Cisco Systems A9014CFD manual MAC Flap Control, Configuring MAC FLap Control

Chapter 7 Configuring Gigabit Ethernet Interfaces

MAC Flap Control

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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:

Cisco ASR 901 Series Aggregation Services Router Software Configuration Guide

OL-23826-09 7-5