Placing clusters on the same rank in Graphviz

You may use the newrank graph attribute (added in GraphViz 2.30) to activate the new ranking algorithm which allows defining rank=same for nodes which belong to clusters.

Add the following line at the top:

newrank=true;

Add the following line after the cluster definitions:

{ rank=same; router1; router2; }

Here's the resulting graph:

You may simply modify the edge between the routers:

router1 -> router2[constraint=false];

constraint indicates whether the edge should be used in the ranking of the nodes.