OSI Layer 3: Routed and Routing Protocols
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Enhanced IGRP (EIGRP)
Enhanced IGRP (EIGRP) is a hybrid link-state and distance-vector routing
protocol that was created to resolve some of the problems with IGRP.
Problems include the fact that the entire route table is sent when changes are
made in the network and that there is a lack of formal neighbor relation-
ships with connected routers. EIGRP is a hybrid of both link-state and
distance-vector routing algorithms, which brings the best of both worlds
together.
EIGRP allows for equal-cost load balancing, incremental routing updates,
and formal neighbor relationships, all of which overcome the limitations
of IGRP. This enhanced version uses the same distance-vector information
as IGRP, yet with a different algorithm. EIGRP uses DUAL (Diffusing-
Update Algorithm) for metric calculation.
EIGRP's specific features are detailed in Table 2.8. The features offered by
EIGRP make it a stable and scalable protocol. Just as IGRP is a Cisco
proprietary protocol, so is EIGRP.
To aid in calculating the best route and load sharing, EIGRP utilizes sev-
eral databases of information. These databases are as follows:
The route database, where the best routes are stored
T A B L E 2 . 8
EIGRP Features
Feature
Description
Route tagging
Distinguishes routes learned via different EIGRP
sessions.
Formal neighbor
relationships
Uses the Hello protocol to establish peering.
Incremental rout-
ing updates
Only changes are advertised, instead of the entire
route table.
Classless routing
EIGRP supports subnet and VLSM information.
Configurable
metrics
Metric information can be set through configura-
tion commands.
Equal-cost load
balancing
Allows traffic to be sent equally across multiple
connections.
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