I would like these two nodes to appear on the same level:
digraph
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: