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Chapter 5
IP Routing
If a network is directly connected, the router will always use the interface
connected to the network. If an administrator configures a static route, the
router will believe that route over any other learned routes. You can change
the administrative distance of static routes, but, by default, they have an
AD of 1.
If you have a static route, a RIP-advertised route, and an IGRP-advertised
route listing the same network, then by default, the router will always use the
static route unless you change the AD of the static route.
Routing Protocols
There are three classes of routing protocols:
Distance vector The distance-vector protocols find the best path to a
remote network by judging distance. Each time a packet goes through a
router, that's called a hop. The route with the least number of hops to the
network is determined to be the best route. The vector indicates the direc-
tion to the remote network. Both RIP and IGRP are distance-vector
routing protocols.
T A B L E 5 . 2
Default Administrative Distances
Route Source
Default AD
Connected interface
0
Static route
1
EIGRP
90
IGRP
100
OSPF
110
RIP
120
External EIGRP
170
Unknown
255 (this route will
never be used)
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