How to remove border around subgraphs

this question has been asked here:

the answer given was to use style=invis but the problem with it as mentioned in one of the comments is that it removes the subgraph label as well which is not desired. so how can one remove just the border around subgraphs and nothing else?

use peripheries=0

digraph D {
	subgraph cluster1 {
		label="c 1"
		a b c
	subgraph cluster2 {
		d e f
		label="c 2"

produces this:

color=transparent also works. that’s what I did.