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Routing Protocol Basics

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Link state In link-state protocols, also called shortest-path-first proto-
cols, the routers each create three separate tables. One of these tables
keeps track of directly attached neighbors, one determines the topology of
the entire internetwork, and one is used as the routing table. Link-state
routers know more about the internetwork than any distance-vector rout-
ing protocol. OSPF is an IP routing protocol that is completely link state.
For more information on OSPF, please see the Sybex CCNP: Routing
Study Guide.

Hybrid The hybrid protocols use aspects of both distance vector and
link state--for example, EIGRP. For more information on EIGRP, please
see the Sybex CCNP: Routing Study Guide.

There is no set way of configuring routing protocols for use with every

business. This is a task that is performed on a case-by-case basis. However,
if you understand how the different routing protocols work, you can make
good business decisions. This book and the equivalent exam only cover dis-
tance-vector routing protocols and theory.

Distance-Vector Routing Protocols

The distance-vector routing algorithm passes complete routing tables to
neighbor routers. The neighbor routers then combine the received routing
table with their own routing tables to complete the internetwork map. This
is called routing by rumor, because a router receiving an update from a
neighbor router believes the information about remote networks without
actually finding out for itself.

It is possible to have a network that has multiple links to the same

remote network. If that is the case, the administrative distance is checked
first. If the AD is the same, the protocol will have to use other metrics to
determine the best path to use to that remote network.

RIP uses only hop count to determine the best path to an internetwork. If

RIP finds more than one link to the same remote network with the same hop
count, it will automatically perform a round-robin load balancing. RIP can
perform load balancing for up to six equal-cost links.

However, a problem with this type of routing metric arises when the

two links to a remote network are different bandwidths but the same
hop count. Figure 5.3, for example, shows two links to remote network
172.16.10.0.

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