OSPF: LSA Type 1 (why it’s unique), explanation of SPF Tree, and a handy show command for exam day!


When you first configure a router in an Area via an OSPF instance, you are creating a Link State Database, that already contains entries for different LSA Types.

Each router considers itself to be the Root of what is called the “Shortest Path First Tree”, which is a mapping via LSA’s from neighbor routers, which are run against the SPF or Shortest Path First Algorithm to make sure all routers are synchronized.

This operation also is the reason where there are rules regarding route filtering, and where summarization can and cannot be done.

Before we start, LSA’s are transported by LSU’s, AKA Link State Update packets.

LSA Type 1 – Known as “Router LSA” – Each router creates a Type 1 LSA for itself and floods it through the same OSPF Area that it is in. It is unique in the way it creates a single LSA for the connected OSPF interface links, and won’t flood multiple LSA’s for each individual link.

This type of LSA cannot be filtered or Summarized, as it is the core of the local Routers own Link State Database, and will stay in it’s own Area as it isn’t concerned with building other Areas SPF Tree – It only cares about it’s own Areas Tree.

The LSA Type 1 will always keep it’s own RID no matter how far it travels, so an OSPF router even 10 hops away knows that X router owns the links being advertised.

Speaking of which, if you want to see the LSA’s for a particular router from your LS DB, type in the command “sh ip ospf database router x.x.x.x” where x.x.x.x is the remote RID and it will show you the links learned from that remote neighbor only.

I will stop here, I’ve stickied a post that has a 3 part link to a complete explanation of all LSA’s, and I highly advise myself (and yourself if you’re studying for the exam) to give it a once over before walking into the exam room!


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