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As demonstrated through the examples in the
preceding section, convergence is absolutely critical to a network's
capability to respond to operational fluctuations. The key factor in
convergence is communications among the routers in the network. Routing
protocols are responsible for providing this function. Specifically, these
protocols are designed to enable routers to share information about routes
to the various destinations within the network.
Note: Route Flapping,
one symptom of network instability that may arise is known as route
flapping. Route flapping is just the rapid vacillation between two, or
more, routes. Flapping happens during a topology change. All the routers
in the network must converge on a consensus of the new topology. Toward
this end, they begin sharing routing information.
In an unstable network, a router (or routers) may be unable to decide
on a route to a destination. Remember that during convergence a router may
alter its primary route to any given destination as a result of the last-
received update. In complex, but unstable networks with redundant routes,
a router may find itself deciding on a different route to a given
destination every time it receives an update. Each update nullifies the
previous decision and triggers another update to the other routers. These
other routers, in turn, adjust their own routing tables and generate
"new" updates. This vicious cycle is known as flapping. You may
find it necessary to power down affected routers and slowly develop
convergence in your network, one router at a time.
Unfortunately, all routing protocols are not created equal. In fact,
one of the best ways to assess the suitability of a routing protocol is to
evaluate its capabilities to calculate routes and converge relative to
other routing protocols. It should be obvious from the previous list of
factors that convergence times may be difficult for you to calculate with
any degree of certainty. Your router vendor may be able to assist you with
this process, even if the vendor provides you with general estimates only.
A routing protocol's convergence capability is a function of its
capability to calculate routes. The efficacy of a routing protocol's route
calculation -
is based on several factors:
- Whether the protocol calculates, and stores, multiple routes to each
destination
- The manner in which routing updates are initiated
- The metrics used to calculate distances or costs
Storing Multiple Routes
Some routing protocols attempt to improve their operational efficiency
by only recording a single route (ideally, the best route) to each known
destination. The drawback to this approach is that when a topology change
occurs, each router must calculate a new route through the network for the
impacted destinations. Examples will be seen in the labs and upcoming
Routing Protocol Chapters.
Other protocols accept the processing overheads that accompany larger
routing table sizes and store multiple routes to each destination. Under
normal operating conditions, multiple routes enable the router to balance
traffic loads across multiple links. If, or when, a topology change
occurs, the routers already have alternative routes to the impacted
destinations in their routing tables. Having an alternative route already
mapped out does not necessarily accelerate the convergence process. It
does, however, enable networks to more gracefully sustain topology
changes.
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