Topology: Three NE80 routers A, B and C. A is mounted at B and the intermediate of A and B operates OSPF routing protocol. B connects C. The intermediate of B and C operates ISIS and OSPF. Introduce OSPF at B and C into ISIS. Introduce static route and direct route at A into OSPF.
Phenomenon: Part of attached network segment is problematic. Problematic network segment can only ping through A, but cannot B and C.
1. Check OSPF status of A and B and it is full. The link of A and B can be ironed out.
2. Check IP planning and there is no overlap.
3. Check route of problematic network segment at B. The next hop should point to A, but it points to C.
(1) At B the route of problematic network segment is in the way of ISIS.
(2) At B debug isis protocol and it is found that these routes is sent by C to B with ISIS.
(3) Carefully check negotiation packet of C, B tells C the problematic network segment through OSPF. C introduces the route to ISIS and then tells B through ISIS.
B learns the route of problematic network segment through A. Priority 150 (external route of OSPF), B learns the route from C, priority 25. The priority of C is higher. So the route at B points to C.
4. Cancel OSPF protocol between B and C and only operate ISIS protocol. B will not tell C the route through OSPF and C will not introduce it to ISIS or even distributes to B. Then the problem is solved.
Possible reasons are as follows:
1. The intermediate of A and B operates OSPF aabnormally. The route of A cannot transmit to B through OSPF.
2. IP address planning of problematic network segment overlaps.
3. Routing protocol planning is false.
The reason is that B and C operate two routing protocols and are configured with introduction. Generally, it is suggested to configure one routing protocol at the same interface of two routers.