Diagrams — Physical Is Not Enough!

In my billion years in the industry, when I’ve asked for network diagrams, I’ve inevitably received a physical diagram — a diagram that shows where stuff is plugged in. This is fine and dandy and has lots of information, but that’s not really enough these days. In the times of Arthur, when every piece of network gear did a single thing, you only needed to know where things were plugged in. In the modern era, devices do more — a switch can route and house wireless, an ASA can terminate VPNs and be a switch — so you need more than just where the cables run.

Logical diagrams show layer-3 (the IP networks) and how those are interconnected. From that, the diagram inherits the data paths as well — how does the packet get from network A to network B and back. You can’t see that with physical diagrams in a lot of cases.

Here’s a physical diagram of a single Internet router and a 6509 with an FWSM. It literally shows a router and a switch. How many IP networks do I have? How many firewall interfaces do I have? How many layer-3 interfaces on the 6509 am I using? This logical diagram, however, shows the same network from layer-3. A big difference, eh? Didn’t know I had 8 networks, did you?

Don’t go replacing all your physical diagrams with logical ones, though. You still have to know where things are plugged in, so keep the physical…just add a logical as well.

Tip of the day: Use Visio tabs for logical and physical diagrams on the same document.

Aaron Conaway

I shake my head around sometimes and see what falls out. That's what lands on these pages.

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4 comments for “Diagrams — Physical Is Not Enough!

  1. April 24, 2008 at 7:54 pm

    I’ve never been quite clear exactly on what the differences are between physical and logical diagrams. So, literally, what more do you need to know about than the physical connections existing between the devices. Does drawing clouds on your diagrams really impart much specificity?

  2. April 24, 2008 at 8:31 pm

    Hi, bubba-jay. Physical diagrams show how devices are physically connected to each other. Logical diagrams show how networks are connected together. It’s different sets of information that are both very important. As in the example, a physical diagram won’t reveal the underlying evil that is the layer-3 interfaces of the chassis switch and firewall module. A logical diagram, however, won’t necessarily help you if the janitor trips over a cable and plugs it in wrong. So yes, drawing clouds does impart some specificity. 🙂

  3. April 24, 2008 at 8:40 pm

    Fair enough, but I’d say what you’re calling logical is really a 20,000 foot high view of your network, instead of being ‘logical’. Your logical diagram had a much wider view of your network than the two links covered by the physical diagram.

  4. LBSources
    April 26, 2008 at 9:44 am

    I myself am into blending the 2 .. I think it also depends on the visio skills one has with putting these things together.. I’m by far not a Visio expert so they do take time for me to put together. At least some of the more detail ones. But for me the logical info must be included somewhere in the diagrams.

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