Editing Administrative distance

{{Short description|Number of arbitrary unit used in network routing decisions}}

'''Administrative distance''' ('''AD''') or '''route preference'''<ref>{{cite web |title=Route Preferences |url=https://www.juniper.net/documentation/software/cable/junosg30/swconfig30-interfaces/html/protocols-overview4.html |access-date=2018-06-18 |publisher=[[Juniper Networks]]}}</ref> is a number of [[arbitrary unit]] assigned to [[dynamic route]]s, [[static route]]s and directly-connected routes. The value is used in [[Router (computing)|routers]] to rank [[Routing|routes]] from most preferred (low AD value) to least preferred (high AD value).<ref name="Understanding Route Redistribution">{{citation |title=Understanding Route Redistribution |url=https://www.cs.cmu.edu/~4D/papers/rr-icnp07.pdf |author1=Franck Le |author2=Geoffrey G. Xie |author3=Hui Zhang}}</ref><ref name=cisco>Cisco Systems (2013),  [http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094195.shtml What is Administrative Distance?], retrieved 14 September 2013</ref> 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]].<ref name="Understanding Route Redistribution"/>

== Overview ==
The administrative distance (AD) value is assigned by the [[Router (computing)|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.<ref name="Understanding Route Redistribution" /> While a router may run multiple [[routing protocol]]s 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.<ref name="Understanding Route Redistribution" /> [[Cisco]] routers simply ignore the values and fall back to the default values, which are never the same.<ref name="ReferenceA">Cisco Systems(n.d.), Information About  Routing, Cisco Systems Inc, retrieved 0109 2014</ref>

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 route]]s have an administrative distance of 1, making them preferred over routes issued by a [[dynamic routing|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.<ref name="Understanding Route Redistribution" />

== Default administrative distances ==

=== Cisco ===
The following table lists the default administrative distances for various routing protocols used on [[Cisco Systems|Cisco]] routers.<ref name=cisco />

{| class="wikitable" style="text-align:center"
! Routing protocol !!  Administrative distance
|- 
| Directly connected interface || 0{{efn|Only the interface itself has an administrative distance of 0, since a route cannot have a distance of less than 1.}}<ref name="Cisco AD">{{citation |title=Default AD |url=http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/15986-admin-distance.html#topic2 |author1=Cisco}}</ref>
|- 
| Static route || 1
|- 
| Dynamic Mobile Network Routing (DMNR)  || 3
|- 
| [[EIGRP]] summary route || 5
|- 
| External [[BGP]] || 20
|- 
| [[EIGRP]] internal route || 90
|- 
| [[IGRP]] || 100
|- 
| [[OSPF|Open Shortest Path First]] (OSPF) || 110
|- 
| [[IS-IS|Intermediate System to Intermediate System]] (IS-IS) || 115
|- 
| [[Routing Information Protocol]] (RIP) || 120
|- 
| [[Exterior Gateway Protocol]] (EGP) || 140
|- 
| [[On Demand Routing|ODR]]|| 160
|- 
| [[EIGRP]] external route || 170
|- 
| Internal [[BGP]] || 200
|-
| [[Next Hop Resolution Protocol]] (NHRP) || 250<ref name="Cisco NHRP">{{citation |title=NHRP |url=https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_nhrp/command/reference/irn_book/nhrp-commands--a-through-z.html#wp8534493760 |author1=Cisco}}</ref>
|-
| Default static route learned via DHCP || 254{{citation needed|date=October 2018}}
|- 
| Unknown and unused || 255{{efn|An administrative distance of 255 will cause the router to remove the route from the routing table and not use it.}}
|}

{{notelist}}

=== Juniper ===
The following table lists the default administrative distances for various routing protocols used on Juniper routers.<ref name="Juniper AD">{{citation |title=Default AD |url=https://www.juniper.net/documentation/en_US/junos/topics/reference/general/routing-protocols-default-route-preference-values.html |author1=Juniper}}</ref>

{| 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
|- 
| [[Routing Information Protocol|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 ===
{{unreferenced section|date=March 2023}}
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
|}
{{notelist}}

=== FortiGate ===
The following table lists the default administrative distances for various routing protocols used on [[Fortinet]] FortiGate routers.<ref name="FortiGate">{{citation |title=Technical Note : FortiGate IP route selection, and how to change the administrative distance (preference) of a routing protocol |url=https://community.fortinet.com/t5/FortiGate/Technical-Note-FortiGate-IP-route-selection-and-how-to-change/ta-p/191612 |author1=Fortinet}}</ref>

{| class="wikitable" style="text-align:center"
! Routing protocol !!  Administrative distance
|- 
| Directly connected || 1
|- 
| Static routes || 10<ref name="FortiGate Static">{{citation |title=FortiGate CLI Reference Static Routes|url=https://docs.fortinet.com/document/fortigate/7.6.1/cli-reference/200835411/config-router-static#:~:text=Maximum%20length%3A%2035-,distance,10,-dst|author1=Fortinet}}</ref>
|- 
| External [[BGP]] || 20<ref name="FortiGate BGP">{{citation |title=FortiGate CLI Reference BGP|url=https://docs.fortinet.com/document/fortigate/7.6.1/cli-reference/225427711/config-router-bgp#:~:text=Disable%20setting.-,distance%2Dexternal,200,-ebgp%2Dmultipath|author1=Fortinet}}</ref>
|- 
| [[OSPF|Open Shortest Path First]] (OSPF) || 110<ref name="FortiGate OSPF">{{citation |title=FortiGate CLI Reference OSPF|url=https://docs.fortinet.com/document/fortigate/7.6.1/cli-reference/103419153/config-router-ospf#:~:text=10-,distance,110,-distribute%2Dlist%2Din|author1=Fortinet}}</ref>
|- 
| [[Routing Information Protocol]] (RIP) || 120
|- 
| Internal & Local [[BGP]] || 200<ref name="FortiGate BGP"/>
|-
| Kernel || 255<ref name="FortiGate Router">{{citation |title=FortiGate CLI Reference Router Settings|url=https://docs.fortinet.com/document/fortigate/7.6.1/cli-reference/263404019/config-router-setting#:~:text=Maximum%20length%3A%2014-,kernel%2Droute%2Ddistance,255,-show%2Dfilter|author1=Fortinet}}</ref>
|- 
|}

== 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.<ref name=cisco /> The Cisco Internetwork 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 89 so that it used in preference to OSPF.<ref name=cisco />

<syntaxhighlight lang="text">
R1#enable

R1#configure terminal

R1(config)#router rip

R1(config-router)#distance 89
</syntaxhighlight>

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.<ref name=cisco /> The example below shows how to configure the administrative distance to 254 to specify that it should only be used as a last resort.

<code>R1(config)# '''ip route 10.0.0.0 255.0.0.0 backupLink 1 254'''</code>

In the event that two routing protocols are configured with the same administrative distance, the [[Cisco Systems|Cisco]] router will ignore the configured values and instead use the default values.<ref name="ReferenceA"/>

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 Systems|Cisco]] router.<ref>Cisco Systems (n.d),  [http://www.cisco.com/en/US/docs/switches/datacenter/sw/5_x/nx-os/unicast/configuration/guide/l3_route.html Configuring  Static Routing], Cisco Systems Inc., retrieved 14 September 2013</ref><ref>Cisco Systems (n.d),  [http://www.cisco.com/en/US/docs/switches/datacenter/sw/6_x/nx-os/unicast/command/reference/l3_cmds_show.html#wp1688356 Show  Commands], Cisco Systems Inc., retrieved 14 September 2013</ref> 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<ref>{{cite web |title=Administrative Distance and Metric |url=https://etutorials.org/Networking/Integrated+cisco+and+unix+network+architectures/Chapter+8.+Static+Routing+Concepts/Administrative+Distance+and+Metric/ |access-date=2021-12-23}}</ref><ref>{{cite web |title=Understand the significance of administrative distance and metrics when working with routers |url=https://www.techrepublic.com/article/understand-the-significance-of-administrative-distance-and-metrics-when-working-with-routers/ |access-date=2021-12-23}}</ref><ref>{{cite web |title=Administrative distance & metric |url=https://study-ccna.com/administrative-distance-metric/ |access-date=2021-12-23}}</ref>

== References ==
{{Reflist}}

[[Category:Routing]]