Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 96
Routing Protocols—Distance Vector

Information is kept in the router that allows to it to know all the networks or subnets in its domain and the path to get to those networks. The information is about networks and their paths. This information is grouped together into a table—a table is the same thing as a table in Microsoft Word. It contains a grouping of like information to be use as a whole. There are two standard methods for building these tables: distance vector and link state. Link state will be covered later. Distance vector means that the information sent from router to router is based on an entry in a table consisting of <vector, distance>. Vector means the network number and distance means what it costs to get there. The routers exchange this network reachability information with each other by broadcasting their routing table information consisting of these distance-vector entries. This broadcast is local and each router is dependent on other routers for correct calculation of the distance.

Each entry in the table is a network number (the vector) and the amount of routers (distance) that are between it (the router) and the final network (indicated by the network number). This distance is sometimes referred to as a metric. For example, if the source station wants to transmit a packet to a destination station that is four hops away, there are probably four routers separating the two networks.

Any time a datagram must traverse a router (thereby passing through a new network number) it is considered a hop (metric). For RIP, the maximum diameter of the internet is 15 routers (hops). A distance of 16 is an indication that the network is not reachable. In other words, if the network is more than 15 routers away, it is considered unreachable. Remember: This is RIP and RIP is an IGP, which is under one domain. The Internet itself encompasses many domains and the diameter of the Internet is much larger than 15 hops.

As shown in the slide, each router will contain a table with starting entries of those networks that are directly attached to it. For a router that has only two network connections (there are no other routers on the internet), the initial entries in the table would look like the following:

There are actually more header entries in a routing table, but the significant portions are shown in the slide. From this table, we know that networks 134.4.0.0 and 134.3.0.0 are directly connected to this router. Network 134.4.0.0 is assigned to port 1 of the router and 134.3.0.0 is directly attached to port 2. It is running the RIP protocol, and xxx indicates how long the route has before it is deleted from the table.

Routing Protocols—Distance Vector