In which situation does MAC address flapping occur on CE switches

For S series switches (except the S1700), MAC address flapping occurs when a MAC address is learned by two interfaces in the same VLAN and the MAC address entry learned later overrides the earlier one. For example, in the MAC address entry with the MAC address 0011-0022-0034 and VLAN 2, the outbound interface is changed from Port1 to Port2. In this case, MAC address flapping occurs. MAC address flapping can cause an increase in the CPU usage on the switch. Generally, MAC address flapping occurs when a loop exists on the network. S series switches support MAC address flapping detection and take configured actions after detecting MAC address flapping.

The maximum number of MAC address flapping times that CE series switches can detect is 65535. When the number of flapping times is greater than 65535, the MoveNum field is still 65535 in the display mac-address flapping command output. In most cases, there is a low probability that MAC address flapping occurs many times. If the number reaches the threshold, you can run the reset mac-address flapping record command to clear the detection record on CE series switches.

In versions earlier than V100R005C10: A CE switch reports an alarm after detecting MAC address flapping for three times. The number of times cannot be configured. In V100R005C10 and later: By default, a CE switch reports an alarm after detecting MAC address flapping for 10 times. The number of times can be configured using the mac-address flapping detection [ security-level { low | middle | high } ] command.

The default aging time of flapped MAC addresses is 300 seconds. You can run the error-down auto-recovery cause mac-address-flapping interval interval-value command to configure it. After the command is executed, if MAC address flapping stops and the interface becomes Up after the delay, the CE switch no longer detects MAC address flapping during the remaining aging time. This is because the aging time does not expire. If detection on MAC address flapping is required, it is recommended that the delay for an interface to automatically become Up be set greater than the aging time of flapped MAC addresses.