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Verifying That MAC Limiting Is Working Correctly

MAC limiting protects against flooding of the Ethernet switching table by setting a limit on the number of MAC addresses that can be learned on a single Layer 2 access interface (port)..

Junos OS provides two methods for MAC limiting for port security:

  • Maximum number of MAC addresses—You configure the maximum number of dynamic MAC addresses allowed per interface. When the limit is exceeded, incoming packets with new MAC addresses can be ignored, dropped, or logged. You can also specify that the interface be shut down or temporarily disabled.

  • Allowed MAC addresses—You configure specific “allowed” MAC addresses for the access interface. Any MAC address that is not in the list of configured addresses is not learned, and the switch logs an appropriate message. The allowed MAC method binds MAC addresses to a VLAN so that the address is not registered outside the VLAN. If an allowed MAC setting conflicts with a dynamic MAC setting, the allowed MAC setting takes precedence.

Junos OS also allows you to set a MAC limit on VLANs. However, setting a MAC limit on VLANs is not considered a port security feature, because the switch does not prevent incoming packets that cause the MAC limit to be exceeded from being forwarded; it only logs the MAC addresses of these packets.

To verify MAC limiting configurations:

Verifying That MAC Limiting for Dynamic MAC Addresses Is Working Correctly

Purpose

Verify that MAC limiting for dynamic MAC addresses is working on the switch.

Action

Display the MAC addresses that have been learned. The following sample output shows the results when two packets were sent from hosts on ge-0/0/1 and five packets requests were sent from hosts on ge-0/0/2, with both interfaces set to a MAC limit of 4 with the default action drop:

Meaning

The sample output shows that with a MAC limit of 4 for each interface, the packet for a fifth MAC address on ge-0/0/2 was dropped because it exceeded the MAC limit. The address was not learned, and thus an asterisk (*) rather than an address appears in the MAC address column in the first line of the sample output.

Verifying That MAC Limiting for a Specific Interface Within a Specific VLAN Is Working Correctly

Purpose

Verify that MAC limiting for a specific interface based on its membership within a specific VLAN is working on the switch.

Action

Display the detailed statistics for MAC addresses that have been learned:

Meaning

The VLAN membership limit shows the number of packets that were dropped because of the VLAN membership MAC limit for interface ge-0/0/28.0 was exceeded. In this case, 20 packets were dropped.

Verifying That Allowed MAC Addresses Are Working Correctly

Purpose

Verify that allowed MAC addresses are working on the switch.

Action

Display the MAC address cache information after allowed MAC addresses have been configured on an interface. The following sample shows the MAC address cache after 5 allowed MAC addresses were on interface ge-0/0/2. In this instance, the interface was also set to a dynamic MAC limit of 4 with the default action drop.

Meaning

Because the MAC limit value for this interface was set to 4, only four of the five configured allowed addresses were learned and thus added to the MAC address cache. Because the fifth address was not learned, an asterisk (*) rather than an address appears in the MAC address column in the last line of the sample output.

Verifying Results of Various Action Settings When the MAC Limit Is Exceeded

Purpose

Verify the results provided by the various action settings for MAC limits—drop, log, shutdown and none—when the limits are exceeded.

Action

Display the results of the various action settings.

Note:

You can view log messages by using the show log messages command. You can also have the log messages displayed by configuring the monitor start messages with the monitor start messages command.

  • drop action—For MAC limiting configured with a drop action and with the MAC limit set to 5:

  • log action—For MAC limiting configured with a log action and with MAC limit set to 5:

  • shutdown action—For MAC limiting configured with a shutdown action and with MAC limit set to 3:

  • none action—If you set a MAC limit to apply to all interfaces on the switch, you can override that setting for a particular interface by specifying this action for that interface. See Override a MAC Limit Applied to All Interfaces.

Meaning

For the drop action results—The sixth MAC address exceeded the MAC limit. The request packet for that address was dropped. Only five MAC addresses have been learned on ge-0/0/2.

For the log action results—The sixth MAC address exceeded the MAC limit. No MAC addresses were blocked.

For the shutdown action results—The fourth MAC address exceeded the MAC limit. Only three MAC addresses have been learned on ge-0/0/2. The interface ge-0/0/1 is shut down.

For more information about interfaces that have been shut down, use the show ethernet-switching interfaces command.

Note:

You can configure the switch to recover automatically from this type of error condition by specifying the port-error-disable statement with a disable timeout value. The switch automatically restores the disabled interface to service when the disable timeout expires. The port-error-disable configuration does not apply to already existing error conditions. It impacts only error conditions that are detected after port-error-disable has been enabled and committed. To clear an already existing error condition and restore the interface to service, use the clear ethernet-switching port-error command.

Verifying That Interfaces Are Shut Down

Purpose

Verify that an interface is shut down when the MAC limit is exceeded.

Action

For more information about interfaces that have been shut down because the MAC limit was exceeded, use the show ethernet-switching interfaces command.

Note:

You can configure interfaces to recover automatically when the MAC limit has been exceeded by specifying the port-error-disable statement with a disable timeout value. The switch automatically restores the disabled interface to service when the disable timeout expires. The port-error-disable configuration does not apply to preexisting error conditions—it affects only error conditions that are detected after the port-error-disable statement has been enabled and the configuration has been committed. To clear a preexisting error condition and restore the interface to service, use the clear ethernet-switching port-error command.

Customizing the Ethernet Switching Table Display to View Information for a Specific Interface

Purpose

You can use the show ethernet-switching table command to view information about the MAC addresses learned on a specific interface.

Action

For example, to display the MAC addresses learned on ge-0/0/2 interface, type:

Meaning

The MAC limit value for ge-0/0/2 was set to 1, and the output shows that only one MAC address was learned and thus added to the MAC address cache. An asterisk (*) rather than an address appears in the MAC address column in the first line of the sample output.