Graphviz is an open source graph visualization software. Imagine if we have to describe our network topology to a colleague without the benefit of a picture. We might say, our network consists of three layers: core, distribution, and access. The core layer comprises two routers for redundancy, and both of the routers are full-meshed toward the four distribution routers; the distribution routers are also full-meshed toward the access routers. The internal routing protocol is OSPF, and externally, we use BGP for peering with our service 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 by describing the graph in the text format that Graphviz can understand, then we can feed the file to the Graphviz program to construct the graph...