Sec Form 4 Filing - SUMMIT PARTNERS L P @ A10 Networks, Inc. - 2021-12-07

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person SUMMIT PARTNERS L P 2. Issuer Name and Ticker or Trading Symbol A10 Networks, Inc. [ ATEN] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director _____ 10% Owner _____ Officer (give title below) X __ Other (specify below) Manager of GP of 10% owner (Last) (First) (Middle) 222 BERKELEY STREET, 18TH FLOOR 3. Date of Earliest Transaction (MM/DD/YY) 12/07/2021 (Street) BOSTON, MA02116 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) _____ Form filed by One Reporting Person __ X __ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock 12/07/2021 S 700,000( 1 ) D $ 15.05 8,877,890( 2 ) I See Footnotes.( 3 )( 4 )

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other SUMMIT PARTNERS L P 222 BERKELEY STREET 18TH FLOOR BOSTON, MA02116 Manager of GP of 10% owner SUMMIT PARTNERS GE VIII, L.P. 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA02116 X GP of 10% owner Summit Partners GE VIII, LLC 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA02116 X GP of 10% owner Summit Investors Management, LLC 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA02116 Manager and GP of 10% Owner Summit Partners Growth Equity Fund VIII-A, L.P. 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA02116 X X Summit Partners Growth Equity Fund VIII-B, L.P. 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA02116 X X Summit Investors I, LLC 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA02116 X X Summit Investors I (UK), L.P. 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA02116 X X MANNION MARTIN J C/O SUMMIT PARTNERS 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA02116 X X Signatures Summit Partners, L.P., by Summit Master Company, LLC, its GP, by /s/ Robin W. Devereux, Member 12/09/2021 Signature of Reporting Person Date Summit Partners GE VIII, L.P., by Summit Partners GE VIII, LLC, its General Partner, by Summit Partners, L.P., its Manager, by Summit Master Company, LLC, its General Partner, by /s/ Robin W. Devereux, Member 12/09/2021 Signature of Reporting Person Date Summit Partners GE VIII, LLC, by Summit Partners, L.P., its Manager, by Summit Master Company, LLC, its General Partner, by /s/ Robin W. Devereux, Member 12/09/2021 Signature of Reporting Person Date Summit Investors Management, LLC, by Summit Master Company, LLC, its Managing Member, by Robin W. Devereux, Member 12/09/2021 Signature of Reporting Person Date Summit Partners Growth Equity Fund VIII-A, L.P., by Summit Partners GE VIII, L.P., its GP, by Summit Partners GE VIII, LLC, its GP, by Summit Partners, L.P., its Manager, by Summit Master Company, LLC, its GP, by /s/ Robin W. Devereux, Member 12/09/2021 Signature of Reporting Person Date Summit Partners Growth Equity Fund VIII-B, L.P., by Summit Partners GE VIII, L.P., its GP, by Summit Partners GE VIII, LLC, its GP, by Summit Partners, L.P., its Manager, by Summit Master Company, LLC, its GP, by /s/ Robin W. Devereux, Member 12/09/2021 Signature of Reporting Person Date Summit Investors I, LLC, by Summit Investors Management, LLC, its Manager, by Summit Master Company, LLC, its Managing Member, by Robin W. Devereux, Member 12/09/2021 Signature of Reporting Person Date Summit Investors I (UK), L.P., by Summit Investors Management, LLC, its General Partner, by Summit Master Company, LLC, its Managing Member, by Robin W. Devereux, Member 12/09/2021 Signature of Reporting Person Date Robin W. Devereux, POA for Martin J. Mannion 12/09/2021 Signature of Reporting Person Date

Explanation of Responses:

( 1 )The Common Stock sold as follows: 510,318 shares in the name of Summit Partners Growth Equity Fund VIII-A, L.P.; 186,436 shares in the name of Summit Partners Growth Equity Fund VIII-B, L.P.; 2,984 shares in the name of Summit Investors I, LLC; 262 shares in the name of Summit Investors I (UK), L.P. ( 2 )The Common Stock held as follows: 6,362,818 shares in the name of Summit Partners Growth Equity Fund VIII-A, L.P.; 2,324,553 shares in the name of Summit Partners Growth Equity Fund VIII-B, L.P.; 37,202 shares in the name of Summit Investors I, LLC; 3,273 shares in the name of Summit Investors I (UK), L.P.; 150,044 shares (including shares underlying RSUs) in the name of Mr. Chung, which are held for the benefit of Summit Partners, L.P. ( 3 )Summit Partners, L.P. is the managing member of Summit Partners GE VIII, LLC, which is the general partner of Summit Partners GE VIII, L.P., which is the general partner of each of Summit Partners Growth Equity Fund VIII-A, L.P. and Summit Partners Growth Equity Fund VIII-B, L.P. Summit Master Company, LLC is the managing member of Summit Investors Management, LLC, which is the manager of Summit Investors I, LLC and the general partner of Summit Investors I (UK), L.P. Summit Partners GE VIII, LLC, as the general partner of Summit Partners GE VIII, L.P., and Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC has each delegated investment decisions, including voting and dispositive power, to Summit Partners, L.P. and its Investment Committee responsible for voting and investment decisions with respect to the Company. ( 4 )Summit Partners, L.P., through a two-person Investment Committee responsible for voting and investment decisions with respect to the Company, currently composed of Martin J. Mannion and Mr. Chung, has voting and dispositive authority over the shares of Common Stock held by each of the entities described in the foregoing footnotes (2) and (3) and therefore may be deemed to beneficially own such shares. In addition, Mr. Chung is a member of Summit Master Company, LLC. Each of the Summit entities, Mr. Mannion and Mr. Chung disclaims beneficial ownership of the shares of Common Stock (including shares underlying RSUs) reported herein, except to the extent of their pecuniary interest therein.

Remarks: Form 2 of 2

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