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24-port NWay Ethernet Switch User’s Guide
118 Switch Management and Operating Concepts
Routing table inconsistency is a fundamental problem that occurs
with any distance-vector protocol in which update messages
carry only pairs of destination network and distance to that
network.
The slow convergence problem is solved using a technique known
as split horizon update. When using split horizon, a router does
not propagate information about a route back over the same
interface from which the route arrived. With split horizon, no
routing loop appears. Instead, after a few rounds of routing
updates, all routers will agree that the network is unreachable.
However, the split horizon heuristic does not prevent routing
loops in all possible topologies as one of the exercises suggests.
Another way to think of the slow convergence problem is in
terms of information flow. If a router advertises a short route to
some network, all receiving routers respond quickly to install
that route. If a router stops advertising a route, the protocol
must depend on a timeout mechanism before it considers the
route unreachable. Once the time out occurs, the router finds an
alternative route and starts propagating that information.
Unfortunately, a router cannot know if the alternate route
depended on the route that just disappeared. Thus, negative
information does not always propagate quickly.
Another technique used to solve the slow convergence problem
employs hold down. Hold down forces a participating router to
ignore information about a network for a fixed period of time
following the receipt of a message that claims a network is
unreachable. Typically, the hold down period is set to 60 seconds.
The idea is to wait long enough to ensure that all machines
receive the message that a network is unreachable and that the
message is not out of date. It should be noted that all machines
participating in a RIP exchange need to use identical hold down
period, or routing loops can occur. The disadvantage of a hold
down technique is that if routing loops occur, they will be