On 30/Oct/15 15:34, Matthew Petach wrote:
It is rather nice that IS-IS does not require level-2 to be contiguous, unlike area 0 in OSPF. It is a valid topology in IS-IS to have different level-2 areas connected by level-1 areas, though you do have to be somewhat careful about what routes you propagate into-and-back-out-of the intervening level-1 area.
I found Route Leaking in IS-IS to be a moot endeavour because if one wants to keep absolute routing inside the IGP, you'll want to have the core and Loopback interface addresses in the IGP, particularly if you're running an MPLS network. In such a case, the only real gain you get from multi-level IS-IS is a little quietness re: the LSP's being propagated within a particular Level-1 Area. However, things like PRC (Partial Route Calculation) and iSPF (Incremental SPF) help a lot here when you have a flat Level-2 IS-IS domain. Mark.