The Control Plane: Link State Routing

-Another way to create a routing table with the most efficient path between two routers or ‘nodes’ is by using link-state routing.

Link state routing works in two phases: reliable flooding and route calculation. Let’s look at phase I now.

Phase I: Reliable Flooding

Neighbor Discovery

When a link-state router boots, it first needs to discover its neighbors. To do this:

• Each router sends a HELLO message every $N$ seconds on all of its interfaces. This message contains the router’s unique address.

• As its neighboring routers also send HELLO messages, the router discovers its neighbors.

• These HELLO messages are only sent to neighbors that are directly connected to a router, and a router never forwards the HELLO messages that they receive.

• HELLO messages are also used to detect link and router failures. A link is considered to have failed if no HELLO message has been received from the neighboring router for a period of $k \times N$ seconds.