I earned my CCNA a good few years back, but I still remember all the pencilled notes in the OSPF LSA section of my Todd Lammle study guide. I found the Router LSA information hard to learn, mainly because I didn’t understand why it was so complex. In this post we’re going to take a closer look at the Router LSA, specifically the point-to-point (P2P) link. On completion, you should better understand why the Router LSA seems complex and you will better understand the role of the Router LSA.
OSPF has a complex view of the network
Let’s take an example of the P2P between Router_A and Router_B. In your network diagram you might draw the network below, with the P2P link in blue:
The CLI output from Router A (below) shows the self-originated Router LSA. The section highlighted in the blue box describes the RouterA to RouterB (A-B) link. Note that this link was pre-configured using “ip ospf network point-to-point”.
Pretty heavy; two stanzas to describe the A-B link. It might be easier to understand if we re-align our diagram with the way OSPF sees the network. In the updated diagram below, each router represents the A-B link in the Router LSA using a point-to-point (link-type 1) and a stub network (link-type 3).
Okay, so the diagram now matches the CLI output, but why are we describing the A-B link twice. To understand that, we need to better understand the purpose of the Router LSA.
Router LSA – Working a double-shift
Each OSPF router needs to build complete view of the interconnected nodes (or routers) and links within an area, before running the SPF algorithm. It relies upon the flooded Router LSAs to provide the full link-state within that area.
Ideally the router LSA would be a purist; describing only it’s link-state information. But it has a dual-role and also describes it’s locally connected prefixes . The Router LSA is thus describing topology (or link-state) information and prefixes as defined below.
1) Local topology – What routers am I connected to, over which links. e.g. P2P, transit or virtual links.
2) Prefixes – What are my locally connected subnets. OSPF calls these ‘stub networks‘. I still find the name confusing though, I like to think of stub networks as ‘IP destinations’
But why have a stub link?
RouterA uses a P2P link type and a stub network link type for the A-B link as it is both part of the OSPF topology and it is a destination.
If I wanted to ping the RouterA link interface from a remote router then I need its subnet to be advertised. Both routers can reach the 192.168.1.0/24 subnet, so they both advertise it using a stub-network link-type.
The two main takeaways from this post are:
- The Router LSA describes P2P links using two links; a P2P link-type describing the topology and a stub network link-type describing the subnet (or prefix).
- The stub network is included so that you can reach the P2P link ip addresses from remote routers.
 In OSPFv3 the prefix information has been split off into a new LSA type; the Intra-Area-Prefix LSA.
 I’ve excluded unnumbered links from this example as I don’t want to overcomplicate things.