D-Link DES-3326 User Manual page 133

24-port NWay Ethernet Switch User's Guide
one hop from a directly connected network, two hops from a
network that is reachable through one other router, and so on.
Thus, the number of hops, or hop count, along a path from a
given source to a given destination refers to the number of
routers that a datagram encounters along a path.
Both active and passive RIP participants listen to all broadcast
messages, and update their tables according to the distance-
vector algorithm described earlier.
RIP specifies a few rules to improve performance and reliability.
Once a router learns a route from another router, it must apply
hysteresis, meaning that it does not replace the route with an
equal cost route. In other words, to prevent oscillation among
equal cost paths, RIP specifies that existing routes should be
retained until a new route has a strictly lower cost.
RIP specifies that all listeners must timeout routes they learn
via RIP. When a router installs a route in its table, it starts a
timer for that route. The timer must be restarted whenever the
router receives another RIP message advertising the route. The
route becomes invalid if 180 seconds pass without the route being
advertised again.
There are three potential errors that can arise using the RIP
algorithm. First, because the algorithm does not explicitly detect
routing loops, RIP must either assume participants can be
trusted or take precautions to prevent such loops. Secont, to
prevent instabilities RIP must use a low value for the maximum
possible distance (RIP uses 16). Thus, for internets in which
legitimate hop counts approach 16, managers must divide the
internet into sections or use an alternative protocol. Third, the
distance-vector algorithm used by RIP can create a slow
convergence or count to infinity problem, in which
inconsistencies arise because routing update messages propagate
slowly across the network.
Switch Management and Operating Concepts
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