D-Link DGS-3308 Series User Manual page 59

8-port Gigabit Ethernet Switch User's Guide
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, R IP must either assume participants can be trusted or take precautions to prevent such loops.
Second, 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.
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 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
down periods are so long that the timers used by higher level protocols can expire and lead to broken connections. Despite
these well-known problems, many groups continue to use RIP and an IGP in wide area networks.
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