See also maintenance item 0271.
Clause 35.5 talks about bridges having a default configuration. The term "default configuration" in particular, and the term "default" in general, are used to refer to the default value for an implementation; it's what a vendor ships when someone orders an 802.1Q Bridge. In 34.5, however, and in the paragraph cited in maintenance item 0271, the term "default configuration" is used in the sense of, "If you decide to use this feature in your network, here is a good plan for configuring your bridges." This isolated misuse of the term "default configuration" can cause, and in fact seems to have caused, confusion for readers who follow the words of the standard, instead of the intent of the contributors/editors. The state that, if my product supports the CBS ("supports" means "has implemented", not "has this feature turned on"), then its required out-of-the-box configuration has the CBS turned on for priorities 2 and 3. This may be true of IEEE Std 802.1BA, but it is certainly not true for 802.1Q. (It would mean that an full-featured 802.1Q bridge black-holes priorities 2 and 3 until it has been configured and/or runs MSRP. 34.5 is a recommendation to network administrators, not to implementers.