What is IS-IS?

September 11, 2014 — Leave a comment

Rasesh Patel

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

  1. IS-IS inter area = “ia”
  2. IS-IS Level 1 = “i L1”
  3. 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

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