Tài liệu What Is Administrative Distance? pdf

Cisco - What Is Administrative Distance?

What Is Administrative Distance?

Introduction

Most routing protocols have metric structures and algorithms that are not compatible with other

protocols. In a network where multiple routing protocols are present, the exchange of route information

and the capability to select the best path across the multiple protocols are critical.

Administrative distance is the feature used by routers to select the best path when there are two or more

different routes to the same destination from two different routing protocols. Administrative distance

defines the reliability of a routing protocol. Each routing protocol is prioritized in order of most to least

reliable (believable) using an administrative distance value.

Selecting the Best Path

Administrative distance is the first criterion that a router uses to determine which routing protocol to use

if two protocols provide route information for the same destination. Keep in mind that administrative

distance has only local significance, that is, it is not advertised in routing updates.

Note: The smaller the administrative distance value, the more reliable the protocol. For example, if a

router receives a route to a certain network from both Open Shortest Path First (OSPF) (default

administrative distance - 110) and Interior Gateway Routing Protocol (IGRP) (default administrative

distance - 100), the router will choose IGRP because it is more reliable. This means the IGRP version of

the route would be added to the routing table.

If you lose the source of the IGRP-derived information (for example, because of a power shutdown), the

software uses the OSPF-derived information until the IGRP-derived information reappears.

The table below lists the administrative distance default values of the protocols that Cisco supports.

Route Source Default Distance Values

Connected interface 0

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