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:
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
... Continue reading in our forumShree,
While you can choose to make the Administrative Distance to almost any number you choose, you should have a good reason for changing, and you should make the change across all routers under your control that use the protocol in question. There are legitimate circumstances where you need to change an AD (often times with multiple redistribution points), but in general this is not done.
The AD number acts as a sort of ranking on how trustworthy the information is from the routing protocol. For example, OSPF routes (AD = 110) are considered far more trust
... Continue reading in our forumHello Marit
Administrative distances are indeed assigned by the vendor, and the values you see in the lesson are values assigned by Cisco. This does make sense when you see that EIGRP is given a better AD that OSPF. Although they are assigned by vendors, they are not entirely based on preference of the proprietary over the open architecture protocols. The default ADs are specifically chosen
... Continue reading in our forumVery helpful Laz, thanks a lot!