TCP/IP Tutorial and Technical Overview - IBM Redbooks

EIGRP processes the information in the topology table to determine the best
paths to each destination network. EIGRP implements an algorithm known as
Diffusing Update ALgorithm (DUAL).
Route recomputation
For a specific destination, the successor is the neighbor router currently used for
packet forwarding. This device has the least-cost path to the destination and is
guaranteed not to be participating in a routing loop. A feasible successor
assumes forwarding responsibility when the current successor router fails. The
set of feasible successors represent the devices that can become a successor
without requiring a route recomputation or introducing routing loops.
A route recomputation occurs when there is no known feasible successor to the
destination. The successor is the neighbor router currently used for packet
forwarding. The process starts with a router sending a multicast query packet to
determine if any neighbor is aware of a feasible successor to the destination. A
neighbor replies if it has an feasible successor.
If the neighbor does not have a feasible successor, the neighbor can return a
query indicating it also is performing a route recomputation. When the link to a
neighbor fails, all routes that used that neighbor as the only feasible successor
require a route recomputation.
5.7.2 EIGRP packet types
EIGRP uses five types of packets to establish neighbor relationships and
advertise routing information:
► Hello/acknowledgement: These packets are used for neighbor discovery.
They are multicast advertised on each network segment. Unicast responses
to the hello packet are returned. A hello packet without any data is considered
an acknowledgement.
► Updates: These packets are used to convey reachability information for each
destination. When a new neighbor is discovered, unicast update packets are
exchanged to allow each neighbor to build their topology table. Other types of
advertisements (for example, metric changes) use multicast packets. Update
packets are always transmitted reliably.
► Queries and replies: These packets are exchanged when a destination enters
an active state. A multicast query packet is sent to determine if any neighbor
contains a feasible successor to the destination. Unicast reply packets are
sent to indicate that the neighbor does not need to go into an active state
because a feasible successor has been identified. Query and reply packets
are transmitted reliably.
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