OSPF LSA Types Cisco for IPv4 Explained

This post will explain the Cisco OSPF LSA Types hopefully in an easy to understand format. These are my study notes for the CCIE R&S Lab exam.

The main LSA types are:

LSA type 1 (Router)

Every router generates a type 1 LSA with the state of each of their links, there are 3 types of links

Stub -none or one neighbor
Transit – more than one neighbor
Point to Point – links with other routers

Basically an LSA type 1 describes all the links that a router has, including loopbacks.

The link state ID is the Router ID

LSA type 2 (Network)

Type 2 LSA’s are present on a multi-access link – a DR exclusively generates type 2 LSAs and the tell the other routers about the network and the routers connected to them

Both LSA types 1 and 2 exist in all areas and are never flooded outside of the area. Other LSAs are advertised within an area depending on the area type, these are all described below.

The link state ID is the interface IP of the DR

LSA type 3 (Summary)

Generated by the ABR to describe a route to neighbors outside the area. (Inter-area routes)

For type-3 LSAs, the Link-State ID is the IP address of the subnet that is being advertised.

LSA type 4 (ASBR Summary)

The link state ID is the Router ID of the ASBR that is being advertised.

External routes – LSA type 5

The link state ID is the IP address of the subnet that is being advertised.

NSSA external routes – LSA type 7

To summarise:

LSA Type 1 – Router – contains router links and state and is flooded into the area of origin

LSA Type 2 – Network – generated by DR – lists all attached routers – flooded into the area of origin.

LSA Type 3 – Network Summary – generated by ABR’s sent into an area to advertise prefixes to other areas – flooded throughout the Autonomous System.

LSA Type 4 – ASBR Summary – generated by ABR’s – advertises the ASBR – flooded throughout the Autonomous System

LSA Type 5 – AS External – generated by ASBR – advertises external destination – flooded throughout the Autonomous System

LSA Type 7 – NSSA External – generated by the ASBR in a not so stubby area – advertises external destination.