# Declarative Diagramming (D2) Output Module

*d2* output module create a description of network topology in [D2 diagram scripting language](https://d2lang.com/tour/intro). You can use that description with [D2 commands](https://d2lang.com/tour/install) to create topology diagrams.

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

A single formatting modifier can be used to specify the graph type:

* **topology** (default) -- Include, physical links, and multi-access- and stub subnets. When the network topology contains BGP information, the graph groups nodes into autonomous systems. Alternatively, you could set **defaults.outputs.graph.groups** attribute to use topology **[groups](../groups.md)** to group graph nodes.
* **bgp** -- Include autonomous systems, nodes, and BGP sessions. With the **rr** option (specified with `netlab create -o graph:bgp:rr`), RR-client sessions are drawn as directed arrows.

```{tip}
The network topology graph description contains nodes and links but no placement information. D2 is pretty good at figuring out how to draw the required graph, but it pays out to test out the [layout engines](https://d2lang.com/tour/layouts).
```

## Modifying Graph Attributes

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.node_interfaces** (default: False) -- add list of interfaces and their IP addresses to nodes[^DG].
* **outputs.d2.as_clusters** (default: True) -- use BGP autonomous systems to cluster nodes in topology graph. BGP AS clusters are always used in BGP graphs.
* **outputs.d2.rr_sessions** (default: True) -- draw IBGP sessions between BGP route reflectors and clients as directional connections.

[^DG]: The results look disgusting. If you find a better way to get it done please submit a PR. Thank you!

## Modifying Shape and Connection Attributes

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

You can also style `lan` shapes, `container` object (groups or autonomous systems) and `ibgp` or `ebgp` sessions.

You can use any D2 style attribute in these default settings. The following printout lists the system defaults within **defaults.output.d2** dictionary:

```
router:
  shape: oval

switch:
  shape: hexagon

lan:
  shape: rectangle
  style.border-radius: 8

ibgp:
  style:
    stroke: '#613913'
    stroke-width: 4
  source-arrowhead:
    shape: arrow
  target-arrowhead:
    shape: arrow

ebgp:
  style:
    stroke: '#b21a1a'
    stroke-width: 4
  source-arrowhead:
    shape: arrow
  target-arrowhead:
    shape: arrow
```

## Specifying D2 Attributes

You could specify D2 attributes in your topology file, in per-user topology defaults, or in system defaults ([more details](../defaults.md)). You could also specify them with `-s` parameter of **netlab create** command ([more details](../netlab/create.md)).

Use the `netlab create -o yaml:defaults.outputs.d2` to show the actual D2 defaults including topology file defaults and user defaults, for example:

```
$ netlab create -o yaml:defaults.outputs.d2
as_clusters: true
ebgp:
  source-arrowhead:
    shape: arrow
  style:
    stroke: '#b21a1a'
    stroke-width: 4
  target-arrowhead:
    shape: arrow
ibgp:
  source-arrowhead:
    shape: arrow
  style:
    stroke: '#613913'
    stroke-width: 4
  target-arrowhead:
    shape: arrow
interface_labels: false
lan:
  shape: rectangle
  style:
    border-radius: 8
node_address_label: true
router:
  shape: oval
rr_sessions: true
switch:
  shape: hexagon
```