Configuring the MAC Address Table

This chapter contains the following sections:

Information About MAC Addresses

To switch frames between LAN ports, the switch maintains an address table. When the switch receives a frame, it associates the media access control (MAcC) address of the sending network device with the LAN port on which it was received.

The switch dynamically builds the address table by using the MAC source address of the frames received. When the switch receives a frame for a MAC destination address not listed in its address table, it floods the frame to all LAN ports of the same VLAN except the port that received the frame. When the destination station replies, the switch adds its relevant MAC source address and port ID to the address table. The switch then forwards subsequent frames to a single LAN port without flooding all LAN ports.

You can also enter a MAC address, which is termed a static MAC address, into the table. These static MAC entries are retained across a reboot of the switch.

In addition, you can enter a multicast address as a statically configured MAC address. A multicast address can accept more than one interface as its destination.

The address table can store a number of unicast and multicast address entries without flooding any frames. The switch uses an aging mechanism, defined by a configurable aging timer, so if an address remains inactive for a specified number of seconds, it is removed from the address table.

Configuring MAC Addresses

Configuring a Static MAC Address

You can configure MAC addresses for the switch. These addresses are static MAC addresses.


Note

You can also configure a static MAC address in interface configuration mode or VLAN configuration mode.

SUMMARY STEPS

    1.    switch# configure terminal

    2.    switch(config-)# mac-address-table static mac_address vlan vlan-id {drop | interface {type slot/port} | port-channel number} [auto-learn]

    3.    (Optional) switch(config-)# no mac-address-table static mac_address vlan vlan-id


DETAILED STEPS
     Command or ActionPurpose
    Step 1 switch# configure terminal
     

    Enters configuration mode.

     
    Step 2 switch(config-)# mac-address-table static mac_address vlan vlan-id {drop | interface {type slot/port} | port-channel number} [auto-learn]
     

    Specifies a static address to add to the MAC address table.

    If you enable the auto-learn option, the switch will update the entry if the same MAC address is seen on a different port.

     
    Step 3 switch(config-)# no mac-address-table static mac_address vlan vlan-id
          		(Optional)

    Deletes the static entry from the MAC address table.

     

    This example shows how to put a static entry in the MAC address table: 

    switch# configure terminal

    switch(config)# mac-address-table static 12ab.47dd.ff89 vlan 3 interface ethernet 2/1

    You can use the mac-address-table static command to assign a static MAC address to a virtual interface.

    Configuring the Aging Time for the MAC Table

    You can configure the amount of time that an entry (the packet source MAC address and port that packet ingresses) remain in the MAC table. MAC aging time can be configured in either interface configuration mode or in VLAN configuration mode.


    Note

    If the Cisco Nexus 3000 switch is used as a Layer 2 or Layer 3 termination switch, Cisco recommends that you set the mac-address-table aging-time to 1800 (higher than the default ARP aging time of 1500 seconds) on all VLANs.

    Cisco Nexus 3000 switch does not support per-VLAN cam aging timers.

    SUMMARY STEPS

      1.    switch# configure terminal

      2.    switch(config)# mac-address-table aging-time seconds [vlan vlan_id]


    DETAILED STEPS
       Command or ActionPurpose
      Step 1switch# configure terminal 

      Enters global configuration mode.

       
      Step 2switch(config)# mac-address-table aging-time seconds [vlan vlan_id]  

      Specifies the time before an entry ages out and is discarded from the MAC address table.

      The seconds range is from 0 to 1000000. The default is 1800 seconds. Entering the value 0 disables the MAC aging. If a VLAN is not specified, the aging specification applies to all VLANs.

       

      This example shows how to set the aging time for entries in the MAC address table to 1800 seconds (30 minutes): 

      switch# configure terminal
switch(config) # mac-address-table aging-time 1800
switch(config) #

      Clearing Dynamic Addresses from the MAC Table

      You can clear all dynamic entries in the MAC address table.

      Command

      Purpose

      switch(config)# clear mac-address-table dynamic {address mac-addr} {interface [type slot/port | port-channel number} {vlan vlan-id}

      Clears the dynamic address entries from the MAC address table.

      This example shows how to clear the dynamic entries in the MAC address table: 

      switch# clear mac-address-table dynamic

      Verifying the MAC Address Configuration

      To display MAC address configuration information, perform one of these tasks:

      Command

      Purpose

      switch# show mac-address-table aging-time

      Displays the MAC address aging time for all VLANs defined in the switch.

      switch# show mac-address-table

      Displays the contents of the MAC address table.

      This example shows how to display the MAC address table: 

      switch# show mac-address-table

      VLAN      MAC Address       Type    Age       Port
      ---------+-----------------+-------+---------+------------------------------
      1         0018.b967.3cd0    dynamic 10        Eth1/3
      1         001c.b05a.5380    dynamic 200       Eth1/3
      Total MAC Addresses: 2

      This example shows how to display the current aging time: 

      switch# show mac-address-table aging-time

      Vlan  Aging Time
      ----- ----------
      1     300
      13    300
      42    300