I know I haven’t posted much lately, with studying for the Cisco BSCI and working on my kitchen floor, but I always get a bit of a kick when reviewing OSPF routing protocol, specifically when talking about stub areas. This is because of the naming of these various types of stub areas:
Totally Stubby Area
Not So Stubby Area (NSSA)
Totally Stubby Not So Stubby Area (sounds kinda contradictory, doesn’t it)
That’s a lot of subbiness going on here. What is it all about, anyway? In a nutshell, the various stub areas in OSPF are configured as such to limit the types of LSA (link-state advertisements) that the router receives, reducing its routing table size. The particulars of each type are a lot to get into in a post, but visit Cisco’s site on the topic to learn more.
Luckily the configuration itself is fairly straightforward. All commands are within the OSPF router config mode.
For a stub area:
area area-id stub (where area-id is the number label of the area in question)
For a Totally Stubby Area:
area area-id stub no-summary (no-summary only required if it is on an Area Border Router, or ABR)
For the Not So Subby Area
area area-id nssa
And for Totally Stubby Not So Stubby Area
area area-id nssa no-summary (again, no-summary only required on an ABR)
Of course, as stated, there’s a bit more to it, but Cisco generally does a good job with information on various topics on their web site. The article also mentions Virtual Links. All areas in an OSPF network are required to be connected to Area 0 (the backbone area). Virtual links are a sort of “band-aid” you can use when an OSPF area cannot be physically connected to Area 0.