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BFD Flapping Due to Duplex Mismatch

Publication Date:  2014-07-30 Views:  82 Downloads:  0
Issue Description
Fault Symptom:
Data/Voice Services effected of BSCs

Alarm Information: (BFD Fluctuation alarms on NE40EX3-B1 & NE40EX3-B2 both were appearing continously)

Jun 16 2014 10:16:10 NE40EX3-B2 %%01BFD/4/STACHG_TOUP(l)[0]:Slot=2;BFD session changed to Up. (SlotNumber=2, Discriminator=8200, FormerStatus=Init, Applications=AUTO, BindInterfaceName=Vlanif406, ProcessPST=False)

Jun 16 2014 10:16:07 NE40EX3-B2 %%01BFD/4/STACHG_TODWN(l)[1]:Slot=2;BFD session changed to Down. (SlotNumber=2, Discriminator=8200, Diagnostic=DetectDown, Applications=AUTO, ProcessPST=False, BindInterfaceName=Vlanif406, InterfacePhysicalState=Down, InterfaceProtocolState=Down)

Jun 16 2014 10:10:47 NE40EX3-B2 %%01BFD/4/STACHG_TOUP(l)[2]:Slot=2;BFD session changed to Up. (SlotNumber=2, Discriminator=9410, FormerStatus=Init, Applications=AUTO, BindInterfaceName=Vlanif413, ProcessPST=False)

Jun 16 2014 10:10:44 NE40EX3-B2 %%01BFD/4/STACHG_TODWN(l)[3]:Slot=2;BFD session changed to Down. (SlotNumber=2, Discriminator=9410, Diagnostic=DetectDown, Applications=AUTO, ProcessPST=False, BindInterfaceName=Vlanif413, InterfacePhysicalState=Down, InterfaceProtocolState=Down)

Version Information:


Patch Information:


Network Overview:

General connectivity is given below:

VRRP is configured between NE40-X3-B1 and NE40-X3-B2.

Few port were configured full duplex on both NEs and few were set to auto negotiation.

At BSC end both master and slave ports are configured to be full duplex.

Handling Process

As there were continuous BFD Alarms for on both NEs. IP Connectivity was fine. So this issue must be related to Physical Layer/Access Layer. 

  • Interface status was checked. (Command: display interface Gi 2/0/0)
    • Bandwidth utilization was normal
    • CRC were increasing.
  • CRC increasing reason was checked.
    • As interface was Rj45 so no optical power issue.
    • Duplex was checked and found Half (auto negotiation)
  • BSC team confirmed that duplex is Full at their end.

So initially it was found that there must be duplex mismatch issue. However when configurations were checked, on Gi 2/0/3 it was found that port was Full Duplex configured on B1 but still there were alarms. So following analysis was made:

There are three scenario of BFD Alarms on Vlanif on NE40.

1) Duplex mismatch on both NE40-X3-B1 and NE40-X3-B2 with BSC So alarms will appear for Vlan allowed on port (Gi 2/0/0) of both NEs.

2) Duplex mismatch on NE40-X3-B1 (VRRP Master) and BSC Master Card only. So alarm will appear for Vlan allowed on port (Gi 2/0/1) of NE40-X3-B1 only. Alarm will not appear for respective Vlan on NE40-X3-B2.

3) Third scenario is interesting. Here Duplex mismatch is between NE40-X3-B2 (VRRP Slave) and BSC Master Card only. In this case BFD alarm will appear for Vlan allowed on Port (Gi 2/0/3) on NE40-X3-B1 and NE40-X3-B2 both, however there is no duplex mismatch on NE40-X3-B1. Reason for alarm on B1 is that its learning MAC for BSC IP from B2 and BFD packets are going to BSC from B1 via B2. (Refer to Figure 4)

Root Cause
Root cause of issue was duplex mismatch on NE40-X3-B1 and NE40-X3-B2 with BSC. At BSC end every port was configured to be half. But at NE40 few ports were configured to be full duplex and few auto negotiation. These Auto negotiated port were negotiated at Half duplex.
Duplex settings should be configured Full on both sides. It must be used as best practice to hard code especially on Rj45 Ethernet connectivity with other Products. 
Proper plan should be made for each connectivity of NE ports which include Duplex Settings/IP/MTU/Broadcast settings.