I used to manage a Fortune 500 company network of 3,000 remote branch offices and 5 large data centers. I learned that creating accurate and clear documentation was critical to supporting the network with a minimum of staff.
To start with, I learned that what you do not draw is just as important as what you do draw. For example, creating 3,000 unique branch office drawings is not practical. On any given day there were 10 new offices opening, 10 closing, and 10 consolidating together to save costs.
Standards are key
In this case, creating strict standards for the branches became more important than drawing detailed L2 diagrams. Creation of a Layer 3 IP plan that allows for scale and assignment of ports based on purpose. For example, port 1 on router 1 always connects with switch 1 port 1, and switch ports 44 and 45 are always used for switch-to-switch uplinks.
Do not intermingle L2 and L3 diagrams
I can't tell you how many times one of my engineers drew a diagram that mixes L2 cabling with L3 routing. These drawings are almost always overly complex and do not do a good job of showing either the cabling or the routing. If you look at a drawing and cannot tell what is going on, there's a good chance it's mixing L2 and L3 environments.
Layer 2 — the physical layer diagram
Simply done, these diagrams should show the specific hardware being drawn and the cabling for the ports being used for important network purposes: the uplinks, the cross-connects, and WAN circuits. Different colors can be used to denote the different purpose of a cable. Matching these to real-life cable colors is also great if you can do it.
The L2 diagram should show only information known at the MAC level. Here is an example of a simple office network with two internet providers and a stackable switch core. The drawing clearly shows what port is connected to what port without any extraneous data.
Layer 3 — the logical topology diagram
The logical diagram should show how things are connected on a Layer 3 basis. In this kind of diagram, Layer 2 switches disappear to become just shared lines connecting two devices.
This Layer 3 example diagram shows the same office with two internet providers, except that this time it shows how the office has its own public block of IPs and a registered BGP ASN. There is a VLAN to connect the WAN to LAN devices and several VLANs for the inside network spaces.
In summary
Hopefully this brief explanation of how to draw simple diagrams that do not mix L2 and L3 topologies will help your team document the networks you're supporting in a more clear and concise way.