Editing Link-state routing protocol (section)

==Distributing maps==
{{Tone|date=October 2023|section}}
The first main stage in the link-state algorithm is to give a map of the network to every node. This is done with several subsidiary steps. First, each node needs to determine what other ports it is connected to over fully working links; it does this using ''reachability protocol'' that it runs periodically and separately with each of its directly connected neighbours.

Each node periodically (and in case of connectivity changes) sends a short message, the [[link-state advertisement]], which:

* Identifies the node that is producing it.
* Identifies all the other nodes (either routers or networks) to which it is directly connected.
* Includes a 'sequence number', which increases every time the source node makes up a new version of the message''.''

This message is sent to all the nodes on a network. As a necessary precursor, each node in the network remembers, for every one of ''its'' neighbors, the sequence number of the last link-state message which it received from that node. When a link-state advertisement is received at a node, the node looks up the sequence number it has stored for the source of that link-state message; if this message is newer (i.e., has a higher sequence number), it is saved, the sequence number is updated, and a copy is sent in turn to each of that node's neighbors. This procedure rapidly gets a copy of the latest version of each node's link-state advertisement to every node in the network.

The complete set produces the graph for the map of the network. The link-state message giving information about the neighbors is recomputed and then flooded throughout the network whenever there is a change in the connectivity between the node and its neighbors, e.g., when a link fails.