Let’s discuss some OSPF design fundamentals, some of which I mentioned in an earlier post, but which are worth reviewing again.
- OSPF links are grouped into contiguous areas. “Contiguous” is key here. If you create an area 9, let’s say, than all of the area 9 links must be able to reach all of the other area 9 links through area 9 links alone. If you were to have to traverse the area 0 backbone to reach the another area 9 link, then you don’t have a contiguous area. Rather, you’ve got 2 area 9’s (and some challenges ahead).
- Area Border Routers (ABRs) connect the backbone area and one or more areas together.
- Autonomous System Boundary Routers (ASBRs) take routes from another routing protocol and redistribute them into OSPF.
- You can use a single OSPF area. However, splitting your OSPF design into multiple areas has these benefits:
- The LSDB for an area is usually smaller, asking less of router memory.
- SPF calculations within an area will be quicker, under the assumption that the LSDB is smaller. Why are area LSDBs smaller? Because ABRs don’t forward type 1 or 2 LSAs into an area; instead type 3 summary LSAs are send into an area.
- When a link fails in one area, routers in other ares only have to do a partial recalculation (more on that in a bit).
- You can summarize routes at border and boundary routers. Summarized routes imply smaller LSDB, and lower SPF computation overhead.
At this point, we need to review the several different types of LSAs:
- 1 (router) – there is 1 of these per router, containing the RID and all interface IPs. Stub networks are included. (Logically, this is going to tell other routers what networks this router has direct connections to.)
- 2 (network) – there is 1 of these per transit network. The DR on a subnet creates these. It contains the subnet and router interfaces connected to that particular subnet. (This is how a router will know all the possible next hops to reach a remote subnet without having to form a full adjacency to every router he sees a hello from.)
- 3 (net summary) – ABRs use these to tell one area about another area’s type 1 and 2 LSA’s. The subnet and cost information is included, but the topology – routers connected to this subnet – are not included.
- This is fundamental to OSPF design. To go from one area to another, you must traverse area 0. To get to area 0, you have to go through an ABR. So why should an ABR pass along topology information into another area? That information is irrelevant.
- A router receiving an type 3 LSA calculates the cost by using the cost to reach the advertising ABR, plus the cost in the LSA itself.
- This in mind, if a network in one area changes (goes up/down or there’s a topology change), routers in other areas don’t have to run a full SPF calculation – only a partial – to update their routing tables. Routers within the area where the topology change happened will receiving a type 1 or 2 LSA and run a full SPF calculation.
- 4 (ASBR summary) – An advertisement of a host route used to reach an ASBR. If redistributing an E1 (exernal type 1, internal and external metric considered) route, the router will send an LSA type 4 for itself, plus the LSA type 5 for the subnet.
- 5 (AS external) – ASBRs create these to describe external routes redistributed into OSPF. If redistributing an E2 (external 2, only external metric considered) route, the ASBR creates a type 5 LSA and floods it, with no accompanying type 4 LSA.
- 6 (group membership) – MOSPF, Cisco IOS does not support this type.
- 7 (NSSA external) – ASBRs inside of a not-so-stubby-area will create these instead of type 5 LSAs.
- 8 (external attributes) – Cisco IOS does not support this type.
- 9-11 (opaque) – generic for future OSPF extension.
A mention of stubby areas is also warranted. The point of a stubby area is to reduce the number of LSAs that will be advertised into or out of an area. Say we’ve got area 54 and area 0. Traffic leaving area 54 will have to traverse area 0, right? Well then, why not have the area 0-54 ABR advertise a default route into area 54 rather than a bunch of type 3 and/or 5 LSA’s? And depending on just how picky we want to be, why should we let area 54 advertise any type 5 LSA’s from an ASBRs that might be in there? You get the idea. There are several different stubby area types, each type with its own unique combination of what LSA types are allowed into and out of the area through the ABR.
Below, type 5 (extenal) and type 3 (summary) are stopped (or allowed) at the ABR from being injected INTO the area. Type 7’s (NSSA external) are stopped (or allowed) OUT OF the area.
Type 5 IN – stopped; Type 3 IN – allowed; Type 7 OUT – stopped.
- Totally stubby
Type 5 IN – stopped; Type 3 IN – stopped; Type 7 OUT – stopped.
- Not-so-stubby area (NSSA)
Type 5 IN – stopped; Type 3 IN – allowed; Type 7 OUT – allowed.
- Totally NSSA
Type 5 IN – stopped; Type 3 IN stopped; Type 7 OUT – allowed.
- All stub areas stop type 5’s.
- All totally stubby areas stop type 3’s.
- All not-so-stubby areas allow type 7 external LSA’s to be injected into area 0.