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OSI Layer 3: Routed and Routing Protocols
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The route with the best metric (lower is better) is chosen as the primary
route. The backup route is chosen by selecting the route with the second-
lowest metric. Primary routes are moved to the route table after selection. It
is possible to have more than one primary route.
EIGRP uses the same vector information as IGRP: bandwidth, delay, reliabil-
ity, load, and MTU. Bandwidth and delay are the two metrics used by default;
the others can be configured manually. Configuring reliability, load, and MTU
can cause the topology table to be calculated more often.
Updates and Changes
EIGRP also has link-state properties. One of these properties is that it prop-
agates only changes in the route table instead of sending an entire new route
table to its neighbors. When changes occur in the network, a regular dis-
tance-vector protocol sends the entire route table to neighbors. By avoiding
sending the entire route table, less bandwidth is consumed. Neighboring
routers don't have to reinitialize the entire route table, which would cause
convergence issues. They just have to insert the new route changes. This is
one of the big enhancements over IGRP.
Updates can follow two paths. If a route update contains a better metric
or a new route, the routers simply exchange the information. If the update
contains information that a network is unavailable or the metric is worse
than before, an alternate path must be found. The flow chart in Figure 2.15
describes the steps that must be taken to choose a new route.
T A B L E 2 . 9
Topology Table Information
Status
Route--Adjacent Router's Address
(Metrics)
Number of
Successors
Feasible
Distance
P
10.10.10.0/24
via 10.1.2.6
(3611648/3609600)via 10.5.6.6
(4121600/3609600)via 10.6.7.6
(5031234/3609600)
1
(Router C)
3611648
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