Is filing of dir-11 compulsory?

Yes.It is Mandatory as per Section 168(1).

For your ready reference,

Section 168:
(1) A director may resign from his office by giving a notice in writing to the company and the Board shall on receipt of such notice take note of the same and the company shall intimate the Registrar in such manner, within such time and in such form as may be prescribed and shall also place the fact of such resignation in the report of directors laid in the immediately following general meeting by the company:(DIR12)
Provided that a director shall also forward a copy of his resignation along with detailed reasons for the resignation to the Registrar within thirty days of resignation in such manner as may be prescribed.(DIR11)