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Detailed OSPF anti-loop mechanism on the AR router – type-5 LSA anti-loop (REPRINT)

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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).



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.


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!

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