Hello, everyone!
This post is about the detailed OSPF anti-loop mechanism on the AR router – type 5 LSA anti-loop, being an authorized reprint by author zhushigeng (Vinsoney).
TYPE-5 LSA ANTI-LOOP
When an OSPF router imports an external route into an OSPF domain, it becomes an ASBR. The imported external route floods in entire OSPF domain as the Type-5 LSA. There are two prerequisites for a router to use the Type-5 LSA to calculate routes. One is to receive the Type-5 LSA, and the other is known where the the ASBR which generated the Type-5 LSA is. A router that accesses the same area which the ASBR belongs can calculate the shortest path to the ASBR based on the flooded Type-1 LSA and Type-2 LSA in the area to, then they can calculate the external route out. The routers in other areas are not so lucky, because the Type-1 LSA generated by the ASBR can only flood in the area where it is located, so the Type-4 LSA is required. Therefore, the routers in other areas can calculate the shortest path to the ASBR after obtaining the Type-4 LSA, and then use the Type-5 LSA generated by the ASBR to calculate the external route. The Type-5 LSA will be flooded to the entire OSPF domain. On the face of it, it does not have any anti-loop capability, but in reality, it is not needed because it can rely on Type-1 LSA and Type-4. LSA to achieve anti-loop.
EXTERNAL ROUTE TAG
In addition, the External Route Tag field in the Type-5 LSA is used for the defense of external routes in the MPLS VPN scenario. It is not discussed in depth here.
This would be all on the detailed OSPF anti-loop mechanism on the AR router – type 5 LSA anti-loop. Thanks for reading this post!
