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You are here: Home / Blogs / Network Documentation Series: Logical Diagram

Network Documentation Series: Logical Diagram

John W Kerns June 9, 2015

Visio_Topology_TemplateIn this article, I will do a walk-through of a logical network diagram. As I also said in the Physical Diagram article: I prefer to use the term “logical” instead of “L3” because it is more easily understood by somebody unfamiliar with the OSI model. It also removes the assumption (made by many non-technical people) that “L1” and “L3” diagrams are incomplete without a “L2” diagram. So I just call them “Physical” and “Logical” to avoid the confusion.

You can download this article’s template file using the link to the right.

Logical Diagram Stencils

Stencils in a logical diagram should be, obviously, logical representations of the network nodes.Physical Stencil vs Generic Icon Keeping that in mind, I propose that it doesn’t make sense to use a physical network device stencil, which is essentially a picture of the actual device (with ports, fans, rack ears…), in a logical diagram. I prefer, instead, to use generic icons which represent different types on network devices in a network.

My favorite resource for generic topology icons is Cisco. You can download a library here or visit the Network Topology Icons Homepage. I am also including the most common icons in the template linked at the top of the page.

Subnets

Subnets are the cornerstone of a logical network diagram. They represent an IP network where nodes can hold L3 addresses and communicate via IP.

Logical Diagram SubnetsThere are three important pieces of information to hold in a subnet object: VLAN Name, VLAN ID, and assigned IP block in CIDR format. The VLAN name and ID information assume that the subnet is contained within a VLAN on a switch. When this is not the case (like with a point-to-point link between two routers), omit the VLAN ID and name and include only the subnet. These different pieces of information are distinguished on the subnet with different font types.

Connectors

Connectors in a logical diagram connect a device to a subnet and represent a layer-3 (or sometimes layer-2) presence on the subnet. There is no need in this diagram for different colored connectors, so always use a solid black pattern. The exception to this rule is non-routable subnets (like heartbeat or vMotion), where the VLAN exists on a switch, but the switch has no layer-3 presence; in this case, use a dot or dash patterned connector.Logical Diagram Connectors

Each connector is labeled with its IP information specific to that VLAN. Here are a couple of rules I typically follow when labeling the connectors:

  1. If the connector is connecting a layer-3 switch to a VLAN which resides on it (with a VLAN name and ID), then simply use the last octet (or how ever many you need to specify the address with the mask in mind) and it can be assumed that that connector represents an SVI on the layer-3 switch
  2. If a FHRP (HSRP, VRRP, GLBP, etc) is present on the subnet, list the physical addresses of the nodes with different colors (see the section about the Legend) along with the FHRP VIP in black
  3. Whenever possible, list the name (or an abbreviated name) of the interface which has the logical connection to the subnet (A Cisco router might be something like “Gi0/0”). It may also be appropriate to list the interface alias when the device uses one (like a firewall with named interfaces).
  4. When multiple connectors are required to connect a single subnet to multiple devices which are grouped together, you can use one connector and connect it to the grouping box. In this case, define multiple IPs on the connector for the different devices.

***NOTE***
In cases when a layer-3 switch holds VLANs where it has no layer-3 presence (like when a public internet block is switched on a L3 switch, and not routed), but other devices in the diagram do hold a layer-3 presence on that subnet: attach the subnet to that switch with a dash connector and hide the connector with a layer so it doesn’t obstruct the rest of the diagram. You can find a quick tutorial on Visio layers here. I have included an example of this in the template.

Grouping Boxes

Logical Diagram Grouping BoxesOne of the first things you may notice on the template document are the colored background grouping boxes labeled “Internet Edge”, “WAN Edge”, etc. These boxes are used to group similar devices (or subnets) into a category or named section of the network when they have related purposes.

Grouping boxes can be used to take a complex topology, with many devices interconnected in confusing ways, and turn it into a simpler and more hierarchical topology; with each device detailed and defined, but serving a role in the larger function of the group.

Device Labeling

Each node (switch, router, firewall, etc) should optimally be labeled with a hostname, and a management IP address. When possible, list a loopback address that has less dependence on the physical interfaces of the device.

Logical Diagram Device LabelingThe obvious exceptions to this rule are things like devices managed by somebody else (where the hostname or management IP are unknown). In the case where there is an active device which is managed by somebody else (ie: CPE router), label it with a non-bold, italicized font (labels for standard devices are bold and non-italicized).

***NOTE***
In a case where you are diagramming a device which has multiple active VRFs: add the different VRFs as different device icons. Label the device icons with the same hostname and management IP, but add a third line with the name of the VRF. You can move around and treat the VRF device icons as separate devices in the network because, logically, they are different devices.

The Legend

Logical Diagram LegendThe legend is a very important part of your logical network diagram. It informs the reader on how to interpret the different pieces of information in the drawing. It also sets the ground rules for you to follow when diagramming the network.

I have included my standard logical diagram legend in the template linked at the top of the page. It has the most commonly used connector types defined, but can always be modified as needed.

Create/Maintain

Up to this point, this article has described the different components of a logical diagram and their meanings. Next we will cover how to create and maintain a logical network diagram. It is assumed at this point that you will be able to use Visio to diagram the logical network topology, using the methods described above once you have the connectivity information. This section will describe the steps to take to get that information.

***NOTE***
Remember that a logical network diagram only records the nodes in the network which have an in-band, layer-3 presence. There is no need to include devices like layer-2 switches, bridges, etc.

Connected Networks

The best way to start on a new logical diagram, in an unknown network, is by logging into a “core” device, or a device which has many connected subnets and has routes to remote networks (either static or dynamic).

Begin by listing out all connected subnets on the device (also list out any VLAN information associated with these subnets). First, add this device to the drawing and label it with the hostname and management IP address. Then translate these subnets and VLAN info into subnet objects in the drawing. As you create each subnet, connect it back to the device with connectors and the appropriate connector labels.

Routing Tables

Logical Diagram Next-HopAfter all directly connected subnets have been created and attached to the device with connectors, list out the device’s routing table. Organize the next-hops and prefixes so that you have a list of next-hop IP’s (without duplicates) and their associated prefixes.

Moving through each next-hop IP, one at a time, add the next-hop as a device in the diagram (I usually add a router icon, since at this point, you don’t know what kind of device it is; then change it once you know) with a connector to a subnet appropriate to its IP. Once the device is added, add a colored bubble, connected to the remote device, containing a list of prefixes associated with that next-hop device.

Rinse & Repeat

After you have completed these steps by documenting all connected subnets, next-hop devices, and prefixes associated with each next-hop: move your attention to one of the next-hop devices and work through its directly connected subnets and routing table. As you discover the directly connected subnets, you should remove those prefixes from the prefix bubble and add them as proper subnets.

Working your way from the core towards the edge of the network will result in reducing the number of prefixes in the prefix bubbles since they will start showing up as subnet icons.

Documentation Tips

Make sure to visit the homepage for this series Network Documentation Series: Preamble and review the generic documentation tips listed there which apply to all network-related documents.

9 Comments

About John W Kerns

John is a network and systems engineer based in the Los Angeles/San Diego area. His background is in two traditionally stovepiped skill sets; systems administration and switching/routing/security. Most of his time is spent as an implementation engineer for a medium sized SoCal VAR. You can visit his blog at blog.packetsar.com or follow him on Twitter @PackeTsar

Comments

  1. What Lies Beneath says

    June 9, 2015 at 8:39 PM

    Hey John, me again. I get it (as this is how it goes mostly) but I find your choice of icons frustrating. The Cisco icons are 3D (mostly) and have a 180 degree perspective, the Visio network ‘pipe’ one a 90 degree perspective. Then the Visio cloud one has a 0 degree perspective and the other Cisco ones have a 135 degree perspective. This does not make for an easy to consume diagram IFAIAK.

    I appreciate you are describing a high degree of detail and you’ve put much thought into this but visually, I find it pretty messy and ultimately, rather ugly to the eye. Your consideration and method are flawless but for this.

    Reply
    • Adeel Ahmad says

      June 10, 2015 at 2:09 PM

      I personally prefer to have a white bordered box to represent a network device. That why I can add further details within the box such as management address, serial number or on a higher-level the function of the device (SW/FW/RR/etc).

      Reply
      • John W Kerns says

        June 10, 2015 at 4:00 PM

        From a practicality standpoint, I agree with you. But illustrated stencils are easier to read by other teams (storage, server, voice, etc) and by upper management. The templates I linked are an attempt at a compromise between form and function.

        Reply
    • John W Kerns says

      June 10, 2015 at 3:55 PM

      I agree buddy. The Cisco icons and the subnet stencils have a sort of “depth” to them. Although it is difficult to show depth on a cylindrical pipe. And the clouds have none.

      There is a pretty limited selection on these types of icons. There have been some fancy ones made for VMWare, but I don’t believe they have all the networking related icons needed in a network diagram.

      If you come across a better set of stencils that have everything needed and are somewhat matching, please let me know. I’d love to improve on my current stencil set.

      Reply
  2. Carlos says

    August 22, 2017 at 1:14 PM

    Hi,

    I love this site and page. very informative, thank you for creating it. I always reference it for to help invoke inspiration for big project documentation on all levels.

    Thanks again,

    Carlos

    Reply
  3. David S says

    September 1, 2017 at 5:27 PM

    The purple “WAN Edge” grouping box in the Template drawing. How did you create the irregular shaped purple colored box like that? That helps a lot. Sometimes on a crowded drawing, having too many big rectangular boxes doesn’t work right. Being able to make irregular shaped colored boxes like that would really help me out.

    Reply
    • John W Kerns says

      September 1, 2017 at 6:30 PM

      Yea this one takes a little more effort. You pretty much create a custom shape by drawing multiple lines which have ends that overlap (they have to overlap perfectly). After the shape is created (and fills in with white), you just edit the line and fill properties to have no line pattern, round the corners, and make the fill transparent.

      Reply
  4. Bob Schmidt says

    February 28, 2018 at 8:10 AM

    3D icons are old hat. 2D is the way.
    Long gone are the Cisco icons where you drag in modules etc. That level of information is best placed in a table in a detailed design.

    Reply
    • Greg Ferro says

      February 28, 2018 at 10:34 AM

      I work in 2D icons, I don’t have time for 3D. I would rather drink beer or sleep than waste time on 3D diagrams.

      Reply

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