The Relationship Between LSA Types and Special Areas

As interesting as it may be, the details of OSPF can be a bit daunting. A big part of that is that there are types of this and types of that all over the place. Two important types that you’ll use a lot and should know pretty well are area types and LSA types. I’ve come up with a pretty good way to learn both and to correctly associate the two.

To save you some time if you are already familiar with OSPF and just want to get the skinny, here it is (we’ll recap at the end too):

1. Type 1 and 2 LSAs don’t cross ABRs, ever.
2. “stub” means type 5 LSAs go away.
3. If you see the word “totally” then type 3 and 4 are out too.
4. If you see the words “not so” then type 7 are in use and you are dealing with an ASBR somewhere in the stub area.

OSPF Special Area Types

OSPF has a few extra types of areas aside from your standard, run-of-the-mill area. All of the special area types revolve around the term “stub”. A stub area is an area that only has 1 way in and out (as far as the OSPF process is concerned). Their purpose typically has to do with minimizing the utilization of memory, CPU, and bandwidth by limiting/filtering certain types of link-state announcements the areas send and receive. The flexibility required to make this possible is actually where the link-state announcement (LSA) types come into play.

Aside from a regular OSPF area, Cisco routers support the following area types:

• stub areas
• not-so-stubby areas (NSSA)
• totally stub
• totally stubby not-so-stubby areas

OSPF LSA Types

I’ll keep this part as short as possible, but keep in mind that one could go on for pages discussing LSA type specifics. There are 11 types of LSAs in OSPFv2. For our discussion, I’m going to just tell you what a hand full (plus 1) of the LSA types are and what they are for, then we’ll get down to associating them to the various area types.

Type 1 LSAs are your standard intra-area link-state announcement. There is nothing special about them, they just do their job, punch the clock, and head home. These are the bread and butter of intra-area routing.

Type 2 LSAs only, only, only come from a DR (designated router). Remember those? They are the elected (by priority or by router-id) official of the broadcast network. Type 2 LSAs come from the DR to tell the rest of the world who is attached to the broadcast link.

Type 1 and 2 never cross area borders, and by virtue that the protocol is link-state, you can’t keep them from moving about the area.

Type 3 LSAs are summaries of routes that are sent inter-area.

Type 4 LSAs are also summary routes, but they specifically deal with ASBR (autonmous system boundry routers). In fact, the only thing that they do is provide information about the ASBR.

Type 5 LSAs are used to deliver external information. This is the information that is learned from outside of OSPF and was introduced by redistribution at some point.

Type 7 LSAs are the same as type 5 LSAs. Redundant? Kinda, but hang in there, there is a reason for it. As stated above:

1. Type 1 and 2 LSAs don’t cross ABRs, ever.
2. “stub” means type 5 LSAs go away.
3. If you see the word “totally” then type 3 and 4 are out too.
4. If you see the words “not so” then type 7 are in use and you are dealing with an ASBR somewhere in the stub area.

Now that you know what the LSAs are, what the area types are, and these rules, let’s try to figure out what kind of LSAs are allowed to cross the ABR for the following special area types:

1. stub
2. not-so-stubby
3. totally stubby
4. totally stubby not-so-stubby area

All of these are valid area types on Cisco routers.

So with the 4 rules in mind, starting with example #1. We won’t have type 1 or 2 for sure – they never cross an ABR anyway. “stub” means that we won’t have type 5. As it looks, we’ll be permitting type 3 and 4 (summaries), blocking 5 (info from outside of OSPF), and letting everything else through.

Example 2 is the same as above, only it has our key words of “not-so”. That means that even though we’re not letting type 5 through, we’re allowing type 7. When the type 7 hit the ABR, they get translated into type 5 to go out to the rest of the network. This may seem stupid on the surface, but digging deeper reveals the point. The key is that we’re saving our area from processing all of the type 5 LSAs from all of the other areas. Let’s say we have enough power (processor/memory, etc) to process our own area’s information, but we’re not able to handle dealing with everyone else’s.

Example #3 is the most efficient of any of these. Looking at the rules we see that stub filters type 5, and “totally” filters 3 and 4. Since we know that ABRs don’t transit type 1 and 2, we’re pretty much done. So how does it work? Well, the point of a stub, if you recall, is that there is only 1 exit point for the network. So…. OSPF just sends a default route and that is it. Intra-area routes within the stub area are still going to make their way into the LSDB and the routing table, but the only way out to the rest of the world is via the default route.

Example #4 is like #3, but type 7 LSAs get through for the same reason as example #2. Did you get all that? =) We filter everything, but we have an ASBR somewhere within our totally stubby area. As such, we need routes that are being introduced to us from that avenue to be sent out to the rest of the network. We send type 7 LSAs to the ABR, it changes them to type 5 on the way out, and everyone is happy.