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Cisco ASR 901 Series Aggregation Services Router Software Configuration Guide
OL-23826-09
Chapter 7 Configuring Gigabit Ethernet Interfaces
MAC Flap Control
Gi0/0 9216
Gi0/1 6000
Gi0/2 3000
Gi0/3 9216
Gi0/4 9216
Gi0/5 9216
Gi0/6 9216
Gi0/7 9216
Gi0/8 9216
Gi0/9 9216
Gi0/10 9216
Gi0/11 9216
MAC Flap Control
A MAC flap occurs when a switch receives packets from two different interfaces, with the same source
MAC address. This happens when wrong configurations such as loops are introduced in networks. MAC
flapping can cause CPU hogs and software induced crashes, if preventive action is not taken.
The two main aspects of MAC flap control feature are:
• Identification of MAC Flapping—Identified when MAC movement counter threshold is hit at
specified time intervals.
• Preventive Action—Err-Disabling is done in one of the ports that has MAC flapping.
This feature is disabled by default and can be enabled or disabled through the CLI. You can
configure the maximum number of MAC movements that are allowed in a specified time interval,
beyond which the MAC movement is termed as flapping.
Once the port is err-disabled, it can be administratively brought up using the shut and no shut
commands.
Restrictions and Limitations
• If MAC learning is done in tens of thousands, the CPU may slow down. This feature does not address
the slow down or CPU hog due to MAC learning.
• When the router is learning tens of thousands of MACs, and there are a couple of genuine MAC
movements (not due to a loop), they are not tagged as MAC flapping since these are valid MAC
movements.
• Average MAC Movement issue
For example, let us assume that MAC movement counter is configured for a maximum of 5 MAC
movements in 10 seconds.
If 2000 MACs have contributed for 4 MAC movements each in 10 seconds, the total number of AC
movements will be 8000. Since the individual MAC threshold is not hit in this case, the router does
not take any preventive action. However, this condition may not really occur in practice.
Configuring MAC FLap Control
Complete the following steps to configure MAC Flap control: