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This chapter contains the following sections:
To switch frames between LAN ports, the switch maintains an address table. When the switch receives a frame, it associates the media access control (MAC) 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
You can configure static MAC addresses for the switch. These addresses can be configured in interface configuration mode or in VLAN configuration mode.
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 switch(config) #
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 |
The Cisco Nexus device does not support per-VLAN CAM aging timers. |
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) #
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
Use one of the following commands to verify the configuration:
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