Explanation
IS-IS is a link-state routing protocol that uses the Dijkstra algorithm to compute the shortest paths between nodes in a network. The Dijkstra algorithm maintains three data structures: a tree database, a candidate database, and a link-state database (LSDB). The tree database contains the nodes that have been visited and their shortest distances from the source node. The candidate database contains the nodes that have not been visited yet and their tentative distances from the source node. The LSDB contains the topology information of the network, such as the links and their costs.
The Dijkstra algorithm works as follows:
* The local router moves its own local tuples into the tree database. A tuple consists of a node ID, a distance, and a parent node ID. The local router's tuple has a distance of zero and no parent node.
* The local router moves its neighbors' tuples into the candidate database. The neighbors' tuples have distances equal to the costs of the links to them and parent node IDs equal to the local router's node ID.
* The local router selects the tuple with the lowest distance from the candidate database and moves it to the tree database. This tuple becomes the current node.
* The local router updates the distances of the current node's neighbors in the candidate database by adding the current node's distance to the link costs. If a shorter distance is found, the parent node ID is also updated.
* The algorithm repeats steps 3 and 4 until either the destination node is reached or the candidate database is empty.