Blackstone Infrastructure Advisors L.L.C. New York, NY 10154 Attention: John G. Finley Email: john.finley@blackstone.com Ladies and Gentlemen:

Exhibit 99.11

Xxxxx 00, 0000

Xxxxxxxxxx Infrastructure Advisors L.L.C.

000 Xxxx Xxxxxx

Xxx Xxxx, XX 00000

Attention: Xxxx X. Xxxxxx

Email: xxxx.xxxxxx@xxxxxxxxxx.xxx

Ladies and Gentlemen:

Reference is made to that certain Purchase Agreement (“Purchase Agreement”), dated as of January 30, 2019, by and among Tallgrass Energy Holdings, LLC, a Delaware limited liability company (“Holdings”), the other sellers named therein (together with Holdings, each a “Seller” and collectively, the “Sellers”), Prairie GP Acquiror LLC, a Delaware limited liability company (“GP Acquiror”), Prairie ECI Acquiror LP, a Delaware limited partnership (“Up-C Acquiror 1”), Prairie VCOC Acquiror LP, a Delaware limited partnership (“Up-C Acquiror 2” and, together with Up-C Acquiror 1, “Up-C Acquirors”), and Prairie Non-ECI Acquiror LP, a Delaware limited partnership (“Class A Acquiror” and, together with GP Acquiror and Up-C Acquirors, each an “Acquiror” and collectively, the “Acquirors”), and Xxxxx X. Xxxxxxxxx, Xx., Xxxx X. Xxxxxxx and Xxxxx X. Xxxxxxx, in their respective capacities as Seller Representatives (each, a “Seller Representative” and collectively, the “Seller Representatives”). Each capitalized term used herein but not defined shall have the meaning ascribed to such term in the Purchase Agreement.

The purpose of this letter agreement (this “Side Letter”) is to confirm the understanding of the Sellers and the Acquirors with respect to certain matters related to the Purchase Agreement. Accordingly, in consideration of the mutual agreements contained herein, the parties hereby agree as follows:

1. Xxxxx Trust. The Xxxxxxx X. Xxxxx Revocable Trust, under trust agreement dated August 27, 2013 (“Xxxxx Trust”) will not sell any of its Class A Shares under the Purchase Agreement and is no longer a Party to, or bound by the terms and obligations of, the Purchase Agreement for any purpose. Accordingly, all references to the “Xxxxx Trust” in the Purchase Agreement (including the exhibits thereto) are hereby omitted.

2. DGD Class A Shares. The Xxxxx X. Xxxxxxxxx, Xx. Revocable Trust, a revocable trust under trust agreement dated April 26, 2006 (“DGD Trust”), will not sell any of its Class A Shares under the Purchase Agreement and all references to the “DGD Class A Shares” in the Purchase Agreement are hereby omitted. DGD Trust shall remain a Seller under the Purchase Agreement with respect to its Up-C Interests as set forth on Schedule 2.1 hereto.

3. Seller Notes. Promissory Notes will not be issued by Class A Acquiror, Up-C Acquiror 1 or Blackstone Infrastructure Partners V L.P. to certain of the Sellers at Closing, or otherwise in connection with the Purchase Agreement. Accordingly, all references to “Seller Notes” and “Exhibit D” in the Purchase Agreement are hereby omitted.

4. Purchase Price and Other Schedules.

a. Schedule 2.1 to the Purchase Agreement is hereby replaced in its entirety with the new Schedule 2.1 attached hereto;

b. The reference in the fifth recital to the Purchase Agreement to “26,339,528” TE Units, shall be changed to “21,751,018”;

c. The reference in Section 2.1 of the Purchase Agreement to an aggregate Purchase Price of “$3,272,860,010”, shall be changed to “$3,213,161,149”;

d. Schedule A-1 to the Purchase Agreement is hereby replaced in its entirety with the new Schedule A-1 attached hereto; and

e. Schedule 6.12(b) to the Purchase Agreement is hereby replaced in its entirety with the new Schedule 6.12(b) attached hereto.

5. Wires. Notwithstanding anything to the contrary in the Purchase Agreement, each Seller acknowledges and agrees that the payments at the Closing made pursuant to the funds flow attached hereto as Exhibit E satisfy the Acquirors’ payment obligations under Section 2.1 of the Purchase Agreement.

6. Effect of Side Letter.  Except as otherwise expressly provided herein, all of the respective terms and conditions of the Purchase Agreement remain unchanged and continue in full force and effect.  In the case of any conflict between this Side Letter and the Purchase Agreement, the terms of this Side Letter will govern. This Side Letter shall be deemed to be in full force and effect from and after the execution of this Side Letter by the parties.

7. Governing Law.  This Side Letter shall be governed by and construed in accordance with the laws of the State of Delaware, without regard to the principles of conflicts of laws thereof.

8. Counterparts.  This Side Letter may be executed and delivered (including by facsimile or other electronic transmission) in one or more counterparts, and by the different parties hereto in separate counterparts, each of which when executed shall be deemed to be an original, but all of which taken together shall constitute one and the same agreement.

[Signature Pages Follow]

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If the foregoing correctly sets forth your understanding of our agreement with respect to the matters above, please indicate your acceptance and approval below.

	Very truly yours,
	TALLGRASS ENERGY HOLDINGS, LLC
	By:	/s/ Xxxxx X. Xxxxxxxxx, Xx.
	Name:	Xxxxx X. Xxxxxxxxx, Xx.
	Title:	Chief Executive Officer and President
	TALLGRASS KC, LLC
	By:	/s/ Xxxxx X. Xxxxxxxxx, Xx.
	Name:	Xxxxx X. Xxxxxxxxx, Xx.
	Title:	Manager
	/s/ Xxxxx X. Xxxxxxxxx, Xx.
	XXXXX X. XXXXXXXXX, XX., TRUSTEE OF THE XXXXX X. XXXXXXXXX, XX. REVOCABLE TRUST, UNDER TRUST AGREEMENT DATED APRIL 26, 2006
	TALLGRASS HOLDINGS, LLC
	By:	EMG Fund II Management, LP,
		its manager
	By:	EMG Fund II Management, LLC,
		its general partner
	By:	/s/ Xxxx X. Xxxxxxx
	Name:	Xxxx X. Xxxxxxx
	Title:	Chief Executive Officer

	KIA VIII (RUBICON), L.P.
	By:	KIA VIII (Rubicon) GP, L.P.,
		its general partner
	By:	Xxxxx XX VIII, LLC,
		its general partner
	By:	/s/ Xxxxx X. Xxxxxxx, XX
	Name:	Xxxxx X. Xxxxxxx, XX
	Title:	Managing Member
	KEP VI AIV (RUBICON), LLC
	By:	/s/ Xxxxx X. Xxxxxxx, XX
	Name:	Xxxxx X. Xxxxxxx, XX
	Title:	Managing Member

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	XXXXX VENTURES LLC
	By:	/s/ Xxxxxxx Xxxxx Xxxxx
	Name:	Xxxxxxx Xxxxx Xxxxx
	Title:	Manager
	XXXXX FAMILY FOUNDATION
	By:	/s/ Xxxxxxx Xxxxx Xxxxx
	Name:	Xxxxxxx Xxxxx Xxxxx
	Title:	President
	XXXXX VENTURES, LLC
	By:	/s/ Xxxxxxx Xxxxx
	Name:	Xxxxxxx Xxxxx
	Title:	President
	/s/ Xxxxx Xxxxx Grills
	XXXXX XXXXX GRILLS
	MATAGATOR FOUNDATION
	By:	/s/ Xxxxxxx Xxxxx
	Name:	Xxxxxxx Xxxxx
	Title:	Trustee

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Acknowledged and agreed:
ACQUIRORS:
PRAIRIE GP ACQUIROR LLC
By: Prairie Non-ECI Acquiror LP, its sole member
By: BIP Holdings Manager L.L.C., its general partner
By:	/s/ Xxxx Klimzack
Name:	Xxxx Xxxxxxxx
Title:	Senior Managing Director
PRAIRIE ECI ACQUIROR LP
By:	BIP Holdings Manager L.L.C., its general partner
By:	/s/ Xxxx Klimzack
Name:	Xxxx Xxxxxxxx
Title:	Senior Managing Director
PRAIRIE VCOC ACQUIROR LP
By: BIP Holdings Manager L.L.C., its general partner
By:	/s/ Xxxx Klimzack
Name:	Xxxx Xxxxxxxx
Title:	Senior Managing Director
PRAIRIE NON-ECI ACQUIROR LP
By:  BIP Holdings Manager L.L.C., its general partner
By:	/s/ Xxxx Klimzack
Name:	Xxxx Xxxxxxxx
Title:	Senior Managing Director

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Acknowledged and agreed:

/s/ Xxxxxxx X. Xxxxx

XXXXXXX X. XXXXX, TRUSTEE OF THE XXXXXXX X. XXXXX REVOCABLE TRUST UNDER TRUST AGREEMENT DATED AUGUST 27, 2013

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