Overview
- Link-state, uses Dijkstra algorithm
Router Types
- Level 1: intra-area (can only form relationships with other Level 1 routers).
- Level 2: inter-area (can only form relationships with other Level 2 routers).
- Level 1-2: both.
Routing
- Level 1 ↔ Level 1
- Level 2 ↔ Level 2
- Level 1 ↔ Level 1-2 ↔ Level 2
Other Stuff
- Area borders are between routers (i.e. on the link):
- in contrast to OSPF where the area border is within an ABR
ISIS NET Address / ISIS Multi-Area Adjancency explanation (Case: Unified MPLS)
Network Entity Titles (NETs) are generally 10 bytes long (they can be from 8 to 20 bytes long) and are written as 49.0001.1921.6811.9001.00.
The first three bytes of the address form the area ID. The area ID can be up to 13 bytes long. The first byte of the area ID is the Address Family Identifier of the authority, which is the space assigned to a particular enterprise (equivalent to an IP address space that is assigned to an enterprise). The value of 49 is the well-known Address Family Identifier used for private addressing, which is the equivalent of RFC 1918 addressing for IP protocols. The last two bytes in the area ID identify an IS-IS area within the AS, here 0001 means are 1.
The next six bytes (here, 1921.6811.9001) are the system identifier, which identifies each node(router) on the network. Although IS-IS supports a variable-length system field, in practice this field is always six bytes.The final two bytes of the NET are the NET selector (NSEL) and, for IS-IS, they must always be zero to indicate "this system".