Administrative distance (AD) or route preference is a number of arbitrary unit assigned to dynamic routes, static routes and directly connected routes. The value is used in routers to rank routes from most preferred (low AD value) to least preferred (high AD value). When multiple paths to the same destination are available in its routing table, the router uses the route with the lowest administrative distance.
Router vendors typically design their routers to assign a default administrative distance to each kind of route. For example, on Cisco routers, routes issued by the Open Shortest Path First routing protocol have a lower default administrative distance than routes issued by the Routing Information Protocol. This is because, by default on Cisco routers, OSPF has a default administrative distance of 110 and RIP has a default administrative distance of 120. Administrative distance values can, however, usually be adjusted manually by a network administrator.
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.
|-
| Static route || 1
|-
| Dynamic Mobile Network Routing (DMNR) || 3
|-
| EIGRP summary route || 5
|-
| External BGP || 20
|-
| EIGRP internal route || 90
|-
| IGRP || 100
|-
| Open Shortest Path First (OSPF) || 110
|-
| Intermediate System to Intermediate System (IS-IS) || 115
|-
| Routing Information Protocol (RIP) || 120
|-
| Exterior Gateway Protocol (EGP) || 140
|-
| ODR|| 160
|-
| EIGRP external route || 170
|-
| Internal BGP || 200
|-
| Next Hop Resolution Protocol (NHRP) || 250
|-
| Default static route learned via DHCP || 254
|-
| Unknown and unused || 255
|}
Juniper
The following table lists the default administrative distances for various routing protocols used on Juniper routers.
{| class="wikitable" style="text-align:center"
! Routing protocol !! Administrative distance
|-
| Directly connected interface || 0
|-
| Static routes || 5
|-
| OSPF internal routes || 10
|-
| IS-IS Level 1 Internal || 15
|-
| IS-IS Level 2 Internal || 18
|-
| RIP || 100
|-
| Aggregate (route summary) || 130
|-
| OSPF external routes || 150
|-
| IS-IS Level 1 External|| 160
|-
| IS-IS Level 2 External|| 165
|-
| BGP || 170
|}
Extreme Networks
The following table lists the default administrative distances used on ExtremeXOS / Switch-Engine.
{| class="wikitable"
!Routing protocol
!Administrative distance
|-
|Directly connected
|10
|-
|MPLS
|20
|-
|Blackhole
|50
|-
|Static
|1100
|-
|HostMobility
|1150
|-
|ICMP-Redirect
|1200
|-
|Fabric
|1699
|-
|eBGP
|1700
|-
|iBGP
|1900
|-
|OSPFintra
|2200
|-
|OSPFinter
|2300
|-
|IS-IS
|2350
|-
|IS-IS L1
|2360
|-
|IS-IS L2
|2370
|-
|RIP
|2400
|-
|OSPF AS Ext
|3100
|-
|OSPF Ext1
|3200
|-
|OSPF Ext2
|3300
|-
|IS-IS L1 Ext
|3400
|-
|IS-IS L2 Ext
|3500
|-
|Bootp
|5000
|}
The following table lists the default administrative distances used on Extreme VOSS / Fabric-Engine.
{| class="wikitable"
!Routing Protocol
!Administrative distance
|-
|Local
|0
|-
|Static
|5
|-
|SPBm L1
|7
|-
|OSPFintra
|20
|-
|OSPFinter
|25
|-
|eBGP
|45
|-
|RIP
|100
|-
|OSPF Ext1
|120
|-
|OSPF Ext2
|125
|-
|iBGP
|175
|}
FortiGate
The following table lists the default administrative distances for various routing protocols used on Fortinet FortiGate routers.
{| class="wikitable" style="text-align:center"
! Routing protocol !! Administrative distance
|-
| Directly connected || 1
|-
| Static routes || 10
|-
| External BGP || 20
|-
| Open Shortest Path First (OSPF) || 110
|-
| Routing Information Protocol (RIP) || 120
|-
| Internal & Local BGP || 200
|-
|}
Configuration
Cisco IOS
The network administrator may modify the administrative distance to change the desired ranking of routing protocols. This may be necessary in cases where multiple routing protocols are in use. 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.
<syntaxhighlight lang="text">
Router#enable
Router#configure terminal
Router(config)#ip route 1.1.1.0 255.255.255.0 fastEthernet 0/0
Router(config)#do show ip route
</syntaxhighlight>
The do show ip route command will display the following, confirming that a static route has an administrative distance of 1.
<code>S 1.1.1.0/0 [1/0] via 172.31.0.1</code>
See also
- Metrics (networking), used for choosing a route when administrative distance is the same