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DES-3326S Layer 3 Fast Ethernet Switch User’s Guide
Switch Management and Operating Concepts 121
distance (represented by the number of hops or routers
between the advertising router and the remote network). So,
the vector is the network address and the distance is measured
by the number of routers between the local router and the
remote network.
RIP measures distance by an integer count of the number of
hops from one network to another. A router is one hop from a
directly connected network, two hops from a network that can
be reached through a router, etc. The more routers between a
source and a destination, the greater the RIP distance (or hop
count).
There are a few rules to the routing table update process that
help to improve performance and stability. A router will not
replace a route with a newly learned one if the new route has
the same hop count (sometimes referred to as ‘cost’). So
learned routes are retained until a new route with a lower hop
count is learned.
When learned routes are entered into the routing table, a timer
is started. This timer is restarted every time this route is
advertised. If the route is not advertised for a period of time
(usually 180 seconds), the route is removed from the routing
table.
RIP does not have an explicit method to detect routing loops.
Many RIP implementations include an authorization
mechanism (a password) to prevent a router from learning
erroneous routes from unauthorized routers.
To maximize stability, the hop count RIP uses to measure
distance must have a low maximum value. Infinity (that is, the
network is unreachable) is defined as 16 hops. In other words,
if a network is more than 16 routers from the source, the local
router will consider the network unreachable.