The topology can be referred to the attachment.
traceroute S6506 address on 12XXX of C corporation and choose secondary optimal route of NE40-R1 and NE40-R2.
traceroute result is as follows:
1 220.127.116.11 4 msec 8 msec－－POS1/0/1
18.104.22.168 8 msec－－－－－POS1/0/0
2 22.214.171.124 8 msec－－－－to NE40-r2 (secondary route)
126.96.36.199 4 msec－－－－－----to S8016
188.8.131.52 8 msec
3 184.108.40.206 8 msec－－－－－----to S8016
220.127.116.11 12 msec 8 msec
4 18.104.22.168 8 msec 12 msec *
1. Check FIB of NE40-R1 and it points to 22.214.171.124 (S8016 interface address).
2. Observe routing table and it is stable. Check the route to S6506 on NE40-R1 and there is no multi-route.
3. Check MPLS label and it does not distribute label for corresponding address. Analyze routing table. NE40-R2 and NE40-R1 establish EBGP neighbor with 12XXX. They will distribute 126.96.36.199 route to GSR through BGP. From routing information on 12XXX, there are two BGP routes to destination address 188.8.131.52 and the next-hop of BGP route is 184.108.40.206 and 220.127.116.11, namely, loopback0 address of NE40-R2 and NE40-R1. Check the rotue from 12XXX to these two loopback addresses: there are three equal cost routes to loopback address. The routes to NE40-R2 are 12XXX direct route and two ones from two links of NE40-R1. The routes to NE40-R1 are two routes from NE40-R2 and two GSR direct routes. C corporation the front * of routing table means present valid route. Analyze early traceroute result. Firstly, the first packet passes 18.104.22.168 and it regards that the next-hop to BGP is 22.214.171.124. The packet will pass NE40-R1 and go to NE40-R2. Secondly, the first packet goes to 126.96.36.199; the interface of NE40-R1 and NE40-R2. Thirdly, it reaches 188.8.131.52; the interface of NE40-R2 and S8016. Fourthly, it reaches 184.108.40.206; the interface of S8016 and 6500. However, the second packet passes 220.127.116.11 and it regards that the next-hop to BGP is 18.104.22.168. The packet directly passes NE40-R1. Secondly, the first packet goes to 22.214.171.124; the interface of NE40-R1 and S8016. Thirdly, it reaches 126.96.36.199; the interface of S8016 and 6500. The third packet and the first packet pass the same path.
The problem is solved.
1. The next-hop of FIB is false.
2. There is load balance and route flapping.
3. Route learning is false or it passes label forwarding through MPLS.
Notice the next-hop and route iteration of EBGP. For this case, 12xxx creates two iteration and there is one route iteration on NE40-R1. The destination address of the first iteration on 12XXX is S6506 188.8.131.52 and iterates two next-hop (loopback addresses of two NE40s). Ans then it iterates loopback address of NE40 (three cables of 12XXX, NE40-R1 and NE40-R2).
NE40-R1 iterates next-hop to 184.108.40.206 the next-hop of NE40-R2 loopback address.