Administrative Distance

The administrative distance selects one path to enter into the routing table from several paths offered by multiple routing protocols.

In Figure 10-6, for example, both RIP and EIGRP have paths to the network 140.100.6.0. RIP is offering a metric of 2 hops, and EIGRP is tendering a metric of 768. Without redistribution, no conversion or choice is possible because there is no similar criteria for distinguishing the two paths. Therefore, the metric is ignored, and the administrative distance is used to make the selection.

In Figure 10-6, despite the speed of the Frame Relay being set at 56 kbps instead of the 100 Mbps of FDDI, Router D would select the Frame Relay path based on administrative distance. In this case, manually configuring the administrative distance on Router D would be advisable.

The administrative distance is a rather arbitrary set of values placed on the different sources of routing information. The defaults can be changed, but care should be taken when subverting the natural path selection, and any manual configuration must be done with careful reference to the network design of the organization and its traffic flow.

The administrative distance reflects the preferred choice. The defaults are listed in Table 10-3.

Figure 10-6 Path Selection Using Administrative Distance

Routing protocol = EIGRP

Administration distance path selection

140.100.6.0

140.100.6.0

Administration distance path selection

Routing protocol = RIP

Table 10-3 Default Administrative Distance

Routing Source

Administrative Distance

Connected interface or static route that identifies 0

the outgoing interface rather than the next logical hop

Static route

EIGRP summary route

External BGP

EIGRP

IGRP

OSPF

IS-IS

RIP v1, v2

continues

Table 10-3 Default Administrative Distance (Continued)

Routing Source

Administrative Distance

EGP

140

External EIGRP

170

Internal BGP

200

An unknown network

255 or infinity

The administrative distance can be manually configured. The reason for manually configuring the administrative distance for a protocol such as EIGRP is that it may have a less desirable path compared to one offered by another protocol such as RIP, which has a higher default AD.

The administrative distance is looked at with total disregard of the metrics. In Figure 10-6, this means that the EIGRP path will be selected even though it is the slower and more expensive link. However, the Enhanced IGRP path will be selected.

Another occasion when the administrative distance would select the suboptimal path is that of a directly connected network. A network that is directly connected to the router has precedence in terms of administrative distance. In this instance, however, it is being used as a backup link for redundancy because the directly connected network is not used on a daily basis. Backup links for redundancy are often implemented on serial connections in which the network charges are based on usage. This design is called a floating static route.

NOTE Details on how to change the administrative distance are given in the section "Configuring Redistribution."

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