Switch 1, it will shows all MAC addresses learned on Switch 2 together with the physical port of switch 1.

Corrrect ?
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Accepted Solution

by:
giltjr earned 350 total points
ID: 38766824
Yes.  Think about what the mac address table is.  This is the table the switch uses to determine which port a frame should be forwarded through to get to that mac.  Say you have:

                  PC1
                    /\
                    |
                   \/
SW1 <----> SW2
                   /\
                    |
                   \/
                  PC2


Now:

      SW2 is connected to port 9 on SW1
      PC1 is connected to port 20 on SW2
      PC2 is connected to port 21 on SW2

If SW1 needs to send a frame to PC1 or PC2 it needs to put it on port 9, so PC1 and PC2's MAC addresses will both show as being associated with port 9 on SW1.
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