UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

Form N-CSR

CERTIFIED SHAREHOLDER REPORT OF REGISTERED

MANAGEMENT INVESTMENT COMPANIES

Investment Company Act File Number: 811-22741

CMBS Portfolio

(Exact Name of Registrant as Specified in Charter)

Two International Place, Boston, Massachusetts 02110

(Address of Principal Executive Offices)

Maureen A. Gemma

Two International Place, Boston, Massachusetts 02110

(Name and Address of Agent for Services)

(617) 482-8260

(Registrant’s Telephone Number)

October 31

Date of Fiscal Year End

April 30, 2016

Date of Reporting Period

Item 1. Reports to Stockholders

CMBS Portfolio

April 30, 2016

Portfolio of Investments (Unaudited)

Commercial Mortgage-Backed Securities — 65.9%
Security	Principal Amount (000’s omitted)		Value
A10 Securitization, LLC
Series 2013-1, Class A, 2.40%, 11/15/25(1)	$	7	$	6,821
Americold LLC Trust
Series 2010-ARTA, Class D, 7.443%, 1/14/29(1)		250		281,615
Banc of America Commercial Mortgage Trust
Series 2006-3, Class A4, 5.889%, 7/10/44(2)		527		526,952
Series 2006-6, Class A4, 5.356%, 10/10/45		1,000		1,005,959
Bear Stearns Commercial Mortgage Securities Trust
Series 2006-PW14, Class A4, 5.201%, 12/11/38		1,586		1,602,568
CD Commercial Mortgage Trust
Series 2006-CD3, Class A5, 5.617%, 10/15/48		896		898,479
COMM Mortgage Trust
Series 2006-C8, Class A4, 5.306%, 12/10/46		415		419,647
Series 2014-KYO, Class D, 2.437%, 6/11/27(1)(3)		810		803,433
Credit Suisse Commercial Mortgage Trust
Series 2006-C4, Class A3, 5.467%, 9/15/39		614		615,714
DBUBS Mortgage Trust
Series 2011-LC1A, Class A1, 3.742%, 11/10/46(1)		19		19,330
Extended Stay America Trust
Series 2013-ESH7, Class C7, 3.902%, 12/5/31(1)		615		618,043
GCCFC Commercial Mortgage Trust
Series 2007-GG9, Class A4, 5.444%, 3/10/39		1,683		1,712,060
JPMorgan Chase Commercial Mortgage Securities Trust
Series 2006-CB16, Class A4, 5.552%, 5/12/45		157		156,871
Series 2006-CB17, Class A4, 5.429%, 12/12/43		1,291		1,304,709
Series 2006-LDP7, Class A4, 6.147%, 4/17/45(2)		419		418,123
Series 2006-LDP8, Class A4, 5.399%, 5/15/45		287		287,662
Series 2006-LDP9, Class A3, 5.336%, 5/15/47		1,209		1,227,387
Series 2006-LDP9, Class AM, 5.372%, 5/15/47		1,380		1,376,770
ML-CFC Commercial Mortgage Trust
Series 2006-4, Class A3, 5.172%, 12/12/49		1,225		1,235,433
Morgan Stanley Capital I Trust
Series 2006-IQ12, Class A4, 5.332%, 12/15/43		1,500		1,509,553
Series 2007-IQ15, Class A4, 6.114%, 6/11/49(2)		1,493		1,548,655
Wachovia Bank Commercial Mortgage Trust
Series 2006-C27, Class A3, 5.765%, 7/15/45(2)		390		389,195
Series 2006-C28, Class A4, 5.572%, 10/15/48		917		923,207
Series 2006-C29, Class A4, 5.308%, 11/15/48		1,689		1,716,949
Total Commercial Mortgage-Backed Securities (identified cost $20,844,006)			$	20,605,135

Asset-Backed Securities — 14.1%
Security	Principal Amount (000’s omitted)		Value
American Homes 4 Rent
Series 2014-SFR1, Class C, 2.186%, 6/17/31(1)(3)	$	650	$	636,153
Series 2014-SFR1, Class D, 2.536%, 6/17/31(1)(3)		100		97,540
American Residential Properties Trust
Series 2014-SFR1, Class C, 2.786%, 9/17/31(1)(3)		700		689,956
Colony American Homes
Series 2014-1A, Class C, 2.286%, 5/17/31(1)(3)		250		245,406
Series 2014-1A, Class D, 2.586%, 5/17/31(1)(3)		500		489,165
Dell Equipment Finance Trust
Series 2014-1, Class C, 1.80%, 6/22/20(1)		100		100,045
First Investors Auto Owner Trust
Series 2016-1A, Class A1, 1.92%, 5/15/20(1)		920		921,961
Invitation Homes Trust
Series 2014-SFR1, Class D, 3.036%, 6/17/31(1)(3)		500		495,635
Santander Drive Auto Receivables Trust
Series 2016-1, Class A2A, 1.41%, 7/15/19		750		751,488
Total Asset-Backed Securities (identified cost $4,461,951)			$	4,427,349
Short-Term Investments — 19.8%
Description	Interest (000’s omitted)		Value
Eaton Vance Cash Reserves Fund, LLC, 0.53%(4)	$	6,196	$	6,196,467
Total Short-Term Investments (identified cost $6,196,467)			$	6,196,467
Total Investments — 99.8% (identified cost $31,502,424)			$	31,228,951
Other Assets, Less Liabilities — 0.2%			$	57,208
Net Assets — 100.0%			$	31,286,159

The percentage shown for each investment category in the Portfolio of Investments is based on net assets.

⁽¹⁾  Security exempt from registration pursuant to Rule 144A under the Securities Act of 1933, as amended. These securities may be sold in certain transactions in reliance on an exemption from registration (normally to qualified institutional buyers). At April 30, 2016, the aggregate value of these securities is $5,405,103 or 17.3% of the Portfolio’s net assets.

⁽²⁾  Weighted average fixed-rate coupon that changes/updates monthly. Rate shown is the rate at April 30, 2016.

1 See Notes to Financial Statements.

CMBS Portfolio

April 30, 2016

Portfolio of Investments (Unaudited) — continued

⁽³⁾  Variable rate security. The stated interest rate represents the rate in effect at April 30, 2016.

⁽⁴⁾  Affiliated investment company, available to Eaton Vance portfolios and funds, which invests in high quality, U.S. dollar denominated money market instruments. The rate shown is the annualized seven-day yield as of April 30, 2016.

2 See Notes to Financial Statements.

CMBS Portfolio

April 30, 2016

Statement of Assets and Liabilities (Unaudited)

Assets	April 30, 2016
Unaffiliated investments, at value (identified cost, $25,305,957)	$	25,032,484
Affiliated investment, at value (identified cost, $6,196,467)		6,196,467
Interest receivable		93,328
Interest receivable from affiliated investment		1,990
Receivable from affiliate		4,265
Total assets	$	31,328,534
Liabilities
Payable to affiliates:
Investment adviser fee	$	14,153
Trustees’ fees		191
Accrued expenses		28,031
Total liabilities	$	42,375
Net Assets applicable to investors’ interest in Portfolio	$	31,286,159
Sources of Net Assets
Investors’ capital	$	31,559,632
Net unrealized depreciation		(273,473	)
Total	$	31,286,159

3 See Notes to Financial Statements.

CMBS Portfolio

April 30, 2016

Statement of Operations (Unaudited)

Investment Income	Six Months Ended April 30, 2016
Interest	$	252,035
Interest allocated from affiliated investment		8,225
Expenses allocated from affiliated investment		(347	)
Total investment income	$	259,913
Expenses
Investment adviser fee	$	88,850
Trustees’ fees and expenses		1,136
Custodian fee		14,671
Legal and accounting services		19,199
Miscellaneous		2,658
Total expenses	$	126,514
Deduct —
Allocation of expenses to affiliate	$	21,592
Total expense reductions	$	21,592
Net expenses	$	104,922
Net investment income	$	154,991
Realized and Unrealized Gain (Loss)
Net realized gain (loss) —
Investment transactions	$	(11,187	)
Investment transactions allocated from affiliated investment		12
Net realized loss	$	(11,175	)
Change in unrealized appreciation (depreciation) —
Investments	$	107,617
Net change in unrealized appreciation (depreciation)	$	107,617
Net realized and unrealized gain	$	96,442
Net increase in net assets from operations	$	251,433

4 See Notes to Financial Statements.

CMBS Portfolio

April 30, 2016

Statements of Changes in Net Assets

Increase (Decrease) in Net Assets	Six Months Ended April 30, 2016 (Unaudited)			Year Ended October 31, 2015
From operations —
Net investment income	$	154,991		$	216,972
Net realized loss from investment transactions		(11,175	)		(21,533	)
Net change in unrealized appreciation (depreciation) from investments		107,617			(48,973	)
Net increase in net assets from operations	$	251,433		$	146,466
Capital transactions —
Contributions	$	1,388,191		$	3,341,284
Withdrawals		(3,567,004	)		(4,941,208	)
Net decrease in net assets from capital transactions	$	(2,178,813	)	$	(1,599,924	)
Net decrease in net assets	$	(1,927,380	)	$	(1,453,458	)
Net Assets
At beginning of period	$	33,213,539		$	34,666,997
At end of period	$	31,286,159		$	33,213,539

5 See Notes to Financial Statements.

CMBS Portfolio

April 30, 2016

Supplementary Data

	Six Months Ended April 30, 2016 (Unaudited)			Year Ended October 31,
Ratios/Supplemental Data				2015			2014			2013 (1)
Ratios (as a percentage of average daily net assets):
Expenses(2)(3)		0.65	%(4)		0.65	%		0.65	%		0.65	%
Net investment income		0.96	%(4)		0.64	%		1.03	%		0.72	%
Portfolio Turnover		35	%(5)		27	%		38	%		55	%
Total Return		0.81	%(5)		0.44	%		0.60	%		(0.30	)%
Net assets, end of period (000’s omitted)	$	31,286		$	33,214		$	34,667		$	26,036

⁽¹⁾  The Portfolio commenced operations on November 1, 2012.

⁽²⁾  Excludes the effect of custody fee credits, if any, of less than 0.005%.

⁽³⁾  The investment adviser reimbursed expenses (equal to 0.13%, 0.13%, 0.13% and 0.12% of average daily net assets for the six months ended April 30, 2016 and the years ended October 31, 2015, 2014 and 2013, respectively). Absent this reimbursement, total return would be lower.

⁽⁴⁾  Annualized.

⁽⁵⁾  Not annualized.

6 See Notes to Financial Statements.

CMBS Portfolio

April 30, 2016

Notes to Financial Statements (Unaudited)

1  Significant Accounting Policies

CMBS Portfolio (the Portfolio) is a Massachusetts business trust registered under the Investment Company Act of 1940, as amended (the 1940 Act), as a non-diversified, open-end management investment company. The Portfolio’s investment objective is total return. The Declaration of Trust permits the Trustees to issue interests in the Portfolio. At April 30, 2016, Eaton Vance Multi-Strategy Absolute Return Fund, Eaton Vance Multi-Strategy All Market Fund and Eaton Vance Short Duration Real Return Fund held an interest of 64.9%, 5.9% and 28.2%, respectively, in the Portfolio.

The following is a summary of significant accounting policies of the Portfolio. The policies are in conformity with accounting principles generally accepted in the United States of America (U.S. GAAP). The Portfolio is an investment company and follows accounting and reporting guidance in the Financial Accounting Standards Board (“FASB”) Accounting Standards Codification Topic 946.

A  Investment Valuation — The following methodologies are used to determine the market value or fair value of investments.

Debt Obligations. Debt obligations (including short-term obligations with a remaining maturity of more than sixty days) are generally valued on the basis of valuations provided by third party pricing services, as derived from such services’ pricing models. Inputs to the models may include, but are not limited to, reported trades, executable bid and asked prices, broker/dealer quotations, prices or yields of securities with similar characteristics, interest rates, anticipated prepayments, benchmark curves or information pertaining to the issuer, as well as industry and economic events. The pricing services may use a matrix approach, which considers information regarding securities with similar characteristics to determine the valuation for a security. Short-term obligations purchased with a remaining maturity of sixty days or less are generally valued at amortized cost, which approximates market value.

Affiliated Fund. The Portfolio may invest in Eaton Vance Cash Reserves Fund, LLC (Cash Reserves Fund), an affiliated investment company managed by Eaton Vance Management (EVM). The value of the Portfolio’s investment in Cash Reserves Fund reflects the Portfolio’s proportionate interest in its net assets. Cash Reserves Fund generally values its investment securities utilizing the amortized cost valuation technique in accordance with Rule 2a-7 under the 1940 Act. This technique involves initially valuing a portfolio security at its cost and thereafter assuming a constant amortization to maturity of any discount or premium. If amortized cost is determined not to approximate fair value, Cash Reserves Fund may value its investment securities in the same manner as debt obligations described above.

Fair Valuation. Investments for which valuations or market quotations are not readily available or are deemed unreliable are valued at fair value using methods determined in good faith by or at the direction of the Trustees of the Portfolio in a manner that fairly reflects the security’s value, or the amount that the Portfolio might reasonably expect to receive for the security upon its current sale in the ordinary course. Each such determination is based on a consideration of relevant factors, which are likely to vary from one pricing context to another. These factors may include, but are not limited to, the type of security, the existence of any contractual restrictions on the security’s disposition, the price and extent of public trading in similar securities of the issuer or of comparable companies or entities, quotations or relevant information obtained from broker/dealers or other market participants, information obtained from the issuer, analysts, and/or the appropriate stock exchange (for exchange-traded securities), an analysis of the company’s or entity’s financial condition, and an evaluation of the forces that influence the issuer and the market(s) in which the security is purchased and sold.

B  Investment Transactions — Investment transactions for financial statement purposes are accounted for on a trade date basis. Realized gains and losses on investments sold are determined on the basis of identified cost.

C  Income — Interest income is recorded on the basis of interest accrued, adjusted for amortization of premium or accretion of discount.

D  Federal Taxes — The Portfolio has elected to be treated as a partnership for federal tax purposes. No provision is made by the Portfolio for federal or state taxes on any taxable income of the Portfolio because each investor in the Portfolio is ultimately responsible for the payment of any taxes on its share of taxable income. If one of the Portfolio’s investors is a regulated investment company that invests all or substantially all of its assets in the Portfolio, the Portfolio normally must satisfy the applicable source of income and diversification requirements (under the Internal Revenue Code) in order for its investors to satisfy them. The Portfolio will allocate, at least annually among its investors, each investor’s distributive share of the Portfolio’s net investment income, net realized capital gains and losses and any other items of income, gain, loss, deduction or credit.

As of April 30, 2016, the Portfolio had no uncertain tax positions that would require financial statement recognition, de-recognition, or disclosure. The Portfolio files a U.S. federal income tax return annually after its fiscal year-end, which is subject to examination by the Internal Revenue Service for a period of three years from the date of filing.

E  Expense Reduction — State Street Bank and Trust Company (SSBT) serves as custodian of the Portfolio. Pursuant to the custodian agreement, SSBT receives a fee that may be reduced by credits, which are determined based on the average daily cash balance the Portfolio maintains with SSBT. All credit balances, if any, used to reduce the Portfolio’s custodian fees are reported as a reduction of expenses in the Statement of Operations. Effective September 1, 2015, SSBT began imposing fees on certain uninvested cash balances and discontinued credits on cash deposit balances.

F  Use of Estimates — The preparation of the financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities at the date of the financial statements and the reported amounts of income and expense during the reporting period. Actual results could differ from those estimates.

G  Indemnifications — Under the Portfolio’s organizational documents, its officers and Trustees may be indemnified against certain liabilities and expenses arising out of the performance of their duties to the Portfolio. Under Massachusetts law, if certain conditions prevail, interestholders in the Portfolio could

7

CMBS Portfolio

April 30, 2016

Notes to Financial Statements (Unaudited) — continued

be deemed to have personal liability for the obligations of the Portfolio. However, the Portfolio’s Declaration of Trust contains an express disclaimer of liability on the part of Portfolio interestholders and the By-laws provide that the Portfolio shall assume the defense on behalf of any Portfolio interestholder. Moreover, the By-laws also provide for indemnification out of Portfolio property of any interestholder held personally liable solely by reason of being or having been an interestholder for all loss or expense arising from such liability. Additionally, in the normal course of business, the Portfolio enters into agreements with service providers that may contain indemnification clauses. The Portfolio’s maximum exposure under these arrangements is unknown as this would involve future claims that may be made against the Portfolio that have not yet occurred.

H  Interim Financial Statements — The interim financial statements relating to April 30, 2016 and for the six months then ended have not been audited by an independent registered public accounting firm, but in the opinion of the Portfolio’s management, reflect all adjustments, consisting only of normal recurring adjustments, necessary for the fair presentation of the financial statements.

2  Investment Adviser Fee and Other Transactions with Affiliates

The investment adviser fee is earned by Boston Management and Research (BMR), a subsidiary of EVM, as compensation for investment advisory services rendered to the Portfolio. The fee is computed at an annual rate of 0.55% of the Portfolio’s average daily net assets up to $1 billion and is payable monthly. On net assets of $1 billion and over, the annual fee is reduced. For the six months ended April 30, 2016, the Portfolio’s investment adviser fee amounted to $88,850 or 0.55% (annualized) of the Portfolio’s average daily net assets. Pursuant to a voluntary expense reimbursement, BMR was allocated $21,592 of the Portfolio’s operating expenses for the six months ended April 30, 2016. The Portfolio invests its cash in Cash Reserves Fund. EVM does not currently receive a fee for advisory services provided to Cash Reserves Fund.

Trustees and officers of the Portfolio who are members of EVM’s or BMR’s organizations receive remuneration for their services to the Portfolio out of the investment adviser fee. Trustees of the Portfolio who are not affiliated with the investment adviser may elect to defer receipt of all or a percentage of their annual fees in accordance with the terms of the Trustees Deferred Compensation Plan. For the six months ended April 30, 2016, no significant amounts have been deferred. Certain officers and Trustees of the Portfolio are officers of the above organizations.

3  Purchases and Sales of Investments

Purchases and sales of investments, other than short-term obligations and including maturities and paydowns, aggregated $9,886,320 and $11,240,716, respectively, for the six months ended April 30, 2016.

4  Federal Income Tax Basis of Investments

The cost and unrealized appreciation (depreciation) of investments of the Portfolio at April 30, 2016, as determined on a federal income tax basis, were as follows:

Aggregate cost	$	32,400,031
Gross unrealized appreciation	$	5,738
Gross unrealized depreciation		(1,176,818	)
Net unrealized depreciation	$	(1,171,080	)

5  Line of Credit

The Portfolio participates with other portfolios and funds managed by EVM and its affiliates in a $625 million unsecured line of credit agreement with a group of banks, which is in effect through September 2, 2016. Borrowings are made by the Portfolio solely to facilitate the handling of unusual and/or unanticipated short-term cash requirements. Interest is charged to the Portfolio based on its borrowings at an amount above either the Eurodollar rate or Federal Funds rate. In addition, a fee computed at an annual rate of 0.10% on the daily unused portion of the line of credit is allocated among the participating portfolios and funds at the end of each quarter. Because the line of credit is not available exclusively to the Portfolio, it may be unable to borrow some or all of its requested amounts at any particular time. The Portfolio did not have any significant borrowings or allocated fees during the six months ended April 30, 2016.

6  CMBS Risk

Commercial mortgage-backed securities (CMBS) include securities that reflect an interest in, and are secured by, mortgage loans on commercial real property. The Portfolio’s investments in CMBS are subject to credit, interest rate, prepayment and extension risk. CMBS are subject to risk of default on the underlying mortgage. CMBS issued by non-government entities may offer higher yields than those issued by government entities, but also may be subject

8

CMBS Portfolio

April 30, 2016

Notes to Financial Statements (Unaudited) — continued

to greater volatility than government issues. CMBS react differently to changes in interest rates than other bonds and the prices of CMBS may reflect adverse economic and market conditions. Small movements in interest rates (both increases and decreases) may quickly and significantly reduce the value of CMBS.

7  Fair Value Measurements

Under generally accepted accounting principles for fair value measurements, a three-tier hierarchy to prioritize the assumptions, referred to as inputs, is used in valuation techniques to measure fair value. The three-tier hierarchy of inputs is summarized in the three broad levels listed below.

• Level 1 – quoted prices in active markets for identical investments

• Level 2 – other significant observable inputs (including quoted prices for similar investments, interest rates, prepayment speeds, credit risk, etc.)

• Level 3 – significant unobservable inputs (including a fund’s own assumptions in determining the fair value of investments)

In cases where the inputs used to measure fair value fall in different levels of the fair value hierarchy, the level disclosed is determined based on the lowest level input that is significant to the fair value measurement in its entirety. The inputs or methodology used for valuing securities are not necessarily an indication of the risk associated with investing in those securities.

At April 30, 2016, the hierarchy of inputs used in valuing the Portfolio’s investments, which are carried at value, were as follows:

Asset Description	Level 1		Level 2		Level 3		Total
Commercial Mortgage-Backed Securities	$	—	$	20,605,135	$	—	$	20,605,135
Asset-Backed Securities		—		4,427,349		—		4,427,349
Short-Term Investments		—		6,196,467		—		6,196,467
Total Investments	$	—	$	31,228,951	$	—	$	31,228,951

The Portfolio held no investments or other financial instruments as of October 31, 2015 whose fair value was determined using Level 3 inputs. At April 30, 2016, there were no investments transferred between Level 1 and Level 2 during the six months then ended.

8  Termination of Portfolio

On April 27, 2016, the Portfolio’s Trustees approved the termination of the Portfolio. The Portfolio expects to make a pro rata distribution of securities and cash to each interestholder on or before July 31, 2016.

9

CMBS Portfolio

April 30, 2016

Board of Trustees’ Contract Approval

Overview of the Contract Review Process

The Investment Company Act of 1940, as amended (the “1940 Act”), provides, in substance, that each investment advisory agreement between a fund and its investment adviser will continue in effect from year to year only if its continuation is approved at least annually by the fund’s board of trustees, including by a vote of a majority of the trustees who are not “interested persons” of the fund (“Independent Trustees”), cast in person at a meeting called for the purpose of considering such approval.

At a meeting of the Boards of Trustees (each a “Board”) of the registered investment companies advised, administered and/or distributed by Eaton Vance Management or its affiliates (the “Eaton Vance Funds”) held on April 26, 2016, the Board, including a majority of the Independent Trustees, voted to approve continuation of existing investment advisory and sub-advisory agreements for the Eaton Vance Funds for an additional one-year period. In voting its approval, the Board relied upon the affirmative recommendation of its Contract Review Committee, which is a committee comprised exclusively of Independent Trustees. Prior to making its recommendation, the Contract Review Committee reviewed information furnished by each adviser to the Eaton Vance Funds (including information specifically requested by the Board) for a series of meetings of the Contract Review Committee held between February and April 2016. The Contract Review Committee also considered information received at prior meetings of the Board and its committees, as relevant to its annual evaluation of the investment advisory and sub-advisory agreements.

The information that the Board considered included, among other things, the following (for funds that invest through one or more underlying portfolio(s), references to “each fund” in this section may include information that was considered at the portfolio-level):

Information about Fees, Performance and Expenses

• A report from an independent data provider comparing the advisory and related fees paid by each fund with fees paid by comparable funds as identified by the independent data provider (“comparable funds”);

• A report from an independent data provider comparing each fund’s total expense ratio and its components to comparable funds;

• A report from an independent data provider comparing the investment performance of each fund (including, where relevant, yield data, Sharpe ratios and information ratios) to the investment performance of comparable funds over various time periods;

• Data regarding investment performance in comparison to benchmark indices and customized groups of peer funds identified by the adviser in consultation with the Board;

• For each fund, comparative information concerning the fees charged and the services provided by each adviser in managing other accounts (including mutual funds, other collective investment funds and institutional accounts) using investment strategies and techniques similar to those used in managing such fund;

• Profitability analyses for each adviser with respect to each fund;

Information about Portfolio Management and Trading

• Descriptions of the investment management services provided to each fund, including the investment strategies and processes it employs;

• The procedures and processes used to determine the fair value of fund assets and actions taken to monitor and test the effectiveness of such procedures and processes;

• Information about each adviser’s policies and practices with respect to trading, including each adviser’s processes for monitoring best execution of portfolio transactions;

• Information about the allocation of brokerage transactions and the benefits received by each adviser as a result of brokerage allocation, including information concerning the acquisition of research through client commission arrangements and policies with respect to “soft dollars”;

• Data relating to portfolio turnover rates of each fund;

Information about each Adviser

• Reports detailing the financial results and condition of each adviser;

• Descriptions of the qualifications, education and experience of the individual investment professionals whose responsibilities include portfolio management and investment research for the funds, and information relating to their compensation and responsibilities with respect to managing other mutual funds and investment accounts;

• The Code of Ethics of each adviser and its affiliates, together with information relating to compliance with and the administration of such codes;

• Policies and procedures relating to proxy voting and the handling of corporate actions and class actions;

• Information concerning the resources devoted to compliance efforts undertaken by each adviser and its affiliates (including descriptions of various compliance programs) and their record of compliance;

• Information concerning the business continuity and disaster recovery plans of each adviser and its affiliates;

• A description of Eaton Vance Management’s procedures for overseeing third party advisers and sub-advisers, including with respect to regulatory and compliance issues, investment management and other matters;

10

CMBS Portfolio

April 30, 2016

Board of Trustees’ Contract Approval — continued

Other Relevant Information

• Information concerning the nature, cost and character of the administrative and other non-investment advisory services provided by Eaton Vance Management and its affiliates;

• Information concerning management of the relationship with the custodian, subcustodians and fund accountants by each adviser or the funds’ administrator; and

• The terms of each investment advisory agreement.

Over the course of the twelve-month period ended April 30, 2016, with respect to one or more funds, the Board met ten times and the Contract Review Committee, the Audit Committee, the Governance Committee, the Portfolio Management Committee and the Compliance Reports and Regulatory Matters Committee, each of which is a Committee comprised solely of Independent Trustees, met seven, sixteen, four, nine and eleven times, respectively. At such meetings, the Trustees participated in investment and performance reviews with the portfolio managers and other investment professionals of each investment adviser relating to each fund, and considered various investment and trading strategies used in pursuing each fund’s investment objective, such as the use of derivative instruments, as well as risk management techniques. The Board and its Committees also evaluated issues pertaining to industry and regulatory developments, compliance procedures, fund governance and other issues with respect to the funds, and received and participated in reports and presentations provided by Eaton Vance Management and other fund advisers with respect to such matters. In addition to the formal meetings of the Board and its Committees, the Independent Trustees hold regular teleconferences in between meetings to discuss, among other topics, matters relating to the continuation of investment advisory and sub-advisory agreements.

For funds that invest through one or more underlying portfolios, the Board considered similar information about the portfolio(s) when considering the approval of investment advisory agreements. In addition, in cases where the fund’s investment adviser has engaged a sub-adviser, the Board considered similar information about the sub-adviser when considering the approval of any sub-advisory agreement.

The Contract Review Committee was assisted throughout the contract review process by Goodwin Procter LLP, independent legal counsel for the Independent Trustees. The members of the Contract Review Committee relied upon the advice of such counsel and their own business judgment in determining the material factors to be considered in evaluating each investment advisory and sub-advisory agreement and the weight to be given to each such factor. The conclusions reached with respect to each investment advisory and sub-advisory agreement were based on a comprehensive evaluation of all the information provided and not any single factor. Moreover, each member of the Contract Review Committee may have placed varying emphasis on particular factors in reaching conclusions with respect to each investment advisory and sub-advisory agreement. In evaluating each investment advisory and sub-advisory agreement, including the specific fee structures and other terms of the agreements, the Contract Review Committee was informed by multiple years of analysis and discussion among the Independent Trustees and the Eaton Vance Funds’ advisers and sub-advisers.

Results of the Process

Based on its consideration of the foregoing, and such other information as it deemed relevant, including the factors and conclusions described below, the Contract Review Committee concluded that the continuation of the investment advisory agreement of CMBS Portfolio (the “Portfolio”) with Boston Management and Research (the “Adviser”), an affiliate of Eaton Vance Management, including its fee structure, is in the interests of shareholders and, therefore, the Contract Review Committee recommended to the Board approval of the agreement. The Board accepted the recommendation of the Contract Review Committee based on the material factors considered and conclusions reached by the Contract Review Committee with respect to the agreement. Accordingly, the Board, including a majority of the Independent Trustees, voted to approve continuation of the investment advisory agreement for the Portfolio.

Nature, Extent and Quality of Services

In considering whether to approve the investment advisory agreement of the Portfolio, the Board evaluated the nature, extent and quality of services provided to the Portfolio by the Adviser.

The Board considered the Adviser’s management capabilities and investment process with respect to the types of investments held by the Portfolio, including the education, experience and number of its investment professionals and other personnel who provide portfolio management, investment research, and similar services to the Portfolio. The Board considered the abilities and experience of such investment professionals in investing securities, derivatives, and other instruments to establish investment exposures to commercial mortgage-backed securities. The Board also took into account the resources dedicated to portfolio management and other services, as well as the compensation methods of the Adviser and other factors, such as the reputation and resources of the Adviser to recruit and retain highly qualified research, advisory and supervisory investment professionals. In addition, the Board considered the time and attention devoted to the Eaton Vance Funds, including the Portfolio, by senior management, as well as the infrastructure, operational capabilities and support staff in place to assist in the portfolio management and operations of the Portfolio, including the provision of administrative services.

The Board considered the compliance programs of the Adviser and relevant affiliates thereof. Among other matters, the Board considered compliance and reporting matters relating to personal trading by investment professionals, selective disclosure of portfolio holdings, late trading, frequent trading, portfolio

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CMBS Portfolio

April 30, 2016

Board of Trustees’ Contract Approval — continued

valuation, business continuity and the allocation of investment opportunities. The Board also considered the responses of the Adviser and its affiliates to requests in recent years from regulatory authorities such as the Securities and Exchange Commission and the Financial Industry Regulatory Authority.

The Board considered shareholder and other administrative services provided or managed by Eaton Vance Management and its affiliates, including transfer agency and accounting services. The Board evaluated the benefits to shareholders of investing in a fund that is a part of a large fund complex offering exposure to a variety of asset classes and investment disciplines.

After consideration of the foregoing factors, among others, the Board concluded that the nature, extent and quality of services provided by the Adviser, taken as a whole, are appropriate and consistent with the terms of the investment advisory agreement.

Portfolio Performance

The Board compared the Portfolio’s investment performance to that of comparable funds and appropriate benchmark indices, as well as a customized peer group of similarly managed funds. The Board’s review included comparative performance data for the one-year period ended September 30, 2015 for the Portfolio. The Board took into account the purpose served by the Portfolio as an investment option for other Eaton Vance Funds seeking to participate in performance of the asset classes in which the Portfolio invests. On the basis of the foregoing and other relevant information provided to the Board, the Board concluded that the Portfolio had achieved its performance objective.

Management Fees and Expenses

The Board considered contractual fee rates payable by the Portfolio for advisory and administrative services (referred to collectively as “management fees”). As part of its review, the Board considered the Portfolio’s management fees and total expense ratio for a one year period ended September 30, 2015, as compared to those of comparable funds, before and after giving effect to any undertaking to waive fees or reimburse expenses. The Board also considered factors that had an impact on Portfolio expense ratios, as identified by management in response to inquiries from the Contract Review Committee. The Board considered management’s proposals to take steps to address the longer-term prospects of the Portfolio, such as liquidation or merger with another fund.

After considering the foregoing information, and in light of the nature, extent and quality of the services provided by the Adviser, the Board concluded that the management fees charged for advisory and related services are reasonable.

Profitability and Other “Fall-Out” Benefits

The Board considered the level of profits realized by the Adviser and relevant affiliates thereof in providing investment advisory and administrative services to the Portfolio and to all Eaton Vance Funds as a group. The Board considered the level of profits realized without regard to marketing support or other payments by the Adviser and its affiliates to third parties in respect of distribution services. The Board also considered other direct or indirect fall-out benefits received by the Adviser and its affiliates in connection with their relationships with the Portfolio, including the benefits of research services that may be available to the Adviser as a result of securities transactions effected for the Portfolio and other investment advisory clients.

The Board concluded that, in light of the foregoing factors and the nature, extent and quality of the services rendered, the profits realized by the Adviser and its affiliates are deemed not to be excessive.

Economies of Scale

In reviewing management fees and profitability, the Board also considered the extent to which the Adviser and its affiliates, on the one hand, and the Portfolio, on the other hand, can expect to realize benefits from economies of scale as the assets of the Portfolio increase. The Board acknowledged the difficulty in accurately measuring the benefits resulting from economies of scale, if any, with respect to the management of any specific fund or group of funds. The Board reviewed data summarizing the increases and decreases in the assets of the Portfolio and of all Eaton Vance Funds as a group over various time periods, and evaluated the extent to which the total expense ratio of the Portfolio and the profitability of the Adviser and its affiliates may have been affected by such increases or decreases. Based upon the foregoing, the Board concluded that the Portfolio currently shares in any benefits from economies of scale. The Board also concluded that, assuming reasonably foreseeable increases in the assets of the Portfolio, the structure of the advisory fee, which includes breakpoints at several asset levels, will allow the Portfolio to continue to benefit from any economies of scale in the future.

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CMBS Portfolio

April 30, 2016

Officers and Trustees

Officers of CMBS Portfolio

Thomas H. Luster

President

Payson F. Swaffield

Vice President

Maureen A. Gemma

Vice President, Secretary and

Chief Legal Officer

James F. Kirchner

Treasurer

Paul M. O’Neil

Chief Compliance Officer

Trustees of CMBS Portfolio

Ralph F. Verni

Chairperson

William H. Park

Vice-Chairperson

Scott E. Eston

Thomas E. Faust Jr.*

Cynthia E. Frost

George J. Gorman

Valerie A. Mosley

Helen Frame Peters

Susan J. Sutherland

Harriett Tee Taggart

* Interested Trustee

13

Item 2. Code of Ethics

Not required in this filing.

Item 3. Audit Committee Financial Expert

Not required in this filing.

Item 4. Principal Accountant Fees and Services

Not required in this filing.

Item 5. Audit Committee of Listed Registrants

Not applicable.

Item 6. Schedule of Investments

Please see schedule of investments contained in the Report to Stockholders included under Item 1 of this Form N-CSR.

Item 7. Disclosure of Proxy Voting Policies and Procedures for Closed-End Management Investment Companies

Not applicable.

Item 8. Portfolio Managers of Closed-End Management Investment Companies

Not applicable.

Item 9. Purchases of Equity Securities by Closed-End Management Investment Company and Affiliated Purchasers

Not applicable.

Item 10. Submission of Matters to a Vote of Security Holders

No material changes.

Item 11. Controls and Procedures

(a) It is the conclusion of the registrant’s principal executive officer and principal financial officer that the effectiveness of the registrant’s current disclosure controls and procedures (such disclosure controls and procedures having been evaluated within 90 days of the date of this filing) provide reasonable assurance that the information required to be disclosed by the registrant has been recorded, processed, summarized and reported within the time period specified in the Commission’s rules and forms and that the information required to be disclosed by the registrant has been accumulated and communicated to the registrant’s principal executive officer and principal financial officer in order to allow timely decisions regarding required disclosure.

(b) There have been no changes in the registrant’s internal controls over financial reporting during the second fiscal quarter of the period covered by this report that has materially affected, or is reasonably likely to materially affect, the registrant’s internal control over financial reporting.

Item 12. Exhibits

(a)(1)	Registrant’s Code of Ethics – Not applicable (please see Item 2).
(a)(2)(i)	Treasurer’s Section 302 certification.
(a)(2)(ii)	President’s Section 302 certification.
(b)	Combined Section 906 certification.

Signatures

Pursuant to the requirements of the Securities Exchange Act of 1934 and the Investment Company Act of 1940, the registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.

CMBS Portfolio

By:	/s/ Thomas H. Luster
	Thomas H. Luster
	President
Date:	June 13, 2016

Pursuant to the requirements of the Securities Exchange Act of 1934 and the Investment Company Act of 1940, this report has been signed below by the following persons on behalf of the registrant and in the capacities and on the dates indicated.

By:	/s/ James F. Kirchner
	James F. Kirchner
	Treasurer
Date:	June 13, 2016

By:	/s/ Thomas H. Luster
	Thomas H. Luster
	President
Date:	June 13, 2016