MAC address table

The switch and mux of the Cisco Nexus 3550-F Fusion (formerly ExaLINK Fusion) makes use of a MAC address table. The table is built dynamically from the source address of received frames, and can also be populated with static entries by the user.

When a frame is received on a port in a switch object, or on the upstream port of a mux object, the destination address is looked up in the MAC address table, and the frame is sent out on the ports stored in the table. If the address is not in the table, the frame is broadcast on all other ports of the switch object, or on all downstream ports of the mux object.

Frames received on downstream ports of a mux object bypass the MAC address table and are always sent out the upstream port.

Display the MAC address table

The show mac-address-table command can be used to show the contents of the MAC address table. For example:

admin@N3550-F(switch:my_switch)> show mac-address-table
Address      Input                                           Output Type
------------ ----------------------------------------------- ------ ----
643F5F0114A1 A13 A14 B1 B2 B5 B6 B9 B10 B13 B14 C1 C2 C5 C6  A10    L
643F5F0114A2 A10 A14 B1 B2 B5 B6 B9 B10 B13 B14 C1 C2 C5 C6  A13    L
643F5F0114A3 A10 A13 B1 B2 B5 B6 B9 B10 B13 B14 C1 C2 C5 C6  A14    L
643F5F01159C A10 A13 A14 B2 B5 B6 B9 B10 B13 B14 C1 C2 C5 C6 B1     L
643F5F01159D A10 A13 A14 B1 B5 B6 B9 B10 B13 B14 C1 C2 C5 C6 B2     L
643F5F01159E A10 A13 A14 B1 B2 B6 B9 B10 B13 B14 C1 C2 C5 C6 B5     L
643F5F01159F A10 A13 A14 B1 B2 B5 B9 B10 B13 B14 C1 C2 C5 C6 B6     L
  • The address field contains the MAC address for this entry
  • The input field indicates the input ports for which this entry applies.
  • The output field indicates the ports on which a matching frame will be output.
  • The type field indicates whether the entry is learned (L) from received frames or if it is a static (S) entry added by the user.

MAC address learning

By default a switch or mux object will learn routes via MAC source addresses from incoming packets and note the ports they come from. This route learning behaviour can be disabled via:

admin@N3550-F(switch:my_switch)> no mac-address-learning

If required, can be enabled again via:

admin@N3550-F(switch:my_switch)> mac-address-learning

Clearing the MAC address table

The MAC address table for an object can have all learned/dynamic entries cleared using the following command:

admin@N3550-F(switch:my_switch)> clear mac-address-table dynamic
Cleared dynamic MAC address table entries on switch "my_switch"

Note this will not impact any static entries that have been added.

Managing static routes

The mac-address-table static command can be used to add static entries to the MAC address table. For example, suppose we want to configure a mux object to send traffic for the MAC address 64:3F:5F:01:23:45 to port A2:

admin@N3550-F(config-mux:my_mux)> mac-address-table static 643F5F012345 port A2
Added static MAC address "643F5F012345" port "A2" on mux "my_mux"

To remove the static entry, use the no form of the command:

admin@N3550-F(config-mux:my_mux)> no mac-address-table static 643F5F012345 port A2
Removed static MAC address "643F5F012345" port "A2" on mux "my_mux"

Note that all packets received on a downstream port of a mux object are transmitted out the upstream port. Static entries in the MAC address table for mux objects only apply to packets received on an upstream port.

Disabling MAC address learning

The no mac-address-learning command disables MAC address learning for a switch or mux object:

admin@N3550-F(config-mux:my_mux)> no mac-address-learning
Disabled MAC address learning on mux "my_mux"

To re-enable MAC address learning, use the mac-address-learning command:

admin@N3550-F(config-mux:my_mux)> mac-address-learning
Enabled MAC address learning on mux "my_mux"

This page was last updated on Feb-19-2021.