Cisco Administrative Distance

Cisco routers use administrative distance (AD) to select the best path when the same destination network is learned by two or more different routing sources. In simple terms, administrative distance is the trustworthiness of the routing source.

The lower the administrative distance value, the more trustworthy it is. When the same destination network is learned via multiple sources, the route with the lowest administrative distance will be placed in the routing table.

Default Administrative Distance Values

Here are the default administrative distance values for common routing protocols:

• Directly Connected - 0
• Static Route - 1
• Enhanced Interior Gateway Routing Protocol (EIGRP) - 90
• Open Shortest Path First (OSPF) - 110
• Routing Information Protocol (RIP) - 120
• External EIGRP - 170
• Unknown/Unreachable - 255 (infinite)

Verification Commands

You can verify administrative distance using several show commands:

Router# show ip route
Router# show ip protocols
Router# show ip route [network-address]

The show ip route command displays the administrative distance and metric for each route in brackets. For example, [110/65] where 110 is the administrative distance and 65 is the metric.

Modifying Administrative Distance

You can modify the administrative distance for static routes and routing protocols:

Static Route Administrative Distance

Router(config)# ip route 192.168.1.0 255.255.255.0 10.0.0.1 5

This creates a static route with an administrative distance of 5 instead of the default value of 1.

Routing Protocol Administrative Distance

To change the administrative distance for a routing protocol:

Router(config)# router ospf 1
Router(config-router)# distance 95

This changes the OSPF administrative distance from 110 to 95.

Floating Static Routes

Floating static routes use a higher administrative distance than the primary routing protocol to provide backup connectivity. When the primary route fails, the floating static route becomes active.

Router(config)# ip route 0.0.0.0 0.0.0.0 192.168.1.1 200

This creates a default route with an administrative distance of 200, which will only be used if all other routes to the same destination are unavailable.

Best Practices

• Use lower administrative distances for more reliable routing sources
• Keep default values unless there's a specific business requirement
• Document any changes to administrative distance values
• Use floating static routes for backup connectivity
• Be careful when modifying administrative distances as it can impact network convergence

Understanding administrative distance is crucial for CCNA candidates as it helps explain how Cisco routers make routing decisions when multiple paths to the same destination exist.