BSC is at Hubli location and SGSN is Banglore location. There is direct lease line between these two locations. For redundancy purposes, a third location Manglore is connect to Hubli and Banglore. So the primary path for BSC traffic was Hubli to Banglore and the redundant path was Hubli-Manglore-Banglore. The issue was that whenever primary path goes down the Gb link between BSC and SGSN goes down. For details please check attached diagram.
The router used was NE40E.
Gb link shows down at BSC side.
1. Issue detected during redundancy checking.
2. Traceroute shows request droping at PE2 router.
3. Routing was checked at PE2.
4. Routing changed at PE2 router.
The root cause was that whenever seconday link (Hubli-Manglore-Banglore) is active the requests was coming to PE2 routers. The routing on PE2 router for logical IP of BSC was routed towards non active IP(IP of slot15 board) of BSC. so the requests from SGSN were droped at non active card.
Suggestions: The logical IP of BSC should always be routed to active IP of PCU card in both PE routers.