The networking diagram is as follows:
A and B belong to an autonomous system enabled with ISIS and BGP. C and D belong to another autonomous system enabled with OSPF and BGP. EBGP runs between A and C. OSPF and IBGP run between C and D. EBGP runs between D and B. OSPF runs between E and CD. It is required that the 10.0.0.0/16 route be advertised in C so that the backhaul traffic comes back from A and C. However, the traffic comes back from not only A and C but also B and D.
1. Accessing A, the engineer found that the route was learned from C. Accessing B, the engineer found that the route was learned from D.
2. Access C and D to view the configuration.
3. View the configuration of C:
network 10.0.0.0 255.255.0.0
ip route 10.0.0.0 255.255.0.0 null 0
4. View the configuration of D:
BGP 65535 //No network 10.0.0.0/16 segment.
5. Access C and D to view the configuration. The problem arose because C advertised the route to D through IBGP and D, as the transit equipment, advertised the route to B.
The customer was sure that the problem did not arise until recently. Viewing the equipment log, the engineer found the undo ip route 10.0.0.0 255.255.0.0 command performed on D, which caused the problem.
BGP judges the precedence of routes before advertising them. When a static route exists, it is not advertised because its precedence is higher than that of the route learned from IBGP. When this black hole route is deleted, the route learned from IBGP is preferentially chosen. Because C does not import the static black hole route through OSPF, D cannot learn the 10.0.0.0/16 route sent from OSPF. BGP decides that the route learned from IBGP is the optimal. Thus, it advertises the route to B.
Solution: Configure a black hole route on D.
1. Route learning
Remember to analyze the impact of IGP upon BGP in terms of BGP route advertisement on the MAN.