Types of LSAs (Link-state advertisements)

There are several different LSA types in OSPF:

Type 1 LSA – also known as router link advertisement (RLA), a Type 1 LSA is sent by every router to other routers in its area. It contains the router ID (RID), interfaces, IP information, and current interface state. Note that Type 1 LSAs are flooded only across their own area.

Type 2 LSA – also known as network link advertisement (NLA), a Type 2 LSA is generated by designated routers (DRs) to send out information about the state of other routers that are part of the same network. Type 2 LSAs are flooded across their own area only.

Type 3 LSA – also known as summary link advertisement (SLA), a Type 3 LSA is generated by area border routers (ABRs) and sent toward the area external to the one where they were generated. It contains the IP information and RID of the ABR that is advertising an LSA Type 3.

Type 4 LSA – informs the rest of the OSPF domain how to get to the ASBR. The link-state ID includes the router ID of the described ASBR.

Type 5 LSA – also known as AS external link advertisements, a Type 5 LSA is sent by autonomous system boundary routers (ASBRs) to advertise routes that are external to the OSPF autonomous system and are flooded everywhere.

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