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Chapter 6
Querying neighbors for unknown alternate routes
Sending out queries for an alternate route if no route can be found
EIGRP fixes many of the problems associated with IGRP, such as the
propagation of the entire routing table, which is sent when changes occur in
the network topology. One unique characteristic of EIGRP is that it is both
a link-state routing and a distance-vector protocol. How can this be? Let's
look at how this protocol combines the best from both routing protocol
Along with rapid convergence discussed above, EIGRP reduces band-
width usage. It does this by not making scheduled updates but sending
updates only when topology changes occur. When EIGRP does send an
update, the update contains information only on the change in the topology,
which requires a path or metric change. Another plus is the fact that only the
routers that need to know about the change receive the update.
One of the best features is that the routing protocol supports all of the
major Layer 3 routed protocols using protocol-dependent modules (PDMs),
those being IP, IPX, and AppleTalk. At the same time, EIGRP can maintain
a completely loop-free routing topology and very predictable behavior, even
when using all three routed protocols over multiple redundant links.
With all these features, EIGRP must be hard to configure, right? Guess
again. Cisco has made this part easy as well and allows you to implement
load balancing over equal-cost links. So why would you use anything else?
Well, I guess you might if all your routers weren't Cisco routers. Remember,
EIGRP is proprietary and only runs over Cisco routers and internal route
Now that we have mentioned all this, we've sold you on EIGRP, right?
Well, if we stopped right here, you would miss out on many other important
details of the route-tagging process, neighbor relationships, route calcula-
tion, and the metrics used by EIGRP, which will be discussed in the next few
sections. Following that discussion, we will look at how to configure EIGRP,
tune EIGRP, load balance, redistribute routes, and troubleshoot.
Route Tagging
Route tagging is used to distinguish routes learned by the different EIGRP
sessions. By defining a different AS number, EIGRP can run multiple sessions
on a single router. Routers using the same ASN speak to each other and share
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