Does aggregation pro-rata rule apply to mega backdoor rollovers of after-tax 401k funds to Roth IRA?

[vpzvmgir]

Does aggregation pro-rata rule apply to mega backdoor rollovers of after-tax 401k funds to Roth IRA?

I.e., should the rollover amount also include the pre-tax funds in the proportion calculated based on all traditional IRAs owned by the individual?

It is my understanding that such requirement applies to backdoor (not mega) rollovers from a traditional IRA to Roth IRA. If available, please provide a reference to the specific source (IRS publication?) explaining the answer to this question. 

[dmertz]

A 401(k) is not an IRA.  Nothing about a mega backdoor Roth is reportable on Form 8606.  Your traditional IRAs are not involved (unless you mistakenly rolled the funds from the 401(k) over to a traditional IRA).

[Opus 17]

Your 401k plan is required to split the pre-tax and after-tax funds, and rollover the pre-tax funds to a traditional IRA and the after-tax funds to a Roth IRA.  The pro-rata rule does not apply to qualified workplace plans.  After the rollover, you will have a Roth IRA and a traditional IRA, and your traditional IRA will not have any portion of after-tax funds (unless it did already for other reasons).