Cisco IE 4000, Software Configuration Manual

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Configuring IP Unicast Routing
Configuring EIGRP
Configuring EIGRP
Enhanced IGRP (EIGRP) is a Cisco proprietary enhanced version of the Interior Gateway Routing Protocol (IGRP). EIGRP
uses the same distance vector algorithm and distance information as IGRP; however, the convergence properties and
the operating efficiency of EIGRP 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. 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.
EIGRP has these 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 by periodically sending small hello packets. As long as hello packets are
received, the neighbor is alive and functioning. When this status is determined, the neighboring routers 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, 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.
To ensure low convergence time, the reliable transport sends multicast packets quickly when there are
unacknowledged packets pending.

The DUAL finite state machine handles the decision process for all route computations. It tracks all routes advertised
by all neighbors and 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 least-cost path to a destination that is guaranteed not to be part of a routing
loop.
When there are no feasible successors, but there are neighbors advertising the destination, a recomputation must
occur to determine a new successor. The amount of time it takes to recompute the route affects the convergence
time. When a topology change occurs, DUAL tests for feasible successors to avoid unnecessary recomputation.

The protocol-dependent modules are responsible for network layer protocol-specific tasks. An example is the IP
EIGRP module, which is responsible for sending and receiving EIGRP packets that are encapsulated in IP. It is also
responsible for parsing EIGRP packets and informing DUAL of the new information received. Routing decisions are
stored in the IP routing table. EIGRP also redistributes routes learned by other IP routing protocols.
This section includes the following topics:

Default EIGRP Configuration, page 839

Configuring Basic EIGRP Parameters, page 840

Configuring EIGRP Interfaces, page 841

Configuring EIGRP Route Authentication, page 842

Configuring EIGRP Stub Routing, page 844

Monitoring and Maintaining EIGRP, page 845