522 | Layer 2
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To set a MAC learning limit on an interface:
Three options are available with the mac learning-limit command: dynamic, no-station-move, and
station-move.
mac learning-limit dynamic
The MAC address table is stored on the Layer 2 FIB region of the CAM (and the Layer 2 ACL region on
the E-Series). On the C-Series and S-Series the Layer 2 FIB region allocates space for static MAC address
entries and dynamic MAC address entries (all MAC address entries on the E-Series are dynamic). When
MAC Learning Limit is enabled, entries created on this port are static by default. When you configure the
dynamic option, learned MAC addresses are stored in the dynamic region and are subject to aging. Entries
created before this option is set are not affected.
FTOS Behavior: When configuring MAC Learning Limit on a port or VLAN the configuration is
accepted (becomes part of running-config and show mac learning-limit interface) before the system
verifies that sufficient CAM space exists. If the CAM check fails, the a message is displayed:
%E90MH:5 %ACL_AGENT-2-ACL_AGENT_LIST_ERROR: Unable to apply access-list Mac-Limit on
GigabitEthernet 5/84
In this case, the configuration is still present in the running-config and show output. Remove the configuration
before re-applying a MAC learning limit with lower value. Also, ensure that Syslog messages can be viewed on
your session.
Note: The CAM-check failure message beginning in FTOS version 8.3.1.0 is different from versions
8.2.1.1 and earlier, which read:
% Error: ACL returned error
% Error: Remove existing limit configuration if it was configured before
Note: ARP will be resolved the first time after the MAC learning limit is reached in an interface with a
violation log enabled.
Task Command Syntax Command Mode
Specify the number of MAC addresses that the system
can learn off a Layer 2 interface.
mac learning-limit address_limit INTERFACE
Note: An SNMP trap is available for mac learning-limit station-move. No other SNMP traps are available
for MAC Learning Limit, including limit violations.
FTOS Behavior: If you do not configure the dynamic option, the C-Series and S-Series do not detect
station moves in which a MAC address learned off of a MAC-limited port is learned on another port on
same line card. Therefore, FTOS does not take any configured station-move violation action. When a
MAC address is relearned on any other linecard (any line card except the one to which the original
MAC-limited port belongs), the station-move is detected, and the system takes the configured the
violation action.
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