OSPF Router LSA (Type 1 LSA)

Type 1 LSA (also called Router LSA) contains information about directed connected links in the area to which the router belong. They are flooded to all routers in that area. If the router is an ABR (Area Border Router), it generates type 1 LSAs for all the areas to which it is connected and send those LSAs to all neighbors in corresponding areas.
router-lsa

For example in the picture exposed above you see that ABR router has three links, two of them connected in area 0 and another one in area 1. So, it will generate one LSA for area 0 which will include links to R1 and R2, this LSA will be sent to all neighbors in area 0, i.e. to R1 and R2. Another LSA will be build for area 1 that will include links in area 1 which is link to R3, this LSA will be sent to R3 because R3 is only neighbor in area 1. This process also happens from opposite side, R1 with R2 send to ABR their type 1 LSAs with all directly connected links in area 0, R3 does the same with its link connected in area 1. LSA received by ABR from R2 is also transmitted to R1 and vice versa, R1 LSA is also carried to R2. I this way, every router in the area knows about all the other routers in the area and their enabled OSPF links. In next picture I added more details to our OSPF domain, let’s consider it and have a closer look on how router LSAs work .

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