Editing IS-IS (section)

== Areas and levels ==
In IS-IS there is concept of areas, but here it works differently from OSPF. First of all, in contrary to OSPF, in IS-IS area is terminated on router, not link. 

In IS-IS, backbone area consists of contiguous Level 2 routers. Level 1 areas can be thought as stub areas in OSPF, where very limited reachability information is available. L1/L2 routers act like area border routers (ABRs) between L1 routers and L2 routers by keeping two databases - L1 database and L2 database.

Here is very important role of L1/L2 router - if it is connected to L2 router in another area, then it sets ATT (ATTached bit) in its L1 LSP. L1 routers which receive this LSP (with ATT bit) add default route to originator of this LSP. This is different from OSPF, where ABR generates default route to stub area routers and sends it via LSA 3.

Another difference of router in L1 area in IS-IS from router in stub area in OSPF is that L1 router can inject external routes into area, which travels up to L1/L2 router. With that, it resembles NSSA area in OSPF (where you cannot have external routes from backbone area, but you can inject external routes to NSSA area which are then translated to regular LSA 5 external routes by NSSA ABRs). 

However, '''by default''', '''external L1 routes''' are '''not injected''' from L1 to L2. This can be changed by policy on L1/L2 router, which accepts L1 external routes and originates them into L2. 

In case of OSPF, if at some moment it is needed to inject external routes into OSPF domain from stub area, this can be done only by changing area type from stub to NSSA. This will causes tearing down OSPF neighborship. In IS-IS, this happens hit-less, just by adding a new export policy under protocol.