Link-state flooding
Routers share topology in a structured way.
IS-IS routers exchange link-state information, build a database, and run a shortest-path calculation. It is an interior gateway protocol, so it operates within one routing domain rather than choosing policy between independent providers.
Build the model
Four IS-IS ideas
Intermediate system
A router participating in the link-state domain.
Link state
Information about neighbours, links, and metrics.
Levels
Hierarchical areas using Level 1 and Level 2 roles.
TLVs
Extensible fields carrying routing information.
Scale the topology
Levels limit scope; TLVs carry the details.
Level 1 routers focus on reachability inside their local area. A Level 2 router connects areas and carries information across the wider IS-IS domain. A router can participate in both levels, acting as the bridge between local detail and inter-area reachability.
IS-IS puts routing information into TLVs: type-length-value fields. The type says what the field describes, the length says how much data follows, and the value carries the information itself, such as a neighbour, a metric, or an IP prefix.
Mini experiment
Inspect an IS-IS update
Choose the kind of information being exchanged.
A hello exchange establishes adjacency with a neighbouring intermediate system.
Adjacencies let routers exchange link-state information reliably.
Knowledge check
Test your IS-IS model
Answer five questions.