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.
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 : 188.8.131.52
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 184.108.40.206 pw-ttl 1
discriminator local 1
discriminator remote 2
<U-PE2> display current-configuration configuration bfd-lsp-session
bfd pe2tope1 bind pw interface LoopBack0 remote-peer 220.127.116.11 pw-ttl 2
discriminator local 2
discriminator remote 1
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.
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.