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Verifying That MAC Limiting Is Working Correctly

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

JUNOS Software provides two MAC limiting methods:

  • Maximum number of dynamic MAC addresses allowed per interface—When the limit is exceeded, incoming packets with new MAC addresses are dropped.
  • Specific “allowed” MAC addresses for the access interface—Any MAC address that is not in the list of configured addresses is not learned.

To verify MAC limiting configurations:

  1. Verifying That MAC Limiting for Dynamic MAC Addresses Is Working Correctly
  2. Verifying That Allowed MAC Addresses Are Working Correctly
  3. Verifying Results of Various Action Settings When the MAC Limit Is Exceeded
  4. Customizing the Ethernet Switching Table Display to View Information for a Specific Interface

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 action drop: 


Ethernet-switching table:  7 entries, 6 learned

  VLAN                 MAC address         Type         Age    Interfaces

  employee-vlan        *                   Flood          -    ge-0/0/2.0
  employee-vlan        00:05:85:3A:82:77   Learn          0    ge-0/0/1.0
  employee-vlan        00:05:85:3A:82:79   Learn          0    ge-0/0/1.0
  employee-vlan        00:05:85:3A:82:80   Learn          0    ge-0/0/2.0
  employee-vlan        00:05:85:3A:82:81   Learn          0    ge-0/0/2.0
  employee-vlan        00:05:85:3A:82:83   Learn          0    ge-0/0/2.0
  employee-vlan        00:05:85:3A:82:85   Learn          0    ge-0/0/2.0

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 Allowed MAC Addresses Are Working Correctly

Purpose

Verify that allowed MAC addresses are working on the switch.

Action

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


Ethernet-switching table:  5 entries, 4 learned

  VLAN                 MAC address         Type         Age    Interfaces

  employee-vlan        00:05:85:3A:82:80   Learn          0    ge-0/0/2.0
  employee-vlan        00:05:85:3A:82:81   Learn          0    ge-0/0/2.0
  employee-vlan        00:05:85:3A:82:83   Learn          0    ge-0/0/2.0
  employee-vlan        00:05:85:3A:82:85   Learn          0    ge-0/0/2.0
  employee-vlan        *                   Flood          -    ge-0/0/2.0

Meaning

Because the MAC limit value for this interface had been set to 4, only four of the five configured allowed addresses were learned and thus added to the MAC 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, none, and shutdown—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:

    Ethernet-switching table: 6 entries, 5 learned

      VLAN                 MAC address         Type         Age    Interfaces

  employee—vlan        *                   Flood          -    ge-0/0/2.0
  employee—vlan        00:05:85:3A:82:80   Learn          0    ge-0/0/2.0
  employee—vlan        00:05:85:3A:82:81   Learn          0    ge-0/0/2.0
  employee—vlan        00:05:85:3A:82:83   Learn          0    ge-0/0/2.0
  employee—vlan        00:05:85:3A:82:85   Learn          0    ge-0/0/2.0
  employee—vlan        00:05:85:3A:82:88   Learn          0    ge-0/0/2.0
  • log action—For MAC limiting configured with a log action and with MAC limit set to 5:

    user@switch> show ethernet-switching table 
    Ethernet-switching table: 74 entries, 73 learned

      VLAN                 MAC address         Type         Age    Interfaces

  employee—vlan        *                   Flood          -    ge-0/0/2.0
  employee—vlan        00:05:85:3A:82:80   Learn          0    ge-0/0/2.0
  employee—vlan        00:05:85:3A:82:81   Learn          0    ge-0/0/2.0
  employee—vlan        00:05:85:3A:82:82   Learn          0    ge-0/0/2.0
  employee—vlan        00:05:85:3A:82:83   Learn          0    ge-0/0/2.0
  employee—vlan        00:05:85:3A:82:84   Learn          0    ge-0/0/2.0
  employee—vlan        00:05:85:3A:82:85   Learn          0    ge-0/0/2.0
  employee—vlan        00:05:85:3A:82:87   Learn          0    ge-0/0/2.0
  employee—vlan        00:05:85:3A:82:88   Learn          0    ge-0/0/2.0
  . . .
  • shutdown action—For MAC limiting configured with a shutdown action and with MAC limit set to 3:

    user@switch> show ethernet-switching table 
    Ethernet-switching table: 4 entries, 3 learned

      VLAN                 MAC address         Type         Age    Interfaces

  employee—vlan        *                   Flood          -    ge-0/0/2.0
  employee—vlan        00:05:85:3A:82:82   Learn          0    ge-0/0/2.0
  employee—vlan        00:05:85:3A:82:84   Learn          0    ge-0/0/2.0
  employee—vlan        00:05:85:3A:82:87   Learn          0    ge-0/0/2.0
  • 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 Setting the none Action on an Interface to Override a MAC Limit Applied to All Interfaces (CLI Procedure).

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.


user@switch> show ethernet-switching interfaces

Interface    State  VLAN members        Tag   Tagging  Blocking 

 

bme0.32770   down   mgmt                      untagged unblocked

ge-1/0/0.0   down   v1                        untagged MAC limit exceeded

ge-1/0/1.0   up     v1                        untagged unblocked

ge-1/0/2.0   up     v1                        untagged unblocked

me0.0        up     mgmt                      untagged unblocked

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 pre-existing error conditions. It impacts only error conditions that are detected after port-error-disable has been enabled and committed. To clear a pre-existing 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 for a specific interface.

Action

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


user@switch> show ethernet-switching table interface ge-0/0/2.0 
Ethernet-switching table: 1 unicast entries

  VLAN              MAC address       Type         Age Interfaces

  v1                *                 Flood          - All-members

  v1                00:00:06:00:00:00 Learn          0 ge-2/0/0.0

Meaning

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

Published: 2009-07-27

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