Administrative distance is one of those routing concepts that most CCNA students have difficulty with understanding. In this short lesson, I’ll explain to you what administrative distance is and how it works.
Let me show you an example:
Imagine we have a network that is running two routing protocols at the same time, OSPF and EIGRP. Both routing protocols give information to R1.
- EIGRP tells us the router should send IP packets using the path on the top.
- OSPF tells us the router should send IP packets using the path on the bottom.
What routing information are we going to use? Both? Use OSPF or EIGRP?
The answer is that when two routing protocols are giving us information about the same destination network, we have to make a choice…you can’t go left and right at the same time. We need to look at the administrative distance or AD.
Let me show you the administrative distance list: