Link State Example

Both OSPF and Integrated IS-IS use the Hello protocol for establishing neighbor relationships. Those relationships are stored in a neighbor table (also called an adjacencies database). Each router learns a complete network topology from information shared through these neighbor relationships. That topology is stored in the router's link-state database (LSDB), also called the topology table or topology database. Each router uses this topology and the SPF algorithm to create a shortest-path tree for all reachable destinations. Each router selects the best routes from its SPF tree and places them in its routing table (also called the forwarding database).

Link-State Routing Analogy

You can think of the LSDB as being like a map in a shopping mall. Every map in the mall is the same, just as the LSDB is the same in all routers within an area. The one difference between all the maps in a shopping mall is the "you are here" dot. By looking at this dot, you can determine the best way to get to every store from your current location; the best path to a specific store is different from each location in the mall. Link-state routers function similarly: They each calculate the best way to every network within the area, from their own perspective, using the LSDB.

Figure 7-4 shows a network that uses a link-state protocol. Triggered updates, which include data on the state of only links that have changed, are sent in this network.

Figure 7-4 Link-State Routing Sends Changed Data Only When There Is a Change

Link-State CI Table C

Figure 7-4 Link-State Routing Sends Changed Data Only When There Is a Change

Link-State CI Table C

Table A Table D

In link-state protocols, the information about connected links (including the subnets on those links) on all routers is flooded throughout the network or to a specific area of the network. Therefore, all routers in the network have detailed knowledge of the entire network. In contrast, routers running a distance vector routing protocol receive knowledge about only the best routes from their neighbors.

After the initial exchange of all link states and on reaching the full (converged) state of operation, almost no periodic updates are sent through the network. (In OSPF, periodic updates are sent every 30 minutes for each specific route, but not at the same time for all routes, reducing the routing traffic volume.) Triggered updates are flooded through the network only when a change in a link state occurs (the link goes down, comes up, or link parameters that affect routing—such as bandwidth—are changed). Only periodic hello messages are sent between neighbors to maintain and verify neighbor relationships.

Most of the control packets used in link-state operations are sent as multicast packets, which might cause problems when deploying link-state protocols in nonbroadcast multiaccess (NBMA) networks, such as with Frame Relay or ATM topologies.

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