Administrative distance is an arbitrary numerical value assigned to a routing
protocol, a static route or a directly-connected route based on its
perceived quality of routing. The administrative distance value is often
used by Cisco routers to determine the "best" route that should be used when
multiple paths to the same destination exist. A routing protocol with a lower
administrative distance is considered "better" and is given priority over
routing protocols with higher administrative distances. The "better"
route is selected by the router and is inserted into the router's routing table
to be used to route traffic. For example, routes issued by EIGRP are
considered more reliable than routes issued by RIP. This is because EIGRP has
an administrative distance of 90 while RIP has an administrative distance of
120. The administrative distance value may be set manually by a network
administrator. If a network administrator fails to properly set the
administrative distance or configures multiple routing protocols with the same
administrative distance, the router will use the default administrative distance.
Default administrative distances The following table lists the default
administrative distances for various routing protocols used on Cisco routers.
Please note that:- An administrative distance of 255 will
cause the router to disbelieve the route entirely and not use it. An
administrative distance of 255 will cause the router to remove the route
from the RIB and not use it.* Since IOS 12.2, the administrative distance of a
static route with an exit interface is 1. Prior to the release of 12.2 it was
in fact 0. Only the interface itself has an
administrative distance of 0, since a route cannot be less than 1.
Directly connected routes have an administrative distance of 0.
Verifying administrative distance Directly connected routes have an
administrative distance of 1. This can be verified by issuing the show ip route
command in privileged exec mode on the console of the Cisco router. For
example, the following commands can be issued in privileged exec mode on the
router:- Router# configure terminal
Router(config)# ip route 1.1.1.0 255.255.255.0 fastEthernet 0/0
Router(config)# do show ip route The do show ip route command will
display the following, confirming that a static route has an administrative
distance of 1. S 1.1.1.0/0 [1/0] via 172.31.0.1
Modifying the administrative distance on a Cisco router
In many cases, the administrative distance can lead to unexpected results,
or lead to a route being selected that is not necessarily the "best" route.
This is especially true if routing redistribution is being used on the
router. In this case, an administrator would usually modify the administrative
distance manually, so that the desired routing protocols' routes take
precedence over other routes. For example, if an administrator wanted
RIP routes to take precedence over OSPF routes, he may modify the administrative
distance in order to do this. The following commands show an example of
how to do this. R1> enable
R1# configure terminal R1(config)# router rip
R1(config-router)# distance 90 Static routes may be used to provide an
alternate path when a link fails. In order for static routes to be configured
as a backup, the static route's administrative distance would need to be
adjusted. Otherwise, it will take precedence over all routing protocols
and routes issued from a routing protocol will not be inserted into the
routing table. The following shows an example of the
commands an administrator would type when setting the administrative
distance. The value of 254 is the administrative distance for the newly
created static route. R1(config)# ip route 10.0.0.0 255.0.0.0
backupLink 1 254 In the event that two routing protocols
are configured with the same administrative distance, the Cisco
router will ignore the configured values and instead use the default values.
See also Routing
Dynamic routing Routing protocol
Routing table Router
Router Information Protocol References
External links Cisco - What is Administrative Distance?