| 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 exhibits some behaviors of a link-state routing 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). EIGRP combines the advantages of
link-state and distance vector routing protocols. Properly designed and
implemented, an EIGRP network is extremely stable and efficient and
converges rapidly after any topological change.
As a hybrid protocol, some of the
specific advantages of EIGRP include the following:
- Rapid convergence ---
EIGRP 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. EIGRP uses an
algorithm called the Diffusing Update Algorithm (DUAL)
to achieve rapid convergence. A router running Enhanced IGRP stores
backup routes, when available, for destinations so that it can quickly
adapt to alternative routes. If no appropriate route or backup route
exists in the local routing table, EIGRP queries its neighbors to
discover an alternative route. These queries are propagated until an
alternative route is found. The DUAL algorithm guarantees loop-free
operation at every instant throughout a route computation and allows
all routers involved in a topology change to synchronize at the same
time. Routers that are not affected by topology changes are not
involved in recomputations. The convergence time with DUAL rivals that
of any other existing routing protocol.
- Efficient use of bandwidth during
convergence --- EIGRP does not
make periodic updates. Instead, it sends partial updates about a route
when the path changes or when the metric for that route changes. When
path information changes, the DUAL algorithm sends an update about that
link only, rather than about the entire table. In addition, the
information is sent only to the routers that need it, in contrast to
link-state protocol operation, which sends a change update to all
routers in an area. In EIGRP, this is known as a partial, bounded update.
- Minimal consumption of bandwidth when
the network is stable ---
During
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.
- Support for VLSM and CIDR --- EIGRP
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
protocols --- EIGRP 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.
- Multiple network-layer support ---
EIGRP
supports AppleTalk, IP, and Novell NetWare through the use of protocol
dependent modules (PDMs). These modules are responsible for
network-layer-specific protocol requirements.
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