Administrative distance: how a router picks between OSPF, EIGRP and static

When a router learns the same destination network from multiple sources - say OSPF, EIGRP, and a static route all at once - how does it decide which path to use? The answer is administrative distance (AD). Administrative distance is a trustworthiness rating that tells a router which routing source to believe first. Lower AD values win. Understanding AD is essential for controlling routing behavior and troubleshooting unexpected path selection.

What is administrative distance?

Administrative distance is a number from 0 to 255 that represents how trustworthy a routing source is. A value of 0 means absolutely trustworthy (only directly connected interfaces have this). A value of 255 means the route is unreachable and will never be used. When a router receives routes to the same destination from multiple protocols, it installs the route with the lowest AD into the routing table.

AD is not the same as metric. Metric measures the quality of a path within a single routing protocol - hop count, bandwidth, delay, and so on. AD is a protocol-selection tool that operates before metric comparison. If OSPF has AD 110 and EIGRP has AD 90, the router will use EIGRP routes even if OSPF has a lower metric, because EIGRP is trusted more.

Default administrative distance values

Cisco and other vendors assign default AD values based on how reliable each routing source typically is. Directly connected networks are most trustworthy (AD 0), followed by static routes (AD 1). Dynamic protocols vary: EIGRP is trusted more (AD 90) than OSPF (AD 110) or RIP (AD 120).

Routing Source                           Default AD
---------------------------------------------------
Directly connected interface              0
Static route                             1
EIGRP summary route                      5
External BGP (eBGP)                     20
Internal EIGRP (IGRP)                   90
OSPF                                   110
Intermediate System to Intermediate
  System (IS-IS)                       115
RIP version 1 and 2                    120
ODR (On Demand Routing)                160
External EIGRP (EIGRP routes from
  outside the AS)                      170
External BGP (iBGP)                    200
Unknown                                255

Why AD matters in practice

AD becomes critical in networks running multiple routing protocols or mixing static routes with dynamic protocols. A common scenario: you configure a static default route as a fallback (AD 1), but also run OSPF (AD 110). The static route will always be preferred, even if OSPF learns a better path. If you want OSPF to take over during normal operation and only use the static route when OSPF fails, you must increase the static route's AD to 111 or higher.

Another example: a network engineer deploys both EIGRP and OSPF during a migration. EIGRP has AD 90, OSPF has AD 110. Every route learned by both protocols will use the EIGRP path, regardless of OSPF's metric. To balance traffic or control which protocol owns which routes, you can modify AD on a per-protocol or per-route basis.

Modifying administrative distance

Most routing protocols allow you to change the default AD. On Cisco IOS, you typically add a number to the protocol configuration command.

! Increase OSPF AD from 110 to 150
router ospf 1
  distance 150

! Increase EIGRP AD from 90 to 170
router eigrp 100
  distance 170

! Create a static route with custom AD (default is 1)
ip route 10.0.0.0 255.0.0.0 192.168.1.1 200

Use custom AD values carefully. Changing AD can cause unexpected failover behavior, traffic blackholes, or loops if not planned correctly. Document any non-default AD settings in your network.