Input service Distributor GST

Dear,

One of our client going to take ISD registration, I need some information regarding ISD.

1) Do we have to give ISD GST number to relevant supplier so that GST input not shown in GSTR-2B of head office GST number,

2) GSTR-6A are same as GSTR-2B

3) can we choose Cross charge billing to Branch instead of distributing input through ISD

4) How Entries made in HO books and Branch Books