CompTIA Network+ Certification Exam Tutorial:

The Full Mesh Topology

By Chris Bryant, CCIE #12933

With a full-mesh topology, each host has at least two connections to other hosts, and the fault tolerance is about as good as it's going to get! 

A single link failure will not prevent one host from reaching another, as shown in the following illustration where one link has been removed.

Even though the more direct path from Host A to Host B is down, the two hosts can still communicate, as they still have a path to each other.  Obviously, if multiple links go down, we could have trouble!

There are two obstacles to using a full-mesh topology, one theoretical and one real-world.  Full-mesh topologies use more cable than any other topology, but again, getting more cable isn't a huge obstacle.  The real-world difficulty is that a full-mesh isn't always practical due to the amount of work involved as opposed to other topologies, and it's really not a scalable solution either.  Any time you add a host to a full-mesh topology, you've got a lot of work ahead of you to connect every other device in the network to the new host.

In short, it's a nice idea, but it's not always practical.

How Many Connections Are Needed For A Full Mesh?

It's pretty easy to look at a four-computer network segment and determine how many physical connections will be needed to create a full mesh.  But what if you had nine computers?  Or 17?  Instead of drawing it out and counting a bunch of lines, just use this simple formula:

N (N - 1) / 2, with N equaling the number of hosts

Let's test that formula with our four-computer network.  If we plug "4" in for "N", we get this:

4 (4 - 1) / 2

That would be 4 times 3, then divide by two.  Four times three is 12, and 12 divided by 2 is six.  Is that right?

Yes!  :)  This is a simple example, but I think you'll agree that if we had nine computers here, it would be really easy to make a mistake by drawing all the lines and then trying to count them.  Trust me, learning the formula is easier!

To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com