Filter
Top Products
DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
Every 30 seconds, a router running RIP broadcasts a routing update containing a set of pairs of
network addresses and a 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.
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.
RIP Version 1 Message Format
There are two types of RIP messages: routing information messages and information requests. The
same format is used by both types.
63