Administrative distance is one of those routing concepts that most CCNA students have difficulty with to understand. 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 are giving 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:
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So…administrative distance is a static number? EIGRP is always 90 and OSPF is always 110? Where are the numbers derived?
Those are the default values yes. You can change them globally so that the router prefers one routing protocol over another, for example by changing the AD of OSPF to 80. It’s also possible to change the AD on a prefix level.
videos are excellent. i can understand topics very easily. I hope Rene Molenaar updates network lessons with more videos as new features are introduced in cisco .
why would u need to run more than 1 routing protocol in your network. wouldn’t you either run in todays world purely eigrp or ospf? why both?
Hi Ruby,
If you design a network, you will always use one routing protocol (if possible). Here are some scenarios where you might have to use two routing protocols:
- You are migrating your company network with the network from another company. Maybe you are running OSPF and they are running EIGRP. As a temporary solution you could use redistribution and run both protocols.
- You run OSPF on your internal network and are installing a new site. Your SP offers you a MPLS VPN connection but only supports BGP as the routing protocol.
- You use EIGRP on your network b
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