mac address-table notification ….. A helpful tool indeed!

Some of the most common issues with complex and simple networks alike, are layer 2 loops.  These can cause all kinds of strange behavior.  They don’t necessarily need to show STP state-changes, or other extreme things like high traffic utilization on an interface……sometimes they are sort of silently disruptive.  Troubleshooting these types of problems can be difficult.  One of the best ways to see a problem like this is to use mac address-table notification.  Depending on the platform and IOS/NX-OS version, it may or may not be enabled by default.  And in some cases, like with the Nexus 7000, you have to turn up the log level to view them.

To turn it on and check that it’s on, the commands are simple such as:

Switch(config)# mac address-table notification
Switch# show mac-address-table notification
MAC Notification Feature is Enabled on the switch
Interval between Notification Traps : 1 secs
Number of MAC Addresses Added : 1
Number of MAC Addresses Removed : 0
Number of Notifications sent to NMS : 1
Maximum Number of entries configured in History Table : 1
Current History Table Length : 1
MAC Notification Traps are Enabled

Some platforms its on by default, on some its not.  For example I don’t think its on by default on N7K, yet it is on N5K, off on 3550, on on 3750, etc……so you need to check it.  Very helpful for troubleshooting.  You may wish to not turn it on if you have AP’s connected with a lot of roaming clients, as mac address-table changes are normal in these scenarios.  On the N7K, you will want to adjust the logging level to make sure you are capturing notification (5) of the l2fm facility, like so:

Switch(config)# logging level l2fm 5

This will allow you to see things like so:

NOR-NX7K01(config)# 2011 Jun 22 12:25:18 NOR-NX7K01 %L2FM-4-L2FM_MAC_MOVE: Mac 0026.982e.aa41 has moved from Eth3/1 to Eth8/14
2011 Jun 22 12:25:18 NOR-NX7K01 %L2FM-4-L2FM_MAC_MOVE: Mac f866.f201.d841 has moved from Eth3/1 to Eth8/14
2011 Jun 22 12:25:19 NOR-NX7K01 %L2FM-4-L2FM_MAC_MOVE: Mac 0026.982e.aa41 has moved from Eth8/14 to Eth3/1
2011 Jun 22 12:25:19 NOR-NX7K01 %L2FM-4-L2FM_MAC_MOVE: Mac f866.f201.d841 has moved from Eth8/14 to Eth3/1
2011 Jun 22 12:25:22 NOR-NX7K01 %L2FM-4-L2FM_MAC_MOVE: Mac 000b.cdca.3931 has moved from Po9 to Eth8/14
2011 Jun 22 12:25:23 NOR-NX7K01 %L2FM-4-L2FM_MAC_MOVE: Mac 000b.cdca.3931 has moved from Eth8/14 to Po9
2011 Jun 22 12:25:25 NOR-NX7K01 %L2FM-4-L2FM_MAC_MOVE: Mac 0012.79cc.66c8 has moved from Eth8/14 to Po9

I hope you find this helpful.  My personal recommendation is leave mac address-table notification on.

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