May 6, 2010

Following my blog post on OSPF BDR - DR election, I have received various questions regarding OSPF and thus I decided to write a small excerpt to describe the type of LSAs generated whether it is in a stub area etc…

Before I start, it is important to keep in mind, that LSAs are carried into OSPF packet 4 . Those OSPF packet 4 are referred as LSU (Link-State Update) and carries the router’s LSA (one or more).

Now, they are 11 type of LSAs, however LSA 6 - 8/9/10/11 will not be covered in this post but in another post

LSA-1 - Router Link LSA

By default all routers in an OSPF area will be sending LSA updates of type 1. Those LSA never gets forwarded out of the area and are sent by each member to inform the other members of its links states, adjacencies (stub interfaces etc..) and cost to the area.

LSA-2 - Network Link LSA

Back to my previous post, this LSA is only generated by the DR (Designated Router); it is generated to inform the routers within its area, which routers are all part of the same segment.

LSA-3 - Network Summary Link

This LSA is only generated by the ABR and exchanged between the areas. It allows routing and communication between the areas as the ABR agregates the routes it learns.

**LSA-4 - AS External ASBR Summary Link **

Sent from an ABR to the routers in its area, it defines the next hop to reach the external routes advertized by the ASBR

LSA-5 - External Link LSA

This LSA is generated by an ASBR. It contains the routes redistributed into the area. When looking at the routing table, you will see that those routes will have an appendix of E1 or E2.

LSA 7 - NSSA External LSA

This LSA is also generated by the ASBR inside a NSSA, it describes the routes that have been distributed into the NSSA. Furthermore, keep in mind that as the LSA leaves the NSSA to the backbone, it is translated as an LSA type 5 (LSA-5). When looking at the routing table, you will see that those routes have an appendix of N1 or N2.*

As a short summary

  • Standard areas - LSA type { 1 - 2 - 3 - 4 - 5 }

  • Stub areas - LSA type { 1 - 2 - 3 }

  • Totally stubby areas - LSA type { 1 - 2 - 3 }

  • Not-so-stubby areas - LSA type { 5 - 7 }