Network Warrior

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?

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 (AD). 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.

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 ...