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Chapter 2

Protocol Attributes

The topology database, where all route information resides

A neighbor table, which is used to house information concerning other
EIGRP neighbors

Each of these databases exists for IP-EIGRP, IPX-EIGRP, and AT-EIGRP

or AppleTalk-EIGRP. Therefore, it is possible for EIGRP to have nine active
databases when all three protocols are configured on the router.

Route Tagging

EIGRP functions within defined autonomous systems on a router. It is
possible for multiple sessions of EIGRP to run on a single router. Each ses-
sion is distinguished by the AS number assigned to it. Routers that have
Enhanced IGRP sessions running under the same AS number speak to
and share their routing information with the other routers in the same AS.
Routes learned via other routers within the AS are considered to be internal
EIGRP routes. It is also possible for one AS session to learn routes from a
different EIGRP AS session through redistribution (redistribution is covered
later in this section). When this occurs, the routes are tagged as being learned
from an external EIGRP session. Each type of route is assigned its own
administrative distance value.

Neighbor Relationships

The manner in which EIGRP establishes and maintains neighbor relation-
ships is derived through its link-state properties. EIGRP uses the Hello pro-
tocol (similar to OSPF) to establish and maintain peering relationships with
directly connected routers. Hello packets are sent between EIGRP routers to
determine the state of the connection between them. Once the neighbor rela-
tion is established via the Hello protocol, the routers can exchange route
information.

Each router establishes a neighbor table, in which it stores important

information regarding the neighbors that are directly connected to it. The
information consists of the neighbor's IP address, hold time interval, smooth
round trip timer (SRTT), and queue information. These data are used to help
determine when the link state changes.

When two routers initialize communication, their entire route tables are

shared. Thereafter, only changes to the route table are propagated. These
changes are shared with all directly connected EIGRP-speaking routers. The
steps are summarized as follows:

Hello packets are multicast out of all of the router's interfaces.

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