[HCIE-R&S interview experience sharing] What problems may be bidirectional route import bring and how can these problems be resolved?Highlighted

The preceding figure shows the dual-point bidirectional re-advertisement topology. If no control is performed, sub-optimal routes and loops may occur.

The fault occurs when R1 imports network segment 10.0.0.0/24 to OSPF. Import the route on R2 and R3.

In normal cases, R2/R3 imports routes to IS-IS in sequence. Assume that R3 imports routes before R2.

To IS-IS. R2 learns the route 10.0.0.0/24 from both OSPF and IS-IS. So R2 will

Routes learned through IS-IS are preferred. (The preference of OSPF external routes is 150, and that of IS-IS routes is 150.)

The route priority is 15. Therefore, when R2 accesses network segment 10.0.0.0/24, the route R4-R3-R1 is used.

Suboptimal path. To prevent this problem, run the route-policy command on R2 to change the priority of the OSPF ASE route so that the OSPF ASE route takes precedence over the route learned from IS-IS.

Correct route. When the interface connecting R1 to the network 10.0.0.0/24 is disconnected, the external LSA is still in OSPF.

The area ages, but R2 has learned the 10.0.0.0/24 network through IS-IS. Therefore, R2 will

This network is introduced into OSPF, which allows R1 and R3 to learn the network 10.0.0.0/24 through R2. When R2

When accessing network 10.0.0.0/24, traffic is sent along the path R4-R3-R1-R2, causing a loop.

To avoid this situation, we can tag the route entry and then add a specific tag to the route entry.

Row filtering is used to prevent loops.