Administrative Distance, Access Lists, Route Manipulation, and IP Multicast - CCIE Routing and Switching Exam Certification Guide - page 474

Foundation Topics

Administrative Distance
On routers running several IP routing protocols, two different routing protocols might have a
route to the same destination. Cisco routers assign each routing protocol an administrative
distance. The default values are shown in Table 10-1. In the event that two or more routing
protocols offer the same route for inclusion in the routing table, the Cisco IOS selects the route
from the routing protocol with the lowest administrative distance.

Default Administrative Distances for IP Routes

Table 10-1

IP Route Administrative Distance

Connected interface 0

Static route directed to a connected interface 0

Static route directed to an IP address 1

Enhanced Interior Gateway Protocol (EIGRP) summary route 5

External BGP route 20

Internal EIGRP route 90

Interior Gateway Protocol (IGRP) route 100

Open Shortest Path First (OSPF) route 110

Intermediate System-to Intermediate System (IS-IS) route 115

Routing Information Protocol (RIP) route 120

Exterior gateway protocol (EGP) route 140

External EIGRP route 170

Internal BGP route 200

Route of unknown origin 255

The administrative distance establishes a precedence among routing algorithms. A router has
an internal EIGRP route to network 172.20.10.0/24 with the best path out Ethernet 0 and an
OSPF route for the same network out Ethernet 1. Because EIGRP has an administrative
distance of 90 and OSPF has an administrative distance of 110, you enter the EIGRP route in
the routing table, and packets with destinations of 172.20.10.0/24 are sent out Ethernet 0.