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I was watching a video talking about EIGRP protocol and got confused in the network part.

The part of the topology was:

R1 e0/1, with IP of 10.1.12.1/24 connected on R2 e0/0, with IP of 10.1.12.2/24

Now, when configuring the EIGRP, the network inserted was 10.1.1.0, I couldn't understand why is that, shouldn't it be 10.1.12.0?

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    Maybe the video wasn't very good, on R1 the EIGRP network to include e0/1 will be either 10.1.12.0/24 or a supernet of it such as 10.1.0.0/16
    – marctxk
    Sep 14, 2016 at 16:17
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    If you post a link to the video, we could tell for sure. Otherwise, we're guessing.
    – Ron Trunk
    Sep 14, 2016 at 18:18
  • It's not a public video, sorry. I talked to the guy who made the video tho, he said that he must have written wrong, but it didn't matter too much because EIGRP will change to 10.0.0.0
    – Lucas Castellani
    Sep 15, 2016 at 19:27

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Alright remember that when you connect two router interfaces like that way then the network between the two routers is a separate network and has nothing to do with your LAN on each side. So let's assume the following

R1 is site one and has LAN network 192.168.100.0/24 R2 is site two and has LAN network 192.168.200.0/24

Now to connect the two you will want to connect the routers as well so you n need a new network for it. Usually it is a network with /30 so you only have 2 usable IP's and not wasting any IP space. But this video obviously wasn't too detailed.

So for routers as an example you create 10.12.1.0/24 and configure it the way you have shown. Now the routes you will advertise are your R1 and R2 LAN networks. These networks have nothing to do with the IP scheme you have assigned the two router interfaces. Obviously the default gateway on the inside interfaces of your routers will be within the LAN network.

Hope it makes sense.

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    If you don't include the link interfaces in EIGRP, the two routers will not become neighbors.
    – Ron Trunk
    Sep 15, 2016 at 1:40
  • "...it is a network with /30 so you only have 2 usable IP's and not wasting any IP space." Actually, a /30 network wastes half the network addresses. A point-to-point link can use a /31 network, which does not waste any network addresses. See RFC 3021, Using 31-Bit Prefixes on IPv4 Point-to-Point Links from December 2000, nearly 16 years ago. Cisco fully supports /31 networks, and it has for many years.
    – Ron Maupin ♦
    Sep 24, 2016 at 6:21

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