Most routing protocols have metric structures and algorithms that are not compatible with other protocols. In a network with multiple routing protocols, the exchange of route information and the capability to select the best path across the multiple protocols are critical.
Administrative distance is the feature that routers use in order to select the best path when there are two or more different routes to the same destination from two different routing protocols. Administrative distance defines the reliability of a routing protocol . Each routing protocol is prioritized in order of most to least reliable (believable) with the help of an administrative distance value.
Administrative distance is the first criterion that a router uses to determine which routing protocol to use if two protocols provide route information for the same destination. Administrative distance is a measure of the trustworthiness of the source of the routing information. Administrative distance has only local significance, and is not advertised in routing updates . The smaller the administrative distance value, the more reliable the protocol. For example, if a Cisco router receives a route to a certain network from both Open Shortest Path First (OSPF) (default administrative distance – 110) and Interior Gateway Routing Protocol (IGRP) (default administrative distance – 100), the router chooses IGRP because IGRP is more reliable. This means the router adds the IGRP version of the route to the routing table.
Routing decision criteria
1) Valid next hop
upon update receipt, the router verifies a valid next hop
the router then attempts to install the best routing metric (OSPF=cost, ie) into the table
3) Administrative Distance
If multiple routing protocols are running, the route with the lowest AD is used
Longest match preferred; trumps AD
Administrative Distances (default)
Connected interface 0
Static route 1
EIGRP summary route 5
External BGP 20
Internal EIGRP 90
Exterior Gateway Protocol 140
On-demand routing 160
External EIGRP 170
Internal BGP 200
A floating static route is a route artificially increased from it’s default, ie. for EIGRP a floating static could be added with an AD of 93, which would be used in the event of a dynamic EIGRP route (AD 90) not found.
Routing’s classic model, hop-by-hop, keeps a list of destinations with their reachable next hops in the routing table. In recent years the hop-by-hop paradigm has given way to more advanced methods such as MPLS, which enables label lookup to dictate the next hop determination. Traffic engineering considerations have become popular as well, such as QOS, that are not limited to routing table only criteria.