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BFD Session Flapping Occurs Because a non-Huawei Device Cannot Transparently Transmit the BFD Packets Tagged with a VLAN

Publication Date:  2013-01-08 Views:  6 Downloads:  0
Issue Description

BFD, VLAN Transparently transmit

Handling Process

A carrier from M country configures BFD on NE40Es with a non-Huawei device functioning as a transmission device in the middle.

Networking: NE40E (PE)—transmission device—NE40E (PE)

Configure a VLANIF interface between the NE40Es and configure a BFD session to be bound to the VLANIF interface in unicast mode. After the BFD session is set up between the two NE40Es, a large number of BFD session flapping events are recorded in the BFD log with short intervals. The link fails to work properly. The log information is shown as follows.

Apr 28 2012 00:35:49 %%01BFD/4/STACHG_TODWN(l)[0]:Slot=1;BFD session changed to Down. (SlotNumber=1, Discriminator=15680, Diagnostic=DetectDown, Applications=ISISL1, ProcessPST=False, BindInterfaceName=Vlanif2, InterfacePhysicalState=Down, InterfaceProtocolState=Down)

Apr 28 2012 00:35:49 %%01BFD/4/STACHG_TOUP(l)[1]:Slot=1;BFD session changed to Up. (SlotNumber=1, Discriminator=15680, FormerStatus=Init, Applications=ISISL1, BindInterfaceName=Vlanif2, ProcessPST=False)

Apr 28 2012 00:35:49 %%01BFD/4/STACHG_TODWN(l)[2]:Slot=1;BFD session changed to Down. (SlotNumber=1, Discriminator=15680, Diagnostic=DetectDown, Applications=ISISL1, ProcessPST=False, BindInterfaceName=Vlanif2, InterfacePhysicalState=Down, InterfaceProtocolState=Down)

Apr 28 2012 00:35:49 %%01BFD/4/STACHG_TOUP(l)[3]:Slot=1;BFD session changed to Up. (SlotNumber=1, Discriminator=15680, FormerStatus=Init, Applications=ISISL1, BindInterfaceName=Vlanif2, ProcessPST=False)

Apr 28 2012 00:35:46 %%01BFD/4/STACHG_TODWN(l)[4]:Slot=1;BFD session changed to Down. (SlotNumber=1, Discriminator=15679, Diagnostic=DetectDown, Applications=ISISL1, ProcessPST=False, BindInterfaceName=Vlanif2, InterfacePhysicalState=Down, InterfaceProtocolState=Down)

Apr 28 2012 00:35:46 %%01BFD/4/STACHG_TOUP(l)[5]:Slot=1;BFD session changed to Up. (SlotNumber=1, Discriminator=15679, FormerStatus=Down, Applications=ISISL1, BindInterfaceName=Vlanif2, ProcessPST=False)

Root Cause

1.         The on-site Huawei technical support personnel check the BFD configurations on the NE40Es and find no fault.

2.         Then they check the logs and diagnostic records on the NE40Es and find that the BFD session flaps frequently and the BFD session flapping lasts for a long time. DetectDown is displayed in the Diagnostic field in the logs on both NE40Es. The NE40Es do not receive BFD packets from each other. Additionally, the BFD session Up events are also recorded in both logs, which indicates that the BFD session status negation works properly.

3.         BFD packets are sent at a short interval, and the statistics about the sent and received BFD packets increase remarkably. After clearing the interface statistics on the NE40E, the on-site Huawei technical support personnel test the interface on the NE40E. They find a remarkable difference in the number of received packets and sent packets on the interface. The number of sent BFD packets far exceeds the number of received BFD packets.

4.         Then they check the interface on the peer NE40E and find the same problem. Based on the test results, they decide that BFD packets are discarded on the link between the NE40Es.

5.         The on-site Huawei technical support personnel obtain the packet header from the BFD packets. Based on their analysis, the BFD packets exchanged during the BFD session status negotiation are not tagged with a VLAN ID. However, the BFD packets exchanged during the BFD detection are tagged with a VLAN ID.

6.         Then they consult the customer on whether the transit device can transparently transmit the BFD packets tagged with a VLAN ID. Finally, they conclude that the transmission device discards the BFD packets tagged with the VLAN ID.

Solution

When a VLANIF interface and BFD are configured on NE40Es, the BFD packets exchanged during the BFD session status negotiation are not tagged with a VLAN ID, but the BFD packets exchanged during the BFD detection are tagged with a VLAN ID. The transit device cannot transparently transmit the BFD packets tagged with the VLAN ID, which causes the BFD session to flap frequently.

Suggestions
1. Modify the transmission device configuration and enable the transmission device to transparently transmit BFD packets tagged with a VLAN ID. 2. Configure the BFD session to be bound to the VLANIF interface in multicast mode and associate the BFD session with PIS. An example configuration is shown as follows. [RouterA] bfd atob bind peer-ip default-ip interface GE x/x/x ---Indicating single-hop multicast BFD [RouterA-bfd-session-atob] discriminator local 10 [RouterA-bfd-session-atob] discriminator remote 20 [RouterA-bfd-session-atob] min-tx-interval 10 [RouterA-bfd-session-atob] min-rx-interval 10 [RouterA-bfd-session-atob]process-interface-status ---Indicating the association between the BFD session with PIS [RouterA-bfd-session-atob] commit [RouterA-bfd-session-atob] quit

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