D-Link DES-3326 User Manual - page 122
DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
108
Switch Management and Operating Concepts
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