Intermediate System to Intermediate System (IS-IS) = Link State Routing Protocol
Each IS-IS router independently builds a database of the network’s topology, aggregating the flooded network information.
IS-IS is similar to OSPF in theory.
IS-IS Adjacency Levels
- Level2 (L2) [Similar to OSPF Area 0]
Inter or Intra Area adjacency.
Must be contiguous.
- Level1 (L1) [Similar to OSPF NSSA]
Intra area routes only.
Default route out. [advertised by L2 adjacent neighbor]
Route Redistribution is allowed.
IS-IS Path Selection Order
- IS-IS inter area = “ia”
- IS-IS Level 1 = “i L1”
- IS-IS Level 2 = “i L2”
Tools for Fast Convergence
- Failure Detection Time (Fast Hello or BFD)
- Even Propagation Time (Fast-Flood)
- SPF Run Time
- RIB FIB Update Timer