Enhanced Interior Gateway Routing Protocol
IP Routing Fundamentals
Author: Mark Sportack
Publisher: Cisco Press (53)
Enhanced Interior Gateway
Routing Protocol, also known as either EIGRP or Enhanced IGRP, is a relatively
new innovation from Cisco that is based on IGRP. EIGRP shares its predecessor's
distance-vector technology, but differs greatly in its operational mechanics.
Additionally, EIGRP features several important new features. Some of these
features were designed to expand the market potential for EIGRP. Unlike its
predecessor, for example, EIGRP supports both classless and class-based IP
addresses, as well as other network protocols. This allows networks using
CIDR-compliant IP addresses and/or VLSM to use Cisco's distance-vector routing
Other updates were designed to reduce convergence times and improve
network stability. One such update was a new algorithm, the Diffusing Update Algorithm
(DUAL), which enables EIGRP routers to determine whether a path advertised
by a neighbor is looped or loop free and allows a router running EIGRP to
find alternative paths without waiting for updates from other routers. This
helps EIGRP networks converge, without incurring any significant risk of introducing
or propagating routing loops. Other measures were also introduced that reduced
the network intensity of convergence. This chapter examines the architectural
framework of EIGRP, its operational mechanics, and the similarities and differences
between EIGRP and IGRP.
The Internet is a highly dynamic network.
Consequently, its protocol, IP, must be equally dynamic. The IETF, it seems,
never ceases upgrading, modifying, and extending the IP specification. One
of the unfortunate consequences of this sort of activity is that many other
IP-centric technologies are forced to either keep the same pace of innovation
or fall into obsolescence. Frequently, such technologies opt for some point
in between these two extremes. IGRP, despite its success in the market, began
to demonstrate some of the effects of obsolescence relative to the ever-evolving
IGRP is still a highly useful routing protocol, but it just can't
support some of the more complex changes made to IPv4. IGRP is, for example,
inherently a classful IP routing protocol. Therefore, it can't support classless
interdomain routing (CIDR), or even variable-length subnet masks (VLSM).
Rather than modify IGRP, which may have caused migration issues for its customers,
Cisco Systems decided to implement the needed modifications in a new proprietary,
routing protocol. This strategy held many benefits for Cisco's customers.
For example, Cisco could offer customers a modern and robust routing protocol,
EIGRP, which was based on IGRP's proven distance-vector technology.
Equally important, developing EIGRP separately from IGRP offered Cisco
the opportunity to make fundamental improvements to the operational efficiency
of the new routing protocol. Had Cisco attempted to make such changes to IGRP,
the result may have been two very different versions of IGRP. The last benefit
of Cisco's strategy was that there was no impact on any of the existing IGRP
customer networks; IGRP soldiered on, unaffected by the development of EIGRP.
Although EIGRP was developed as a more up-to-date and efficient alternative to IGRP, it was
also explicitly an extension of IGRP. Consequently, the two are designed to
be completely compatible. These two routing protocols even share the same
distance-vector routing technology: EIGRP uses the same composite routing
metric as IGRP. EIGRP also supports all the same distance vectors, and their
mathematical weights, as does IGRP. EIGRP also uses IGRP's Variance feature
to provide unequal-cost load balancing.
There are strong similarities between many of IGRP and EIGRP's basic
components. These include the formula for calculating their respective composite
metrics, as well as equal or unequal-cost load balancing using the Variance
mechanism. If you would like more information on any of these topics, refer
to Chapter 10, “Interior Gateway Routing Protocol.”
There is only one minor difference in the algorithm that calculates
the composite metric: The IGRP metric is 20 bits long, whereas the
EIGRP metric is 32 bits long. This difference results in the EIGRP metric
being larger by a factor of 256 than a comparable IGRP metric for any given
route. The larger metric allows a better and finer mathematical comparison
of potential routes.
This minor difference is easily and automatically compensated for by
EIGRP. EIGRP automatically adjusts the composite metric of IGRP routes and
adjusts its own metric on routes being redistributed to IGRP routers. IGRP's
and EIGRP's metrics are directly comparable. Therefore, they can be used interchangeably
after translation. EIGRP does, however, track the translated IGRP routes as
IGRP doesn't have any concept of internal and external routes. Consequently,
EIGRP routes that are translated and redistributed into an IGRP network are
treated as IGRP routes.
Automatic redistribution between IGRP and EIGRP will only occur if the
two protocols are configured with the same autonomous system (AS) number.
If they have different AS numbers, they will assume that they are part of
different networks (that is, autonomous systems).
EIGRP's automatic metric adjustment mechanism enables IGRP and EIGRP
to be fully integrated in a network via a simple mathematical function. External
IGRP routes that are automatically adjusted by EIGRP can be directly compared
to internal EIGRP routes. Cisco routers will always select the route with
the best metric (adjusted or otherwise) rather than automatically select the
route of any particular protocol. Therefore, an EIGRP router might decide
that the best route is actually an external IGRP route rather
than an internal EIGRP
Migration from IGRP to EIGRP can be done gradually without incurring
any network downtime. EIGRP can be introduced into strategic areas of the
as the backbone. Its automatic metric translation mechanism would enable the
network administrator to replace IGRP with EIGRP in those strategic areas.
Support for IGRP is integral to EIGRP so the network's functionality isn't
compromised. The network administrator can then selectively extend its use
in the network, until the migration to EIGRP is complete. At this point, some
of the more advanced IP architectures, such as VLSM and classless addressing,
can be implemented.
Cisco also instituted several changes
in EIGRP that were designed to improve its operational efficiency relative
to IGRP. These two protocols are interoperable thanks to their mutual distance
vectors, but they operate in very different ways. EIGRP reacts to topological
change differently, advertises routes differently, and even has a different
approach to updating entries in routing tables. In many ways, EIGRP behaves
more like a link-state routing protocol than it does a traditional distance-vector
protocol. Yet, EIGRP uses the distance vectors and composite metric of IGRP.
Consequently, EIGRP is sometimes referred to as a hybrid
routing protocol (or an advanced
distance-vector protocol). It combines the best features of link-state
routing with the best features of distance-vector routing. Properly designed
and implemented, an EIGRP network is extremely stable and efficient and
converges rapidly after any topological change.
Some of the specific advantages of EIGRP include the following:
Minimal consumption of bandwidth when the network is stableDuring normal, stable network operation, the only EIGRP packets exchanged
between EIGRP nodes are hello packets. This simple handshake enables the EIGRP
routers to know that all remains well in the network.
Efficient use of bandwidth during convergenceEIGRP only propagates changes to the routing table, not the entire routing
table. Also, updates are advertised only after a topological change rather
than on a strict, periodic basis. These updates are also only transmitted
to those EIGRP routers that need to know of the change. This is known as a partial,
Rapid convergenceEIGRP routers store every path they have learned to every destination
in the network. Therefore, a router running EIGRP can quickly converge on
an alternative route after any topological change.
Support for VLSM and CIDREIGRP supports
the definition of network and host numbers on any bit boundary, per interface,
for both IP addresses and subnet masks.
Complete independence from routed protocolsEIGRP is
designed to be completely independent of routed protocols. Support for routed
protocols is via individual, protocol- specific modules. Therefore, evolution
of a protocol, such as IP, won't threaten EIGRP with obsolescence. Nor will
such technological advances force a painful revision of EIGRP.
The features highlighted in this list are examined in more detail throughout
this chapter. Combined, they make a compelling case for migrating to EIGRP.
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