Saturday, November 6, 2010


Recall that OSPF has areas for efficiency. Every router in the area needs to know about all the routes and changes in the area. If you want to keep the topology database small, you'd have to implement areas to localize routes and updates.
Also recall that there are multiple router types which you can see from this diagram:

(Yes I'm not using Cisco logos because I used an old image I made previously)

From this point I'd need you to be familiar with the areas and router types because we'll be talking about LSA types.


Yes, recall that after exchange of DBD, LSQs are used to request for LSUs which contain multiple LSAs. But there are many different types of LSAs!

Before we describe anything, we'll go through summarization in OSPF. We'll make use of this topology:

Now back on R1, I have links to the networks:

We can only summarize on ABRs or ASBRs, so we'll need to do this on R2. To summarize Area 1, we hop on over to R2 and type:
area 1 range is the summary mask that summarizes 192.168.[1-7].0/24 to a Once this is done, R2 will begin suppressing routes matching Routers in Area 0 and other areas would only see pointing towards R2. The "area 1 range" command can also be used to hide routes by adding the "not-advertise" keyword behind. The summarized range is suppressed and not advertised.

Now, assume that R4 has the routes:

R3 receives these routes as RIP routes. Now, remember that we can do summarization on ASBRs, so we should be able to summarize on R3, right? However, one thing comes in mind: Since the summarization command is "area x range", what area exactly do we use to specify an external AS?

We don't! Instead, we use the summary-address command! To summarize on R3, we use:
redistribute rip subnets

What this does is that it advertises all RIP routes into OSPF as E2 routes, then suppress and apply the summary-address of to every address matching it.

Now, we can talk about LSAs. LSA are the building blocks of OSPF, and below is a list of the 5 most common LSAs (there are 11 in total):
LSA Type 1: Router LSA
LSA Type 2: Network LSA
LSA Type 3: Summary LSA
LSA Type 4: Summary LSA
LSA Type 5: External LSA

The Type 1 LSA (Router) is generated by routers to talk about 1 specific network within the same area.

The Network LSA is generated by a DR to tell everyone about the other routers connected to the same network segment.

Type 3 LSAs are generated by ABRs to talk about routes and summaries from other areas.

Even though Type 4 LSAs are called Summary LSAs, they are just generated by ASBRs to point to the IP Address of the ASBR in the OSPF domain.

The real routes and summaries from external locations are advertised by ASBRs as Type 5 LSAs.

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