Configuring MAC Address Tables

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 (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.

You cannot enter a multicast address as a statically configured MAC address, both for IP multicast and non-IP multicast MAC addresses. This is not supported by the N3548 platform.

The address table can store a number of unicast 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 Static 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.

Procedure

  Command or Action Purpose
Step 1

switch# configure terminal

Enters global configuration mode.

Step 2

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

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

Step 3

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

(Optional)

Deletes the static entry from the MAC address table.

Use the mac address-table static command to assign a static MAC address to a virtual interface.

Example

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 1/4
switch(config) #

Disabling MAC Address Learning on Layer 2 Interfaces

You can now disable and re-enable MAC address learning on Layer 2 interfaces.

Procedure

  Command or Action Purpose
Step 1

switch# configure terminal

Enters global configuration mode.

Step 2

switch(config)# interface type slot/port

Enters the interface configuration mode for the specified interface.

Step 3

switch(config-if)# [no] switchport mac-learn disable

Disables MAC address learning on Layer 2 interfaces.

The no form of this command re-enables MAC address learning on Layer 2 interfaces.

Note 

In Warp mode, the Cisco Nexus 3500 switch does not flood Layer 3 traffic to the VLAN in which the port configured using switchport mac-learn disable is present, and the traffic is dropped. In Normal mode, the switch should flood the Layer 3 traffic to this VLAN.

Step 4

switch(config-if)# clear mac address-table dynamic interface type slot/port

Clears the MAC address table for the specified interface.

Important 
After disabling MAC address learning on an interface, ensure that you clear the MAC address table.

Example

This example shows how to disable MAC address learning on Layer 2 interfaces: 

switch# configure terminal
switch(config)# interface ethernet 1/4
switch(config-if)# switchport mac-learn disable
switch(config-if)# clear mac address-table dynamic interface ethernet 1/4

This example shows how to re-enable MAC address learning on Layer 2 interfaces: 

switch# configure terminal
switch(config)# interface ethernet 1/4
switch(config-if)# no switchport mac-learn disable

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

The Cisco Nexus device does not support per-VLAN CAM aging timers.

Procedure

  Command or Action Purpose
Step 1

switch# configure terminal

Enters global configuration mode.

Step 2

switch(config)# mac-address-table aging-time seconds

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.

Example

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

Configuring MAC Move Loop Detection

When the number of MAC address moves between two ports exceeds a threshold, it forms a loop. Starting from Cisco NX-OS release 6.0(2)A8(1), you can configure the action of bringing down the port with the lower interface index when such a loop is detected by using the mac address-table loop-detect port-down command. To revert to the default action of disabling MAC learning, use the no form of this command.

Procedure

  Command or Action Purpose
Step 1

switch# configure terminal

Enters global configuration mode.

Step 2

switch(config)# [no] mac address-table loop-detect port-down

Specifies the port-down action for MAC move loop detection. The no form of this command reverts to the default action of disabling MAC learning for 180 seconds.

Step 3

switch(config)# mac address-table loop-detect port-down edge-port

Enables the err-disabled detection for the edge-port on the MAC move loop detection.

Example

This example shows how to configure port-down as the action for MAC move loop detection. 

switch# configure terminal
switch(config)# mac address-table loop-detect port-down

This example shows how to enable the err-disabled detection for the edge-port on the MAC move loop detection. 

switch# configure terminal
switch(config)# mac address-table loop-detect port-down edge-port

Verifying the MAC Address Configuration


Note
On Cisco Nexus 3000 and Cisco Nexus 3548 Series platforms, the self router MAC or HSRP VMAC are dynamically learned by the switch under the following conditions:

  • When there is a transient loop in the network due to which the switch receives its own packets.

  • When there are spoofed packets where the source MAC is same as the Router MAC or HSRP MAC.

This behavior is different from other Cisco Nexus platforms. However, there is no operational impact due to these self MAC entries that are present in the MAC table. Any packet that is destined to the router MAC or HSRP MAC is routed. There is no Layer 2 lookup on these packets.

Use one of the following commands to verify the configuration:

Table 1. MAC Address Configuration Verification Commands

Command

Purpose

show mac address-table aging-time

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

show mac address-table

Displays the contents of the MAC address table.

Note 

IGMP snooping learned MAC addresses are not displayed.

show mac address-table loop-detect

Displays the currently configured action.

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

This example shows how to display the currently configured action: 

switch# configure terminal
switch(config)# show mac address-table loop-detect
Port Down Action Mac Loop Detect : enabled





switch# configure terminal
switch(config)# no mac address-table loop-detect port-down
switch(config)# show mac address-table loop-detect
Port Down Action Mac Loop Detect : disabled