TCP/IP Tutorial and Technical Overview - IBM Redbooks

► To provide more efficient resource utilization. This method of routing table
management requires no network bandwidth to advertise routes between
neighboring devices. It also uses less processor memory and CPU cycles to
calculate network paths.
5.2.2 Distance vector routing
Distance vector algorithms are examples of dynamic routing protocols. These
algorithms allow each device in the network to automatically build and maintain a
local IP routing table.
The principle behind distance vector routing is simple. Each router in the
internetwork maintains the distance or cost from itself to every known destination.
This value represents the overall desirability of the path. Paths associated with a
smaller cost value are more attractive to use than paths associated with a larger
value. The path represented by the smallest cost becomes the preferred path to
reach the destination.
This information is maintained in a distance vector table. The table is periodically
advertised to each neighboring router. Each router processes these
advertisements to determine the best paths through the network.
The main advantage of distance vector algorithms is that they are typically easy
to implement and debug. They are very useful in small networks with limited
redundancy. However, there are several disadvantages with this type of protocol:
► During an adverse condition, the length of time for every device in the
network to produce an accurate routing table is called the convergence time.
In large, complex internetworks using distance vector algorithms, this time
can be excessive. While the routing tables are converging, networks are
susceptible to inconsistent routing behavior. This can cause routing loops or
other types of unstable packet forwarding.
► To reduce convergence time, a limit is often placed on the maximum number
of hops contained in a single route. Valid paths exceeding this limit are not
usable in distance vector networks.
► Distance vector routing tables are periodically transmitted to neighboring
devices. They are sent even if no changes have been made to the contents of
the table. This can cause noticeable periods of increased utilization in
reduced capacity environments.
Enhancements to the basic distance vector algorithm have been developed to
reduce the convergence and instability exposures. We describe these
enhancements in 5.3.5, “Convergence and counting to infinity” on page 185.
RIP is a popular example of a distance vector routing protocol.
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