Cisco CCNA Routing and Switching ICND2 200-101 Official Cert Guide

Administrative Distance

Administrative Distance

OSPF can choose the best route for each subnet based on OSPF’s metric, as discussed in the previous page or two of this chapter. However, IOS must also be able to choose the best route to reach a subnet when the router knows of routes from different sources. To make such a choice, routers use a concept called administrative distance (AD).

First, think about a common case, in which all the routers use OSPF, and only OSPF, as the one routing protocol for IPv4. A router learns routes with OSPF, but it also learns connected routes. It may also have some ip route commands that define static IPv4 routes. ...