Information Technology Reference
In-Depth Information
Administrative Distance
On Cisco routers running more than one routing protocol, it is possible for two different
routing protocols to have a route to the same destination. Cisco routers assign each rout-
ing protocol an administrative distance. When multiple routes exist for a destination, the
router selects the longest match. For example, if to reach a destination of 170.20.10.1
OSPF has a route prefix of 170.20.10.0/24 and EIGRP has a route prefix of 170.20.0.0/16,
the OSPF route is preferred because the /24 prefix is longer than the /16 prefix. It is more
specific.
If two or more routing protocols offer the same route (with same prefix length) for inclu-
sion in the routing table, the Cisco IOS router selects the route with the lowest administra-
tive distance.
The administrative distance is a rating of the trustworthiness of a routing information
source. Table 10-5 shows the default administrative distance for configured (static) or
learned routes. In the table, you can see that static routes are trusted over dynamically
learned routes. Within IGP routing protocols, EIGRP internal routes are trusted over
OSPF, IS-IS, and RIP routes.
Ta b l e 1 0 - 5
Default Administrative Distances for IP Routes
IP Route
Administrative Distance
Connected interface
0
Static route directed to a connected interface
1
Static route directed to an IP address
1
EIGRP summary route
5
External BGP route
20
Internal EIGRP route
90
IGRP route
100
OSPF route
110
IS-IS route
115
RIP route
120
EGP route
140
External EIGRP route
170
Internal BGP route
200
Route of unknown origin
255
The administrative distance establishes the precedence used among routing algorithms.
Suppose a router has an EIGRP route to network 172.20.10.0/24 with the best path out
