Administrative Distance vs. Metric

A “metric” allows a router to choose the best path within a routing protocol.

Distance vector routing protocols use “distance” (usually hop-count) as their metric. Link state protocols utilize some sort of “cost” as their metric.

Only routes with the best metric are added to the routing table. Even if a particular routing protocol (for example, RIP) has four routes to the same network, only the route with the best metric (hop-count in this

example) would make it to the routing table. If multiple equal-metric routes

exist to a particular network, most routing protocols will load-balance.

If your router is running multiple routing protocols, Administrative

Distance is used to determine which routing protocol to trust the most.

Lowest administrative distance wins. Again: if a router receives two RIP routes to the same network, it will use the routes’ metric to determine which path to use. If the metric is identical for both routes, the router will load balance between both paths. If a router receives a RIP and an OSPF route to the same network, it will use Administrative Distance to determine which routing path to choose. The Administrative Distance of common routing protocols (remember, lowest wins):

Connected= 0

Static= 1

EIGRP Summary= 5

External BGP= 20

Internal EIGRP= 90

IGRP= 100

OSPF= 110

IS-IS= 115

RIP= 120

External EIGRP= 170

Internal BGP= 200

Unknown= 255

A route with an “unknown” Administrative Distance will never be inserted

into the routing table.

Administrative Distance

Metric