Cisco 15327 User Manual download pdf (Page 178)

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Ethernet Card Software Feature and Configuration Guide, R7.2

Chapter 11 Configuring Networking Protocols

Configuring EIGRP

Enhanced IGRP (EIGRP) is a Cisco proprietary enhanced version of the Interior Gateway Routing

Protocol (IGRP). Enhanced IGRP uses the same distance vector algorithm and distance information as

IGRP; however, the convergence properties and the operating efficiency of Enhanced IGRP are

significantly improved.

The convergence technology employs an algorithm referred to as the Diffusing Update Algorithm

(DUAL), which guarantees loop-free operation at every instant throughout a route computation and

allows all devices involved in a topology change to synchronize at the same time. Routers that are not

affected by topology changes are not involved in recomputations.

IP EIGRP provides increased network width. With RIP, the largest possible width of your network is

15 hops. When IGRP is enabled, the largest possible width is 224 hops. Because the EIGRP metric is

large enough to support thousands of hops, the only barrier to expanding the network is the

transport-layer hop counter. EIGRP increments the transport control field only when an IP packet has

traversed 15 routers and the next hop to the destination was learned through EIGRP. When a RIP route

is used as the next hop to the destination, the transport control field is incremented as usual.

EIGRP offers the following features:

•

Fast convergence

•

Incremental updates when the state of a destination changes, instead of sending the entire contents

of the routing table, minimizing the bandwidth required for EIGRP packets

•

Less CPU usage than IGRP because full update packets do not need to be processed each time they

are received

•

Protocol-independent neighbor discovery mechanism to learn about neighboring routers

•

Variable-length subnet masks (VLSMs)

•

Arbitrary route summarization

•

EIGRP scales to large networks

EIGRP has four basic components:

•

Neighbor discovery and recovery is the process that routers use to dynamically learn of other routers

on their directly attached networks. Routers must also discover when their neighbors become

unreachable or inoperative. Neighbor discovery and recovery is achieved with low overhead by

periodically sending small hello packets. As long as hello packets are received, the Cisco IOS

software can determine that a neighbor is alive and functioning. When this status is determined, the

neighboring routers can exchange routing information.

•

The reliable transport protocol is responsible for guaranteed, ordered delivery of EIGRP packets to

all neighbors. It supports intermixed transmission of multicast and unicast packets. Some EIGRP

packets must be sent reliably, and others need not be. For efficiency, reliability is provided only

when necessary. For example, on a multiaccess network that has multicast capabilities (such as

Ethernet), it is not necessary to send hellos reliably to all neighbors individually. Therefore, EIGRP

sends a single multicast hello with an indication in the packet informing the receivers that the packet

need not be acknowledged. Other types of packets (such as updates) require acknowledgment, which

is shown in the packet. The reliable transport has a provision to send multicast packets quickly when

there are unacknowledged packets pending. Doing so helps ensure that convergence time remains

low in the presence of varying speed links.

•

The DUAL finite state machine embodies the decision process for all route computations. It tracks

all routes advertised by all neighbors. DUAL uses the distance information (known as a metric) to

select efficient, loop-free paths. DUAL selects routes to be inserted into a routing table based on

feasible successors. A successor is a neighboring router used for packet forwarding that has a

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