MAC address table - show command on Nexus device

Cisco Nexus switches, like all switches, maintain a MAC address table that creates a correspondence between MAC addresses and switchports. To view the MAC address table we use the show mac address-table CLI command.

The output of such a command looks something like this:

switch#show mac address-table
Legend:
* - primary entry, G - Gateway MAC, (R) - Routed MAC, O - Overlay MAC
age - seconds since last seen,+ - primary entry using vPC Peer-Link,
   (T) - True, (F) - False

VLAN     MAC Address      Type      age     Secure NTFY Ports/SWID.SSID.LID
----     -----------      ----      ---     ------ ---- -------------------
10       0000.0c9f.f002   dynamic   10      F      F    Po100
10       0000.0c9f.f003   dynamic   15      F      F    Po100 (R)
20       0000.0c9f.f004   dynamic   5       F      F    vPC Peer-Link
20       0000.0c9f.f005   dynamic   20      F      F    vPC Peer-Link (R)
30       0000.0c9f.f006   dynamic   0       F      F    Eth1/1
30       0000.0c9f.f007   dynamic   0       F      F    Eth1/2

On the Ports column, you may get a particular interface, a port channel, or a vPC link, and each one has a specific meaning.

Remember, MAC addresses are learned when a frame enters a port. The source MAC address is recorded in the MAC address table, and the port on which it entered the switch is also recorded.

The Port column contains the port on which that particular MAC address was learned. Remember, Ethernet frames can enter a switch via a physical port, a port channel, or via the vPC peer link as well. Here are the various values you can see in the Ports column:

  • Physical interface - example: Eth1/2 - This MAC address was learned via a frame that entered the Ethernet 1/2 interface. This is a physical interface on the switch.
  • Port channel - example: Po100 - This MAC address was learned via a frame that entered port channel 100, which is a logical interface composed of multiple physical links.
  • vPC peer-link - The vPC Peer-Link is the special link between the Nexus switches that are part of the vPC domain. The MAC address was learned from the Peer-Link itself.
  • (R) - This indicates that the MAC address is associated with a routed port such as an SVI, a Layer 3 EtherChannel port, or a Layer 3 physical port.

Links:

https://forum.networklessons.com/t/spine-and-leaf-architecture/8418/43?u=lagapides

https://www.cisco.com/c/en/us/td/docs/switches/datacenter/sw/5_x/nx-os/layer2/command/reference/l2_commands.html#wp1023630