Finally figured out traceroute mac on Cisco L3 switches

Just want to put this out there for whoever needs it.

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For those who don't know, the "traceroute mac" command will perform a L2 traceroute to tell you the switch and port that the MAC is found on. Syntax is like this:

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traceroute mac [source_mac_address] [destination_mac_address] 

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Pretty handy, but it wasn't working for me consistently.

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If you're using traceroute mac and it keeps failing, it's probably for one of two reasons:

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1. The MAC you're trying to find is on the L3 switch itself.
2. There's is a VoIP phone or some kind of switch-like thing in between the device and the access switch. Traceroute mac needs to have the same network between switches for it to work. If you change networks (like say the voice VLAN) it will fail.

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I've been racking my brain trying to figure out why it failed on some MACs but no others, and there's not a lot of documentation about it.

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Hope this helps someone in the future!