Routing Protocol Basics
273
administrative domain, which basically means that all routers sharing the
same routing table information are in the same AS.
EGPs are used to communicate between ASs. An example of an EGP is
Border Gateway Protocol (BGP), which is also discussed in CCNP: Routing
Study Guide.
Since routing protocols are so essential to dynamic routing, in the
next section we'll look at some basic information you should know about
these protocols. In the section after that, we'll get nitty-gritty with RIP in
particular.
Routing Protocol Basics
T
here are some fundamentals you should know about routing proto-
cols before we look deeper into RIP. Specifically, you should understand
administrative distances, the three different kinds of routing protocols, and
routing loops.
Administrative Distances
The administrative distance (AD) is used to rate the trustworthiness of rout-
ing information received on a router from a neighbor router. An administra-
tive distance is an integer from 0 to 255, where 0 is the most trusted and 255
means no traffic will be passed via this route.
If a router receives two updates listing the same remote network, the first
thing the router checks is the AD. If one of the advertised routes has a lower
AD than the other, then the route with the lowest AD will be placed in the
routing table.
If both advertised routes to the same network have the same AD, then
routing protocol metrics (such as hop count or bandwidth of the lines) will
be used to find the best path to the remote network. The advertised route
with the lowest metric will be placed in the routing table. However, if both
advertised routes have the same AD as well as the same metrics, then the
routing protocol will load-balance to the remote network.
Table 5.2 shows the default administrative distances that a Cisco router
will use to decide which route to use to a remote network.
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