Administrative distance

Administrative distance (AD) is a number of arbitrary unit assigned to dynamic routes, static routes and directly-connected routes. The value is used by vendor-specific routers to rank routes from most preferred (low administrative distance value) to least preferred (high administrative distance value).^[1][2] When multiple paths to the same destination are available, the router uses the route with the lowest administrative distance and inserts the preferred route into its routing table. Router vendors typically design their routers to assign a default administrative distance to each kind of route that is used, however, this value can usually be set manually by a network administrator.^[1]

On Cisco routers, routes issued by OSPF have a lower (default) administrative distance than routes issued by the Routing Information Protocol. By default, OSPF has a default administrative distance of 110 and RIP has a default administrative of 120.

Overview

The administrative distance (AD) value is assigned by the router on a per-protocol basis. Routers, by design, should not install multiple routes into the routing table as this has the potential to cause routing loops.^[1] While a router may run multiple routing protocols on the same device, it is necessary for the router to implement a process to ensure that multiple routes, pointing to the same destination do not simultaneously exist in the routing table. Each process running on a router advertises its administrative distance value to the local router. The router uses this value to determine which route should be used. Once a route has been selected, the routing information database is updated. If two routes have the same administrative distance, the router uses its vendor-specific algorithm to determine which route should be installed.^[1] Cisco routers simply ignore the values and fall back to the default values, which are never the same.^[3]

The router will usually compare administrative distances to determine which protocol has the lowest value. The router prefers protocols that have a lower assigned administrative distance. For example, OSPF has a default distance of 110, so it is preferred by the router process, over RIP, which has a default distance of 120. The administrator can arbitrarily reconfigure the administrative distances, which affects the ranking of the preferred routes by the routing process. On Cisco routers, static routes have an administrative distance of 1, making them preferred over routes issued by a dynamic routing protocol. The administrative distance is a value that is always only referenced by the local router itself. The administrative distance is not advertised on the network.^[1]

Default administrative distances

Cisco

The following table lists the default administrative distances for various routing protocols used on Cisco routers.^[2]

Please note that:-

• An administrative distance of 255 will cause the router to remove the route from the routing table 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 have a distance of less than 1.
• Directly connected routes have an administrative distance of 0. ^[4]

Configuration

Cisco IOS

The network administrator may modify the administrative distance to change the desired ranking of router protocols. This may be necessary in cases where routing redistribution has to be used, otherwise, routing loops could occur.^[2] The Cisco Internet Operating System enables network administrators to modify the distance by changing the distance value in sub-router configuration mode. In the example below, RIP's administrative distance is changed to 90 so that it used in preference to OSPF.^[2]

R1> enable

Manually configuring the administrative distance is also required when configuring a floating static route. Floating Static routes are used to provide an alternate path when a primary 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.^[2] The example below shows how to configure the administrative distance to 254 to specify that it should only be used as a last resort.

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.^[3]

Verification

Cisco IOS

Verifying the configuration of the administrative distance is done on Cisco equipment using the show ip route command in privileged exec mode on the console of the Cisco router.^[5][6] In the example shown below, the administrative distance is 1. The letter "S" indicates that the route is a static route that has, for all intents and purposes, been added manually to the router process by the administrator and installed into the routing table.

Router# enable

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

See also

• Routing
• Routing protocol

References

External links

• Cisco - What is Administrative Distance? (Ref. Mr. Praveen Suvarna Mundkur)