Topology: Three NE40 routers including A-B-C run OSPF and work in Area 0. Import direct route and static route at A into OSPF, of which the static route contains the X network segment. At B, the network segment X is configured with OSPF route filtration policy which requires that the route from X does not occur to both B and C routers.
Symptom: After configuration, the routing table of C router does contain the route from the network segment X.
1. According to checkup fro configurations, OSPF and OSPF route filtration of three routers are configured correctly, so the problem is not caused by cofigurations.
2. Ports of three routers are all up, and OSPF is full, so the problem is not caused by false running of OSPF.
3. After analysis on packets at Router B, it comes to the following conclusion:
(1) The OSPF route filtration policy configured on router is only valid for routes.
(2) At the network segment X, however, during the transmission of OSPF through router B to C, the route is transmitted as LSA, not route. So, the OSPF policy configured at B cannot prevent the routes from X to C.
4. Configure the OSPF routing polciy on C for network segment X, and the policy takes effect during the process of transformation of LSA to routing table. Check the routing table of C router, and there is no route from X, so the problem is solved.
1. False configurations.
2. False state of OSPF.
3. Unreasonable network planning.
OSPF route filtration policy is only valid for routes, not for LSA. However, during the transmission of OSPF routes, routes exist as LSA, so it should be noted in configuration and planning of OSPF route filtration policy.