OSPF summary LSA (Type 3 LSA)

Summary LSA can be of two flavors: type 3 and type 4. In this tutorial we will focus on type 3 LSA. Type 3 LSAs are generated by ABR. They represent networks from an area and are sent to the rest of the areas in OSPF domain. Type 1 LSAs don’t cross area boundary, so Area Border Router (ABR) uses type 3 LSA to inform other areas about networks learned in its area. Type 3 LSA uses network address as Link ID and router id of advertising router as ADV Router in OSPF database. By default a type 3 LSA contains information about only one network learned in the area, so, ABR (Area Border Router) will generate one type 3 LSA per network learned via type 1 LSA. This can cause serious problems in case there are a lot of networks to advertise, because a lot of LSA will be generated by ABRs. Area border routers can generate Type 3 LSAs that advertise summarized networks instead of single networks. Summarization is highly recommended in large OSPF domains, it can significantly  improve the performance of the network. If you need a guide on how to perform summarization, have a look at OSPF internal route summarization tutorial. For now let’s consider following topology:type-3-lsa

 

Here we have two ABRs. ABR1 will generate type 3 LSAs for network 192.168.0.0/24 in area 10 and send it in area 0. Also it will generate type 3 LSA for networks 192.168.1.0/24 and 192.168.2.0/24 in area 0 and will send them in area 10. ABR2 will perform a similar operation, i.e. it will generate type 3 link state advertisement in area 20 for network 192.168.3.0/24 and send it in area 0, and for networks 192.168.1.0/24 and 192.168.2.0/24 in area 0 that will send in area 20.

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