Graphviz is an open source graph visualization software. Imagine you have to describe your network topology to a colleague without the benefit of a picture. You might say, "Our network consists of three layers: core, distribution, and access. The core layer comprises two routers for redundancy, and both the routers are fully meshed toward the four distribution routers; they are also fully meshed toward the access routers. The internal routing protocol is OSPF, and externally, we use BGP for our provider." While this description lacks some details, it is probably enough for your colleague to paint a pretty good high-level picture of your network.

Graphviz works similarly to the process of describing the graph in text term, then asking the Graphviz program to construct the graph for us. Here, the graph is described in a text format called DOT (https://en.wikipedia.org/wiki/DOT_(graph_description_language)) and Graphviz renders the graph based on the description. Of course, because the computer lacks human imagination, the language has to be precise and detailed.

In this section, we will use the Link Layer Discovery Protocol (LLDP) to query the device neighbors and create a network topology graph via Graphviz. Upon completing this extensive example, we will see how we can take something new, such as Graphviz, and combine it with things we have already learned to solve interesting problems.

Let's start by constructing the lab we will be using.