Routing Protocol Administrative Distance

The previous sections in this chapter provide a high-level overview of IP routing protocols from the perspectives of design, architecture, and operation. The section discusses briefly generic implementation- related issues that impact operation of these protocols on Cisco routers. Details of operation and configuration of each protocol are covered in the protocol-specific chapters.

Cisco IOS Software provides common command resources for configuring and enabling the capabilities of IP routing protocols. Commands such as distance, distribute-list, redistribute, route-map, policy-map, access-list, prefix-list, offset-list, and so forth frequently are referred to as protocol-independent commands because they can be used in diverse ways to enable many features in Cisco IOS Software, including routing protocol capabilities. In their application to routing protocols, protocol-independent commands are used for filtering routes, enabling redistribution, configuring default routes, and imple-menting various routing policies. You can find more detail on these commands online at www.cisco.com; however, this section discusses the distance command and the feature that it supports ‚ administrative distance.

All the IP routing protocols discussed so far can operate concurrently and yet independently on Cisco routers if enabled together. Usually, only one IGP (OSPF or IS-IS) is required to run alongside BGP in an IP network. However, depending on the situation and the history of a network, more than one IGP might be operation to support routing requirements.

Administrative distance is a Cisco-specific method of distinguishing between routes obtained from different routing sources in the same network. It provides a simple mech-anism to differentiate believability of routing information sources. Cisco IOS Software assigns numeric values to routing sources that allow routes from one routing source to be preferred over similar routes from another source. Sources with lower administrative distance values are preferred. When multiple protocols supply the same route, only the route from the source with the lower administrative distance will make it into the routing table. Table 1-5 lists the default administrative distances of IP routing sources. The distance command can be used to modify any of the defaults.

Table 1-5. Administrative Distances of IP Routing Protocols

Route Source	Administrative Distance
Connected interface
Static route out an interface	1
Static route to a next hop	1
EIGRP summary route	5
External BGP	20
Internal EIGRP	90
IGRP	100
OSPF	110
IS-IS	115
RIP-1/RIP-2	120
EGP	140
External EIGRP	170
Internal BGP	200
Unknown	255