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The BFD for PW Session Cannot Be Up Due to the Inconsistent TTL Configuration at the Two Ends of the Device

Publication Date:  2013-09-30 Views:  58 Downloads:  0
Issue Description

Networking: CE-A and CE-B respectively access U-PE1 and U-PE2 through the PPP. U-PE1 and U-PE2 are connected over the MPLS backbone network. A dynamic multi-hop PW is set up between U-PE1 and U-PE2 through an LSP, with S-PE functioning as the transit node. The BFD session is configured on the U-PE1 and the configured TTL value is 1. The BFD session is configured on the U-PE2 and the TTL value is automatically calculated. After the configuration, the BFD session cannot be Up.

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Handling Process

Run the command display mpls l2vc interface interface-type interface-number on U-PE1 and U-PE2. It is found that the values of "session state", "AC state", and "VC state" are all Up. It indicates that the problem is not caused by a PW failure.

<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          : --

Run the command display bfd session all verbose on U-PE1. It is found that the value of "PW TTL" is 1. Run the command display bfd session all verbose on U-PE2. It is found that the value of "PW TTL" is 2. The preceding information indicates that the TTL configured at the two ends are different and it must be changed to the hop number of the PW.

<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
The BFD for PW detects parameters such as the TTL hop number of the PW. Because the TTL at one end is set to 1 and the TTL at the other end is calculated automatically, the PW-TTL at the two ends are different and the BDF cannot be Up.
Solution

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

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

4. Run the commit command to commit the configuration.

5. After the PW TTL value is changed, the BFD session is Up and the problem is resolved.
Suggestions
When configuring the BFD for PW session, if you specified the TTL value manually, ensure that the value of TTL is the number of S-PE nodes added by 1. If you are not clear about this principle, you can use the auto-calculate mode to determine the value of the TTL. Note that remote-peer remote-peer-address is the address of the peer U-PE.

END