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24-port NWay Ethernet Switch User’s Guide
Switch Management and Operating Concepts 119
preserved for the duration of the hold down period. More
important, incorrect routes will be preserved for the hold down
period, even when alternatives exist.
A final technique for solving the slow convergence problem is
called poison reverse. Once a connection disappears, the router
advertising the connection retains the entry for several update
periods, and includes an infinite cost (hop count of 16) in its
broadcasts. To make poison reverse most effective, it must be
combined with triggered updates. Triggered updates force a
router to send an immediate broadcast when receiving a message
that a network is unreachable, instead of waiting for the nest
periodic broadcast. By sending an update immediately, a router
minimizes the time it is vulnerable to believing inaccurate routes.
Unfortunately, while triggered updates, poison reverse, hold
down, and split horizon techniques all solve some problems, they
introduce others. For example, consider what happens with
triggered updates when many routers share a common network.
A single broadcast may change all their routing tables, triggering
a new round of broadcasts. If the second round of broadcasts
changes tables, it will trigger even more broadcasts. A broadcast
storm can result.
The use of broadcast, potential for routing loops, and the use of
hold down to prevent slow convergence can make RIP extremely
inefficient in a wide area network. Broadcasting always takes
substantial bandwidth. Having all machines broadcast
periodically means that the traffic increases as the number of
routers increases. The potential for routing loops can also be
deadly when line capacity is limited. Once lines become
saturated by looping packets, it may be difficult or impossible for
routers to exchange the routing messages needed to break the
loops. Also, in a wide area network, hold doen periods are so
long that the timers used by higher level protocols can expire
and lead to broken connections. Despite these well-known