Relationship Between MAC Address Flapping and Loops

Publication Date:  2015-03-24 Views:  460 Downloads:  0
Issue Description
Applicable Products and Versions:All products and versions.

Relationship Between MAC Address Flapping and Loops.
Alarm Information
A switch sends an alarm indicating MAC address flapping. Efforts are then made to check for loops, but the interface where the loop occurs fails to be located. The MAC address flapping alarm cannot be rectified.

The following provides the alarm information:

Alarm information differs for fixed and modular switches of different versions. The following alarm information is only used as an example.

Nov 21 2013 19:29:33 Quidway L2IFPPI/4/MAC_FLAPPING_ALARM:OID mac-address has flap value.  (L2IfPort=0,entPhysicalIndex=0,  BaseTrapSeverity=4, BaseTrapProbableCause=549, BaseTrapEventType=1,  MacAdd=5654-4c83-05c0,vlanid=712,  FormerIfDescName=GigabitEthernet1/1/16,CurrentIfDescName=GigabitEthernet1/1/12,DeviceName=Quidway)
Handling Process
The preceding alarm information shows that the switch can learn the same MAC address from multiple interfaces. In this case, a loop exists, or multiple Layer 2 devices or terminals share the same MAC address.

If there is a loop on the network, the alarm usually involves many MAC addresses. In addition, traffic is heavy on some interfaces, and there are a large number of broadcast packets. If you disable one interface where the alarm is generated, the alarm is cleared. MAC address flapping occurs regardless of the service traffic volume.

If multiple terminals share the same MAC address, the alarm usually involves only one MAC address or a small number of MAC addresses, and the statistics show that the number of received and sent packets is within a normal range. Change the MAC address learning priority for an interface. If traffic of users connected to this interface becomes abnormal, multiple user terminals are using the same MAC address. In this case, change the MAC addresses of the user terminals. If user traffic remains normal, some Layer 2 devices are using the same MAC address. In this case, check the configuration of the Layer 2 devices and change their MAC addresses.

Root Cause
A loop exists on the network.
There are multiple terminals with the same MAC address.
When MAC address flapping occurs, troubleshoot the fault based on the symptoms. MAC address flapping does not necessarily result from loops, but loops will definitely result in MAC address flapping.