EIGRP - Routing Protocols and Concepts - page 396

EIGRP Concepts Exercise

Roots of EIGRP: IGRP

Cisco developed the proprietary IGRP in 1985 in response to some of the limitations of RIPv1 ,
including the use of the hop count metric and the maximum network size of 15 hops .

Instead of hop count , both IGRP and EIGRP use metrics composed of bandwidth , delay , reliability ,
and load . By default, both routing protocols use only bandwidth and delay . However, because IGRP is
a classful routing protocol, its usefulness is limited in many of today's networks.

Therefore, Cisco enhanced IGRP with a new algorithm, DUAL , and other features.

Table 9-1 summarizes the main differences between a traditional distance vector routing protocol,
such as RIP, and the enhanced distance vector routing protocol, EIGRP. Fill in the missing words and
phrases.

Table 9-1 Comparing Traditional Distance Vector and EIGRP

Traditional Distance Vector Enhanced Distance Vector

Routing Protocols Routing Protocol: EIGRP

Uses the Bellman - Ford or Ford-Fulkerson Uses the Diffusing Update Algorithm (DUAL).

algorithm.

Ages out routing entries and uses . Does not age out routing entries nor use periodic

periodic updates updates.

Keeps track of only the best routes, . Maintains a topology table separate from the

the best path to a destination network routing table, which includes the best path and

any loop - free backup paths.

When a route becomes unavailable, When a route becomes unavailable, DUAL will use

the router must wait for a new routing update. a backup path if one exists in the topology table.

Slower convergence because of Faster convergence because of the absence of

-hold-down timers. hold-down timers and a system of coordinated

route calculations.

Algorithm

Traditional distance vector routing protocols all use some variant of the Bellman - Ford or Ford-
Fulkerson algorithm. These protocols, such as RIP and IGRP , age out individual routing entries and
therefore need to periodically send routing table updates.

EIGRP uses the Diffusing Update Algorithm (DUAL). EIGRP uses a lightweight Hello protocol to
monitor connection status with its neighbors. Only changes in the routing information, such as a new
link or a link becoming unavailable, cause a routing update to occur.

Path Determination

DUAL maintains a topology table separate from the routing table, which includes both the best path to
a destination network and any backup paths that DUAL has determined to be loop - free . In EIGRP,
loop - free means that a route has met a requirement known as the feasibility condition .