I recently came a cross a x509 P(rivate)KI Root Certificate which had a pathLen constrain set on the (self signed) Root Certificate. Since that is not commonly seen I looked a bit around to get a better understanding about how the pathLen basic constrain should be used.

Primary source is RFC 5280 section

The pathLenConstraint field is meaningful only if the cA boolean is asserted and the key usage extension, if present, asserts the keyCertSign bit (Section In this case, it gives the maximum number of non-self-issued intermediate certificates that may follow this certificate in a valid certification path

Since the Root is always self-issued it doesn't count towards the limit, and since it's the last certificate (or the first depending on how you count) in a chain, it's pretty much pointless to configure a pathLen constrain directly on a Root Certificate.

Another relevant resource are the Baseline Requirements of the CA/Browser Forum (currently v2.0.2). Section "Root CA Basic Constraints" describes it as NOT RECOMMENDED for a Root CA.

Last but not least there is the awesome x509 Limbo project which has a section for validating pathLen constrains. Since the RFC 5280 based assumption is that self signed certs do not count, they do not check a case with such a constrain on the Root itself, and what the implementations do about it. So the assumption right now is that they properly ignore it.

Summary: It's pointless to set the pathLen constrain on the Root Certificate, so just don't do it.