Administrative Distance

Cisco routers are capable of supporting several IP routing protocols concurrently. When identical prefixes are discovered from two or more separate sources. Administrative Distance (AD) is used to discriminate between the paths. AD is a poor choice of words; trustworthiness is a better name. Routers use paths with the lower AD.

Table 1-1 lists the default values for various routing protocols. Of course, there are several ways to change AD for a routing protocol or for a specific route.

TABLE 1-1 Routing Protocols and Their Default Administrative Distance

Information Source

AD

Connected

0

Static 1

External BGP (Border Gateway Protocol)

20

Internal EIGRP (Enhanced IGRP)

90

IGRP (Internet Gateway Routing Protocol) LOO

Information Source

AD

OSPF (Open Shortest Path First)

no

IS-IS (Intermediate System to Intermediate System)

115

RIP (Routing Information Protocol)

120

ODR (On Demand Routing)

160

External EIGRP

170

Internal BGP

200

Unknown

255

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