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Cisco MAC address finding

Posted on 2016-10-24
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Last Modified: 2016-11-01
I am trying to locate an host IP address and I got to this switch. Can the experts explain the output below:

sw3750x#show mac address-table address 1234.5678.9012
Unicast Entries
 vlan   mac address     type        protocols               port
-------+---------------+--------+---------------------+--------------------
 100    1234.5678.9012    static ip,ipx,assigned,other Switch                
 101    1234.5678.9012    static ip,ipx,assigned,other Switch            

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Question by:leblanc
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Expert Comment

by:Predrag Jovic
ID: 41857993
That is MAC address of a switch, more specifically - interface VLAN.
MAC address of SVIs are the same for all VLANs.
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Author Comment

by:leblanc
ID: 41858023
I am trying to locate the port where that MAC address came from. So if I understand correctly, the location of the MAC address is the SVI of the Switch. Correct? So I will not be able to find the port number for that MAC address. Correct?
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Expert Comment

by:Predrag Jovic
ID: 41858667
Correct?
Yes. your problem is that gateway is overwriting original MAC address.
So I will not be able to find the port number for that MAC address
Not correct. If you issue
# sh ip arp
On that device where you see that MAC address you can see IP - MAC connection and to what VLAN it belong (IP address range). Then, you can find device by original MAC address prior to L3 device replace it. Just follow it downstream inside of VLAN. :)

MAC address is only locally significant, every L3 device is overwriting it. Only in originating VLAN (network) you can see source's original MAC address.
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by:leblanc
ID: 41859597
That was what I did to find the port that learnt that MAC address and as you can see, the port is Switch.
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Predrag Jovic earned 500 total points
ID: 41859705
If traffic is generated by switch... that's it.
(IP address of an SVI matches source IP address in packets).
GAME OVER :)
If you want to be sure/change what SVI should generate traffic you can configure source-interface per traffic type with command(s):
ip radius source-interface
ip tacacs source-interface
ip ssh source-interface
ip ftp source-interface
etc
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