Enhanced Interior Gateway

Enhanced Interior Gateway
Routing Protocol
Enhanced Interior Gateway Routing Protocol (EIGRP) is better than its little brother, IGRP.
EIGRP allows for incremental routing updates and formal neighbor relationships, which overcome
some of the limitations of IGRP. The enhanced version uses the same type of information as IGRP,
obtained by distance-vector methods, yet with a different algorithm, and scaled for a 32-bit metric,
as opposed to IGRP’s 24-bit metric. EIGRP uses DUAL (Diffusing Update Algorithm) for metric
calculation, which permits rapid convergence. This algorithm allows for the following:
 Automatic backup route determination, if one is available
 Sending out queries for an alternate route if no route can be found
 Support of variable-length subnet masks (VLSMs)
 Reliable communication with neighbors
EIGRP fixes many of the problems associated with IGRP, such as the propagation of the
entire routing table, which IGRP sends when changes occur in the network topology, as well as
periodically, even in the absence of change.
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 types.
Along with rapid convergence discussed earlier, EIGRP reduces bandwidth usage. It does this
not by making scheduled updates, but by sending updates only when a topology change occurs.
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.
Basically, EIGRP establishes neighbor relationships and keeps tables in addition to the routing
table, just like link-state protocols. However, just like distance-vector routing protocols, EIGRP
still routes by rumor and trusts whatever it hears from its adjacent neighbors. Related to this
behavior, unlike link-state routing protocols, EIGRP does not build a hierarchical view of the
entire routing domain with itself as the root of the inverted tree.
Enhanced Interior Gateway Routing Protocol 119
One of the most attractive features of EIGRP is its support of all 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. Besides the
protocol-dependent modules, EIGRP has three other components that make up the four components
of EIGRP:
 Reliable Transport Protocol (RTP)
 Neighbor discovery/recovery
 Diffusing Update Algorithm (DUAL)
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- or unequal-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 runs only on Cisco routers and route
switch processors.
Now that we have mentioned all this, we’ve sold you on EIGRP, right? Well, if we stopped
right here, you’d miss out on many other important details of the route-tagging process, neighbor
relationships, route calculation, 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, and troubleshoot. We will also briefly cover redistributing routes. This
topic will be covered in greater detail in Chapter 10.