Layer 2 Packet Loss Caused by Loops

Publication Date:  2015-03-24 Views:  214 Downloads:  0
Issue Description
Applicable Products and Versions:All products and versions.

    As shown in Figure 4-3, a switch is connected to an enterprise network through a leased line. The switch functions as a Layer 2 aggregation switch, and an NE80 functions as the gateway.

Figure 4-3 Network where layer 2 packet loss occurs

Layer 2 Packet Loss Caused by Loops.
Alarm Information
Enterprise network users complain that the network has a slow response to their service requests. When the NE80 pings a terminal on the enterprise network, packet loss occurs.

Handling Process
1.  Enable MAC address flapping detection on the switch and check alarms.

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

#Jul 28 09:59:34 2012 Switch L2IF/4/mac_flapping_alarm:OID mac-address has flap value . (BaseTrapSeverity=0, BaseTrapProbableCause=0, BaseTrapEventType=4, L2IfPort=549,entPhysicalIndex=1, MacAdd=0025-9e03-02f1,vlanid=107, FormerIfDescName=GigabitEthernet12/0/0,CurrentIfDescName=GigabitEthernet10/0/6,DeviceName= Switch)
The preceding alarm information indicates that MAC address flapping occurs.

2.  Set the NE80 MAC address to a static MAC address on GE12/0/0.

The loop on the downstream network of GE10/0/6 is eliminated.

Root Cause
A loop exists on the downstream network of GE10/0/6. As a result, the MAC address of the NE80 flaps between GE10/0/6 and GE12/0/0 of the switch. When GE10/0/6 learns the MAC address of the NE80, user packets cannot be forwarded to the gateway.

To locate Layer 2 packet loss and intermittent disconnection problems, first check whether MAC address flapping occurs in addition to checking basic factors such as network cables, optical power of optical modules, and interface status. Then configure a static MAC address and check whether the problems are resolved. Configuring a static MAC address only prevents loops. To eliminate loops, configure a loop prevention protocol.