Administrative Distance for CCNA students

Administrative distance is one of those routing concepts that most CCNA students have difficulty with to understand. In this short lesson I’ll explain you what administrative distance is and how it works.

Let me show you an example:

administrative distance

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?

Administrative distance is one of those routing concepts that most CCNA students have difficulty with to understand. In this short lesson I'll explain 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 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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Forum Replies

  1. 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?

  2. 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 forum

  3. Thanks Laz, it kind of helps by giving me a basic concept.

  4. I would add when it comes to more specific routes, for example a router learns 192.168.1.0/24 from OSPF (AD 110) and 192.168.1.0/25 from RIP (AD 120), the router will choose the more specific route, so the RIP one, despite it has a higher (worse) AD .
    So, if we want to reach the host 192.168.1.1 from this router, it will use the /25 route from RIP.

  5. Hello Juan,

    The key thing to remember is that AD only applies when you have two exact routes.

    192.168.1.0/24 and 192.168.1.0/25 are not the same routes.

    Rene

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