Hey there!
This is a case about the fault caused by IS-IS LDP synchronization.
Problem Description
As shown in the following figure, IS-IS and MPLS are enabled between the Cisco PE and Huawei PE. After site B is migrated from the Cisco PE to the Huawei PE, there is no traffic from site A to site B.
Before Migration
After Migration
Handling Process
1) Check the ISIS neighbor status. It is normal and the Cisco PE can be pinged from the Huawei PE.
2) Check the VPN routing table on Huawei PE. No problem is found.
3) The BGP routing table is also normal.
4) The ISIS cost is infinite (65535). Check the ISIS configuration, no problem is found.
Root Cause
When MPLS LDP is configured, the customer does not add a password to the Huawei PE. Therefore, the MPLS LDP peer relationship between Huawei and Cisco devices fails to be established.
The Cisco configuration:
mpls label protocol ldp
mpls ldp neighbor XXXX password 7 XXXX
And the synchronization between LDP and IS-IS is enabled
isis 1
isis ldp-syn
As LDP is down so it increases the ISIS cost as infinity (65535) and no traffic is transmitted on the link.
Solution
Check the password and put it on MPLS LDP peer or disable the ISIS ldp-sync.
Suggestions and Summary
Before interconnection and migration, check MPLS sessions on the peer device. Check and compare the data after the migration.
The number of LDP sessions must be the same before and after the operation.
Check the MPLS LDP password and remote session configuration.
This is what I want to share with you today, thank you!
