| Not only does the topology table track
information regarding route states, but it also can record special
information about each route. EIGRP classifies routes as either
internal or external. EIGRP uses a process called route tagging to
add special tags to each route. These tags identify a route as
internal or external, and may include other information as well.
Internal routes originate from within
the EIGRP AS. External routes originate from outside the system.
Routes learned (redistributed) from other routing protocols, such as
RIP, OSPF, and IGRP are external. Static routes originating from
outside the EIGRP AS and redistributed inside are also external
routes.
All external routes are included in
the topology table and are tagged with the following information:
- The identification number (router
ID) of the EIGRP router that redistributed the route into the
EIGRP network
- The AS number of the destination
- The protocol used in that external
network
- The cost or metric received from
that external protocol
- The configurable administrator tag
The figure shows a specific topology
table entry for an external route.
To develop a precise routing policy,
take advantage of the route tagging and, in particular, the
administrator tag (shown in the shaded portion of the figure). You
can configure the administrator tag to be any number between 0 and
255; in effect, this is a custom tag that you can use to implement a
special routing policy. External routes can be accepted, rejected,
or propagated based on any of the route tags, including the
administrator tag. Because you can configure the administrator tag
as you see fit, the route-tagging feature affords a high degree of
control. This level of precision and flexibility proves especially
useful when EIGRP networks interact with BGP networks, which
themselves are policy-based. You will learn more about BGP in
Chapter 8, BGP, and Chapter 9, Scaling BGP.
|