Fault Symptom
The protocols of IS-IS and BGP and MPLS VPN services are configured on six PE devices. All the PEs use the same RD; PE1 and PE2 serve as RRs. The router ID of PE1 is smaller than that of PE2; the router ID of PE3 is smaller than that of PE4; the router ID of PE5 is smaller than that of PE6. The two CEs belong to the same VPN.
After PE3 is restarted, PE5 and PE6 have to wait for 2 minutes before learning the route to the segment where CE1 resides.
Fault Analysis
PE5 and PE6 learn two equal-cost MPLS VPN routes to CE1 through PE3 and PE4. IGP selects the route passing through PE3 as the optimal route to CE1 because the router ID of PE3 is smaller than that of PE4.
After PE3 is restarted, the two RRs (PE1 and PE2) forward another route to the segment where CE1 resides to PE5 and PE6 when confirming that they cannot establish BGP neighbor relationships with PE3. After IGP convergence is complete, MPLS VPN convergence starts. MPLS VPN convergence is rather slow because its time depends on the number of routing entries in the routing tables of the routers. Usually, MPLS VPN convergence takes about 2 minutes.
Procedure
1. Run the system-view command on PE3 to enter the system view.
2. Run the ip vpn-instance vpn-access command to enter the view of the corresponding VPN instance.
3. Run the route-distinguisher 22:1 command to change the RD for the VPN instance on PE3 to be different from that on the other PEs.
After the configuration, PE5 and PE6 learn two MPLS VPN routes with different next hops to CE1 through PE3 and PE4. One of the routes is active, and the other is inactive.
When PE3 is restarted again, the active route is removed from the MPLS VPN routing table. In this case, the inactive route quickly switches to be an active route without waiting for the finding of a reachable IGP route. Therefore, the convergence speed is rather quick. The fault is rectified.
Summary
MPLS VPN convergence may become slow due to slow IGP convergence. In this case, you can set different RDs on PEs to configure two MPLS VPN routes for backup or alternatively configure VPN FRR to rectify the fault.
