Administrative distance is used to determine the reliability of one source of routing information from another. Some sources are considered more reliable than others; therefore, administrative distance can be used to determine the best or preferred path to a destination network or network node when there are two or more different paths to the same destination from two or more different routing protocols.
In Cisco IOS software, all sources of routing information are assigned a default administrative distance value. This default value in an integer between 0 and 255, with a value of 0 assigned to the most reliable source of information and a value of 255 being assigned to the least reliable source of information. Any routes that are assigned an administrative distance value of 255 are considered untrusted and will not be placed into the routing table.
The administrative distance is a locally significant value that affects only the local router. This value is not propagated throughout the routing domain. Therefore, manually adjusting the default administrative distance for a routing source or routing sources on a router affects the preference of routing information sources only on that router. Table 1-1 shows the default administrative values used in Cisco IOS software.
| Route Source | AD |
| Connected Interfaces | 0 |
| Static Routes | 1 |
| Enhanced Interior Gateway Routing Protocol (EIGRP) Summary Routes | 5 |
| External Border Gateway Protocol (eBGP) Routes | 20 |
| Internal Enhanced Interior Gateway Routing Protocol (EIGRP) Routes | 90 |
| Open Shortest Path First (OSPF) Internal and External Routes | 110 |
| Intermediate System-Intermediate System (IS-IS) Internal and External Routes | 115 |
| Routing Information Protocol (RIP) Routes | 120 |
| Exterior Gateway Protocol (EGP) Routes | 140 |
| On Demand Routing (ODR) Routes | 160 |
| External Enhanced Interior Gateway Routing Protocol (EIGRP) Routes | 170 |
| Internal Border Gateway Protocol (iBGP) Routes | 200 |
| Unreachable or Unknown Routes | 255 |
Tab.1-1. Administrative Distance Values
The default route source administrative distance is displayed in the output of the show ip protocols command. This is illustrated in the following output:
| R1#show ip protocols Routing Protocol is “isis” Invalid after 0 seconds, hold down 0, flushed after 0 Outgoing update filter list for all interfaces is not set Incoming update filter list for all interfaces is not set Redistributing: isis Address Summarization: None Maximum path: 4 Routing for Networks: Serial0/0 Routing Information Sources: Gateway Distance Last Update 10.0.0.2 115 00:06:53 Distance: (default is 115) |
The administrative distance value assigned to an individual route is viewed in the output of the show ip route [prefix] command. This is illustrated in the following output:
| R1#show ip route 150.1.1.0 Routing entry for 150.1.1.0/28 Known via “isis”, distance 115, metric 20, type level-2 Redistributing via isis Last update from 10.0.0.2 on Serial0/0, 00:00:11 ago Routing Descriptor Blocks: * 10.0.0.2, from 150.1.1.2, via Serial0/0 Route metric is 20, traffic share count is 1 |
In addition to allowing administrators to change the default administrative distance values of individual routing protocols, Cisco IOS software also allows administrators to adjust the administrative distance of individual prefixes learned via a dynamic routing protocol. This will be described in detail later in this guide.