Declarative Diagramming (D2) Output Module

d2 output module create a description of network topology in D2 diagram scripting language. You can use that description with D2 commands to create topology- or routing protocol diagrams.

Note: The graph descriptions contain nodes and links, but no placement information. D2 is pretty good at figuring out how to draw the required graph, but it pays off to test out the layout engines. Changing the order of nodes or links might also unclutter the diagrams.

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Creating Graph Description File

The d2 output module is invoked with the netlab graph -e d2 command or by specifying the -o d2 parameter in the netlab create command. It takes an optional destination file name (default: graph.d2).

The -t parameter of the netlab graph command or a formatting modifier in the netlab create -o parameter can be used to specify the graph type:

• topology (default) – Includes point-to-point links, multi-access bridges, and stub subnets. When the network topology contains BGP information, the graph groups nodes into autonomous systems. Alternatively, you could set the defaults.outputs.d2.groups attribute to use topology groups to group graph nodes.

• bgp – Include autonomous systems, BGP routers, and color-coded BGP sessions.

• isis – Create a diagram of IS-IS routing, including areas, color-coded circuit types, and edge subnets (does not work with IS-IS running over VLANs)

Tip

The ‑f parameter of the netlab graph command or a formatting modifier of the ‌netlab create ‑o parameter can include BGP formatting parameters. For example, netlab graph -t bgp -f vrf draws VRF BGP sessions as dotted lines.

Modifying Global, Link, and Node Graph Attributes

You can set the graph title with graph.title or defaults.graph.title topology attribute.

You can use the d2 link and node attributes to change the style of individual nodes or links. The following attributes are built into netlab (but see also Extending D2 Style Attributes):

• d2.color – line color (stroke in D2 lingo)

• d2.fill – fill color (fill in D2 lingo)

• d2.width – line width (stroke-width in D2 lingo)

Tip

The generic link- and node attributes can also be specified as graph.attribute (for example, graph.color) values to use the same settings for GraphViz and D2 graphs.

You can also use the d2.format node attribute to specify D2 shape dimensions or position and the d2.format.style node/link[1] attribute to specify the D2 style of a shape or a connection.

For example, to draw a node as a rectangle 250 units wide and 70 units high, use:

nodes:
  spine:
    d2.format:
      width: 250
      height: 70
      shape: rectangle

To change the fill pattern of a node, use:

nodes:
  spine:
    d2.format:
      style.fill-pattern: grain

Likewise, to turn a link into a dashed line, use:

links:
- interfaces: [ a,b ]
  d2.format.style.stroke-dash: 5

Influencing the Graph Layout

netlab uses the node graph.rank attribute to sort nodes in link definitions to ensure the graph edges are always defined as going from nodes with a lower rank to nodes with a higher rank. The order of nodes in the graph edges influences the D2 layout engine.

The default value of the graph.rank attribute is 100, allowing you to push some nodes (with rank below 100) toward the top of the graph and others (with rank above 100) toward the bottom.

You can also use the graph.rank on links to influence how D2 draws multi-access links.

Finally, the link/interface graph.linkorder attribute allows you to specify the node order in individual links. The default graph.linkorder value is 50 for interfaces and 100 for subnets (multi-access links), resulting in subnets being “below” nodes unless you change the link- or interface graph.linkorder value.

Modifying Graph Appearance

Graphing routines use default topology settings to modify the node- or link parameters of the generated D2 file:

• outputs.d2.interface_labels (default: False) – Add IP addresses to links in topology graph. Results in a cluttered image.

• outputs.d2.groups (default: None) – use the specified list of groups (or all groups when set to True) to create graph clusters

• outputs.d2.node_address_label (default: True) – add node loopback IP addresses or IP addresses of the first interface (for hosts) to node labels.

• outputs.d2.as_clusters (default: True) – use BGP autonomous systems to cluster nodes in the topology graph. BGP AS clusters are always used in BGP graphs.

These default settings modify how the topology graphs look:

• outputs.d2.topology.vlan (default: False) – draw VLAN links in a different color. Use wider lines for VLAN trunk links.

These default settings modify how the BGP graphs look:

• outputs.d2.bgp.all (default: False) – show all lab devices in the BGP graph. By default, the BGP graphs include only lab devices running BGP.

• outputs.d2.bgp.rr (default: True) – draw arrows on BGP sessions to indicate peer-to-peer versus reflector-client sessions

• outputs.d2.bgp.vrf (default: False) – draw VRF BGP sessions as dotted lines

• outputs.d2.bgp.af.af (default: all address families) – when one or more af parameters (valid keys: ipv4, ipv6, vpnv4, vpnv6, 6pe, evpn) are set to True, the graph is limited to BGP sessions of the specified address families.

• outputs.d2.bgp.novrf (default: False) – do not include VRF BGP sessions in the graph

You can specify the above BGP parameters in the graph format CLI argument.

Modifying Shape and Connection Attributes

d2 output module uses graphite.icon device attribute to select the node style defined in defaults.outputs.d2.styles settings:

graphite icon	D2 style
router	oval shape
switch	hexagonal shape

You can also style these objects:

Object	Description
container	Autonomous system/group container formatting
title	Graph title formatting
node	Default device formatting
edge	Link formatting
lan	Multi-access subnet formatting
stub	Stub network formatting
ibgp	IBGP session formatting
ebgp	IBGP session formatting
localas_ibgp	Local-AS IBGP session formatting
confed_ebgp	Confederation EBGP session formatting
vrf	BGP VRF session formatting
level-1	IS-IS level-1-only link
level-2	IS-IS level-2-only link
level-1-2	IS-IS level-1-2 link
vlan_access	VLAN access link
vlan_trunk	VLAN trunk link

Each style definition is a dictionary of D2 shape/connection attributes and their values. To change the style of a D2 shape or connection, use the style dictionary within a style definition.

For example, to change the color of the VRF BGP sessions to red, use:

defaults.outputs.d2.styles.vrf.style.color: red

You could specify D2 attributes in your topology file (where you would have to prefix them with defaults), in per-user topology defaults, or with environment variables (even more details). You could also specify them with the -s parameter of the netlab create command, yet again prefixed with defaults (more details).

To display the current system defaults, use the ‌netlab show defaults outputs.d2.styles command.

Warning

netlab releases 25.07 and older specified D2 style attributes in the ‌defaults.outputs.d2 dictionary. The style attributes recognized by those releases are automatically migrated into the ‌defaults.outputs.d2.styles dictionary.

Extending D2 Style Attributes

You can define your own link/node style attributes:

• Define the attribute within defaults.outputs.d2.attributes dictionary. For example, you might define d2.background node attribute (a string) to add background color to nodes[2]:

defaults.outputs.d2.attributes.node.background: str

• Define a mapping between your attribute and D2 style attribute within the defaults.outputs.d2.style_map dictionary. For example, your d2.background attribute maps into D2 style.fill attribute:

defaults.outputs.d2.style_map.background: fill

Sample Graphs

The lab topologies used for platform integration testing contain numerous examples of graph attributes:

• Leaf-and-spine topology used to create topology graphs and test node- and link attributes and custom formatting.

• EVPN topology used to create BGP graphs

• IS-IS topology used to create IS-IS graphs and test node ranking

You’ll find further graph-creation tips on the ipSpace.net blog.

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[1]

Link attributes are applied to links between nodes or to the node representing a multi-access subnet.

[2]

See Lab Topology Attribute Validation and Valid Data Types for more details.