Logical Diagrams

A logical diagram of a network will show routers, subnets, VLAN, gateways etc. and will facilitate debugging using layer 3. It should closely integrate with the VLAN table.

A logical network diagram provides sufficient information for someone to understand the network at layer 2 and layer 3, such that diagnostics are possible. Typically, switches will not be shown, routers will. Broadcast domains will be shown, these are Ethernets but in a modern network, will normally be VLANs. Each VLAN should show relevant information such as the VLAN ID, name, and any IP address range associated. Routers and gateways should be indicated. Infrastructure servers such as DNS, DHCP, and AAA will normally be indicated. Any zoning should be indicated such as the presence of firewalls.

The gold standard: the diagram should show enough information to allow fault-finding from a console using ping, trace route, etc.

Feel free to annotate to make the diagram more understandable.

At a minimum, I should be able to see:

  1. VLAN ID, IPv4 allocation and description

  2. Gateways/HSRP/VRRP

  3. IP address ranges/DHCP

  4. Known fixed IP addresses

  5. Layer 3 devices, routers

  6. VRFs etc. if you need to use them

I would never show individual PCs, except as a total count.

This diagram should be consistent with the VLAN plan for the site.

“A VLAN plan showing VLAN IDs, Subnets, Routers, etc. This will normally be in a table format and should closely integrate with the Logical Diagram”.

This is best provided as a table, and should be consistent with the logical diagram. It needs to be printed/PDF’d in landscape mode to be clear.

Include VLAN ID and/or location, IP address range, CIDR/Mask, Gateway, native and discard VLANs, number of nodes.

  1. HSRP/VRRP information is essential

  2. Showing DHCP allocations would be useful

  3. Show any utility VLANs (Discard, Trunk, Netman) and indicate if they route

  4. I often include the assumptions for

    1. Eventual total for this design

    2. Maximum number of nodes on Day1 (<Eventual Total * 0.6)

Take a look at this video of a live session from 2022.

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