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The BFD Session Status Fails to Go Up to Monitor a PW Because of the Inconsistency in TTL Configurations

Publication Date:  2013-01-08 Views:  44 Downloads:  0
Issue Description

As shown in the following networking diagram, CE1 is connected to U-PE1, and CE2 is connected to U-PE2 through the PPP. U-PE1 and U-PE2 are connected on the MPLS backbone network. A dynamic multi-segment PW is configured between U-PE1 and U-PE2 through an LSP, with S-PE functioning as the transit node. A BFD session is set up between U-PE1 and U-PE2. The TTL value on U-PE1 is set to 1. The TTL value on U-PE2 is determined by automatic calculation. After the configurations are complete, the BFD session status fails to go Up.

 

 

Handling Process

The on-site Huawei technical support personnel check the configurations on U-PE1 and U-PE2 using the display mpls l2vc interface interface-type interface-number command. The command output shows that the values of session state, AC status, and VC state are all Up. Therefore, the problem is not caused by a PW failure. The command output on U-PE1 is as follows.

<U-PE1> display mpls l2vc

total LDP VC : 1  1 up  0 down

*client interface  : LoopBack0

 Administrator PW  : yes

 session state  : up

 AC status   : up

 VC state    : up

 Label state   : 0

 Token state   : 0

 VC ID    : 100

 VC type    : IP-interworking

 destination   : 3.3.3.9

 local VC label  : 1034   remote VC label  : 1031

 control word   : disable

 forwarding entry  : exist

 local group ID  : 0

 manual fault   : not set

 active state   : active

 link state   : up

 local VC MTU   : 1500   remote VC MTU  : 1500

 tunnel policy name : --

 traffic behavior name: --

 PW template name  : to3

 primary or secondary : primary

 create time   : 0 days, 4 hours, 51 minutes, 2 seconds

 up time    : 0 days, 4 hours, 49 minutes, 57 seconds

 last change time  : 0 days, 4 hours, 49 minutes, 57 seconds

 VC last up time  : 2010/12/07 11:41:02

 VC total up time  : 0 days, 4 hours, 49 minutes, 57 seconds

 CKey     : 23

 NKey     : 22

 Diffserv Mode  : uniform

 Service Class  : --

 Color    : --

 DomainId    : --

 Domain Name   : --

Then they check the TTL configurations on U-PE1 and U-PE2 using the display current-configuration configuration bfd-lsp-session command. The command outputs show that the PW TTL value on U-PE1 is 1 but the PW TTL value on U-PE2 is 2. The TTL configurations on U-PE1 and U-PE2 are inconsistent. The number of PW segments must be modified.

<U-PE1> display current-configuration configuration bfd-lsp-session

#

bfd pe1tope2 bind pw interface LoopBack0 remote-peer 5.5.5.9 pw-ttl 1

e

 discriminator local 1

 discriminator remote 2

 commit

<U-PE2> display current-configuration configuration bfd-lsp-session

#

bfd pe2tope1 bind pw interface LoopBack0 remote-peer 1.1.1.9 pw-ttl 2

e

 discriminator local 2

 discriminator remote 1

 commit

Root Cause

When BFD is used to monitor a PW, BFD monitors various parameters of a PW, including the TTL value. Because the TTL value on U-PE1 is manually set to 1 but the TTL value on U-PE2 is determined by automatic calculation, the TTL values on U-PE1 and U-PE2 are inconsistent, causing the BFD session status to fail to go Up.

Solution

Modify the TTL value on U-PE1 to make the TTL values on U-PE1 and U-PE2 consistent. Perform the following steps:

1.         Run the system-view command on U-PE1 to enter the system view.

2.         Run the undo bfd cfg-name command to delete the BFD session.

3.         Run the bfd cfg-name bind pw interface interface-type interface-number remote-peer remote-peer-address pw-ttl auto-calculate command to set up a new BFD session.

4.         Run the commit command to commit the configuration.

END