When two or more sources are giving you information about a certain prefix you need to choose which information you are going to use. For example OSPF might tell you to go “left” if you want to reach network X, EIGRP might tell you that you need to go “right”. Who do you trust? OSPF or EIGRP? You can’t put both in the routing table for network X.
The administrative distance solves this problem. When two sources give us information about the exact same network we’ll have to make a decision and it’s done by looking at the administrative distance. Let me show you the different values:
The lower the better…as you can see EIGRP has a lower administrative distance (90) than OSPF (110) so we will use EIGRP in my example.
Keep in mind:
- The administrative distance is only local and can be different for each router.
- The administrative distance can be modified.
Especially when we use redistribution we sometimes have to change the administrative distance. Let me show you how you can do this:
R1(config)#router eigrp 12 R1(config-router)#distance eigrp 90 160
Above we have EIGRP and with the distance command I can change the administrative distance for EIGRP globally. Internal EIGRP will keep its AD of 90 but external EIGRP will have an AD of 160. You will see this change in the routing table:
R1#show ip route eigrp 184.108.40.206/24 is subnetted, 1 subnets D EX 220.127.116.11 [160/1734656] via 192.168.12.2, 00:00:30, FastEthernet0/0 D EX 192.168.23.0/24 [160/1734656] via 192.168.12.2, 00:00:30,FastEthernet0/0
You can verify it by looking at the routing table, the external networks on router R1 now have an AD of 160.
We can change the AD of the other routing protocols as well, here are some examples: