We have the following scenario:
IOU 4 has a static route 10.1.9.9 to IOU 9. It redistributes it into OSPF. IOU4 has OSPF adj to IOU5, IOU5 to IOU2.
IOU2 is a BGP neigh to IOU5 and also to IOU4. IOU4 and IOU5 are route reflector clients of IOU2.
The problem starts when we enable BGP synchronization on R5. What do we know about synchronization? the condition is the route in BGP needs to match the same route in IGP to avoid black holes. where R1>R2>R3 but BGP is only enabled on R1 and R3. R2 causes the black hole.
But having the routes match in BGP and IGP is not the only condition, i’m afraid… let’s have a look at this case:
R5 receives the ospf route to IOU9 from IOU4, but it receives the BGP route to IOU9 from IOU2! The route from the internal routing protocol will not get synchronized with the same route in BGP.
The only fix is to change bgp router-id on IOU2 to be the same as OSPF router-id on IOU4.