Show mac-address-table command

From NetworkCommands

A command available on most Cisco switch platforms; used to display the device's layer 2 MAC address table, which contains MAC address to interface mappings (in other words, which interface a particular MAC address can be reached through.)

These mappings are continuously 'learnt' by the device through the inspection of the Ethernet header (which contains source and destination MAC addresses) of all frames entering the switch's interfaces. These are used by the device to switch frames between specific interfaces rather than broadcast them to all interfaces (as a hub does.) However, frames with a destination MAC addresses not in this table are broadcast or 'flooded' to all interfaces.

Usage Syntax

show mac-address-table [options]

The available options are;

• address MAC-address - only display MAC address table information for the specified MAC-address
• interface interface-name/number - only display MAC address table information for entries learned on the specified interface
• vlan vlan-number - only display MAC address table information for entries learned on the specified vlan-number
• dynamic - only display MAC address table information on dynamically learnt entries
• static - only display MAC address table information on static entries
• multicast - only display MAC address table information on multicast entries
• count - display MAC address table statistics including number of multicast, unicast, static and dynamic entries
• aging-time - display the global and per VLAN MAC address table aging times
• protocol protocol-name - only display MAC address table information for entries using the specified protocol-name

Usage Examples

show mac-address-table interface fa0/1 - display only those MAC address table entries learned on interface Fa0/1

show mac-address-table vlan 111 - display only those MAC address table entries learned on interfaces in VLAN 111

show mac-address-table address 00.0B.E4.36.60.E2 - display the MAC address table entry for the specified address, including the interface it was learnt on

Usage Notes

This command is obviously very useful in discovering which switch interface a particular host is connected to, if you have it's MAC address. If you only have a host's IP address, you may be able to find it's MAC address using the show arp command. If the interface a MAC address is associated with is connected to another switch, you will have to repeat this step on that switch (and possibly others.)

Frames with a destination MAC addresses not in this table are flooded to all interfaces.

Typical Output

show mac-address-table:

Unicast Entries
 vlan   mac address     type        protocols               port
-------+---------------+--------+---------------------+--------------------
 210    0011.2003.a87f    static ip,ipx,assigned,other  Switch
 210    0040.ca79.8821   dynamic ip                     GigabitEthernet1/1
 210    00d0.b7bc.3d2c   dynamic ip                     GigabitEthernet3/34
 210    0800.8e05.1bbc   dynamic ip                     GigabitEthernet3/33
 210    0800.8e05.39b7   dynamic ip                     GigabitEthernet3/35

Multicast Entries
 vlan    mac address     type    ports
-------+---------------+-------+-------------------------------------------
 210    ffff.ffff.ffff   system Switch,Gi2/2,Gi2/6,Gi3/33,Gi3/34,Gi3/35,Gi1/1

show mac-address-table count:

MAC Entries for all vlans:
Dynamic Unicast Address Count:                  253
Static Unicast Address (User-defined) Count:    0
Static Unicast Address (System-defined) Count:  14
Total Unicast MAC Addresses In Use:             267
Total Unicast MAC Addresses Available:          32768
Multicast MAC Address Count:                    17
Total Multicast MAC Addresses Available:        16384

Related Commands

Use the configuration mode mac-address-table command command to specify the mac address table aging time; how long a MAC address to interface mapping stays in the mac address table. This command can also be used to configure static MAC address to interface mappings.

The Cisco show arp command can be used to display IP address to MAC address mappings.

Command Equivalents

The Extreme switch command equivalent is the show fdb command

Related Articles

You may find our OSI Model General Diagram useful in understanding where layer 2 MAC addressing fits into the OSI model.

Information on other Cisco commands

Information on Linux commands

Information on F5 BigIP commands

Information on Vyatta commands

Information on Extreme commands

Information on Zebra commands

Information on Secure Platform commands

Information on Blue Coat SGOS commands

Information on Nokia IPSO commands

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