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Network Warrior, 2nd Edition by Gary A. Donahue

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Administrative Distance

Networks often have more than one routing protocol active. In such situations, there is a high probability that the same networks will be advertised by multiple routing protocols. Figure 10-7 shows a network in which two routing protocols are running: the top half of the network is running RIP, and the bottom half is running OSPF. Router A will receive routes for the network 20.0.0.0 from RIP and OSPF. RIP’s route has a better metric, but, as we’ve seen, OSPF has a better means of determining the proper path. So, how is the best route determined?

Competing routing protocols

Figure 10-7. Competing routing protocols

Routers choose routes based on a predetermined set of rules. One of the factors in deciding which route to place in the routing table is administrative distance. Administrative distance is a value assigned to every routing protocol. In the event of two protocols reporting the same route, the routing protocol with the lowest administrative distance will win, and its version of the route will be inserted into the RIB.

The administrative distances of the various routing protocols are shown in Table 10-1.

Table 10-1. Administrative distances of routing protocols

Route type

Administrative distance

Connected interface

0

Static route

1

EIGRP summary route

5

External BGP

20

Internal EIGRP

90

IGRP

100

OSPF

110

IS-IS

115

RIP

120

EGP

140

ODR

160

External EIGRP

170

Internal BGP

200

Unknown

255

A static route to a connected ...

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