Specifications

174 VH-2402-L3 Management Guide 9033691-01
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.
RIP can also be slow to converge (to remove inconsistent,
unreachable or looped routes from the routing table)
because RIP messages propagate relatively slowly through
a network.
Slow convergence can be solved by using split horizon
update, where a router does not propagate information about
a route back to the interface on which it was received. This
reduces the probability of forming transient routing loops.
Hold down can be used to force a router to ignore new route
updates for a period of time (usually 60 seconds) after a new
route update has been received. This allows all routers on
the network to receive the message.
A router can ‘poison reverse’ a route by adding an infinite
(16) hop count to a route’s advertisement. This is usually
used in conjunction with triggered updates, which force a
router to send an immediate broadcast when an update of
an unreachable network is received.