<PAGE>   1
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                UNITED STATES SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549

                                    FORM 10-Q

  (MARK ONE)
      [X]    QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF
             THE SECURITIES EXCHANGE ACT OF 1934

                FOR THE QUARTERLY PERIOD ENDED SEPTEMBER 30, 1999

                                       OR

      []    TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF
            THE SECURITIES EXCHANGE ACT OF 1934

               FOR THE TRANSITION PERIOD FROM ________ TO ________

                        COMMISSION FILE NUMBER: 0-23023

                               MMC NETWORKS, INC.
             (EXACT NAME OF REGISTRANT AS SPECIFIED IN ITS CHARTER)

               DELAWARE                                   77-0319809
    (STATE OR OTHER JURISDICTION OF                   (I.R.S. EMPLOYER
     INCORPORATION OR ORGANIZATION)                   IDENTIFICATION NO.)

                              1134 E. ARQUES AVENUE
                               SUNNYVALE, CA 94086
                         (ADDRESS OF PRINCIPAL OFFICES)
                                   (ZIP CODE)

                                 (408) 731-1600
              (REGISTRANT'S TELEPHONE NUMBER, INCLUDING AREA CODE)

INDICATE BY CHECK MARK WHETHER THE REGISTRANT (1) HAS FILED ALL REPORTS REQUIRED
TO BE FILED BY SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 DURING
THE PRECEDING 12 MONTHS (OR FOR SUCH SHORTER PERIOD THAT THE REGISTRANT WAS
REQUIRED TO FILE SUCH REPORTS), AND (2) HAS BEEN SUBJECT TO SUCH FILING
REQUIREMENTS FOR THE PAST 90 DAYS.

                               YES [X]   NO [ ]

THE NUMBER OF SHARES OUTSTANDING OF THE ISSUER'S COMMON STOCK AS OF NOVEMBER 1,
1999 WAS 31,703,494.

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<PAGE>   2
                               MMC NETWORKS, INC.
                          QUARTERLY REPORT ON FORM 10-Q
                                      INDEX

<TABLE>
<S>                                                                                     <C>
PART I. FINANCIAL INFORMATION

    Item 1. Financial Statements

          Condensed Consolidated Balance Sheets at September 30, 1999
               and December 31, 1998 ..................................................   3

          Condensed Consolidated Statements of Operations for the three and nine
               months ended September 30, 1999 and 1998 ...............................   4

          Condensed Consolidated Statements of Cash Flows for the nine months
               months ended September 30, 1999 and 1998 ...............................   5

          Notes to the Condensed Consolidated Financial Statements ....................   6

     Item 2. Management's Discussion and Analysis of Financial Condition
               and Results of Operations ..............................................   8

PART II. OTHER INFORMATION

        Item 1. Legal Proceedings .....................................................  20

        Item 2. Changes in Securities .................................................  20

        Item 3. Defaults Upon Senior Notes ............................................  20

        Item 4. Submission of Matters to a Vote of Security Holders ...................  20

        Item 5. Other Information .....................................................  20

        Item 6. Exhibits and Reports on Form 8-K ......................................  20

SIGNATURES ............................................................................  22
</TABLE>


                                       2
<PAGE>   3
PART I.  FINANCIAL INFORMATION
ITEM 1.  FINANCIAL STATEMENTS

                               MMC NETWORKS, INC.
                      CONDENSED CONSOLIDATED BALANCE SHEETS
                      (IN THOUSANDS, EXCEPT PER SHARE DATA)
                                   (UNAUDITED)

<TABLE>
<CAPTION>
                                                                                  SEPTEMBER 30,  DECEMBER 31,
                                                                                      1999           1998
                                                                                  -------------  ------------
<S>                                                                               <C>            <C>
ASSETS
Current assets:
      Cash and cash equivalents ..............................................      $ 26,763       $ 31,452
      Short-term investments .................................................        43,165         21,726
      Accounts receivable, net of allowance of $422 and $172 .................        12,184         10,582
      Inventories ............................................................         2,288            630
      Prepaid expenses and other current assets ..............................         3,915          3,372
                                                                                    --------       --------
         Total current assets ................................................        88,315         67,762
Property and equipment, net ..................................................         6,967          5,487
Other assets .................................................................           222            135
                                                                                    --------       --------
                                                                                    $ 95,504       $ 73,384
                                                                                    ========       ========
LIABILITIES AND STOCKHOLDERS' EQUITY
Current liabilities:
      Accounts payable .......................................................      $  6,232       $  4,400
      Accrued expenses .......................................................         4,282          5,753
      Capital lease obligations ..............................................            52            286
                                                                                    --------       --------
         Total current liabilities ...........................................        10,566         10,439
                                                                                    --------       --------
Stockholders' equity:
      Preferred Stock: $0.001 par value; 10,000 shares authorized; no
           shares issued or outstanding ......................................            --             --
      Common Stock: $0.001 par value; 100,000 shares authorized; 31,654
           and 30,135 shares issued and outstanding ..........................            28             26
      Additional paid-in capital .............................................        65,247         55,520
      Notes receivable from stockholders .....................................          (107)          (107)
      Retained earnings ......................................................        19,770          7,506
                                                                                    --------       --------
         Total stockholders' equity ..........................................        84,938         62,945
                                                                                    --------       --------
                                                                                    $ 95,504       $ 73,384
                                                                                    ========       ========
</TABLE>

                 THE ACCOMPANYING NOTES ARE AN INTEGRAL PART OF
               THESE CONDENSED CONSOLIDATED FINANCIAL STATEMENTS.


                                       3
<PAGE>   4
                               MMC NETWORKS, INC.
                 CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS
                      (IN THOUSANDS, EXCEPT PER SHARE DATA)
                                   (UNAUDITED)

<TABLE>
<CAPTION>
                                                              THREE MONTHS ENDED          NINE MONTHS ENDED
                                                                 SEPTEMBER 30,               SEPTEMBER 30,
                                                            ----------------------      ----------------------
                                                              1999          1998          1999          1998
                                                            --------      --------      --------      --------
<S>                                                         <C>           <C>           <C>           <C>
Revenues .............................................      $ 23,149      $ 14,015      $ 58,582      $ 35,655
Cost of revenues .....................................         6,868         4,036        17,336        10,462
                                                            --------      --------      --------      --------
            Gross profit .............................        16,281         9,979        41,246        25,193
Operating expenses:
      Research and development, net ..................         6,360         3,872        16,070        10,670
      Selling, general and administrative ............         3,136         2,406         8,211         6,758
      Litigation settlement ..........................            --            --            --         1,250
                                                            --------      --------      --------      --------
            Total operating expenses .................         9,496         6,278        24,281        18,678
                                                            --------      --------      --------      --------
Operating income .....................................         6,785         3,701        16,965         6,515
      Interest income, net ...........................           844           556         2,189         1,545
                                                            --------      --------      --------      --------
Income before income taxes ...........................         7,629         4,257        19,154         8,060
Provision for income taxes ...........................         2,745         1,275         6,890         2,530
                                                            --------      --------      --------      --------
Net income ...........................................      $  4,884      $  2,982      $ 12,264      $  5,530
                                                            ========      ========      ========      ========
Basic income per share ...............................      $   0.16      $   0.10      $   0.40      $   0.19
                                                            ========      ========      ========      ========
Shares used to compute basic income per share ........        31,417        29,854        30,821        29,561
                                                            ========      ========      ========      ========
Diluted income per share .............................      $   0.14      $   0.09      $   0.36      $   0.16
                                                            ========      ========      ========      ========
Shares used to compute diluted income per share ......        35,283        33,723        34,456        33,745
                                                            ========      ========      ========      ========
</TABLE>

                 THE ACCOMPANYING NOTES ARE AN INTEGRAL PART OF
               THESE CONDENSED CONSOLIDATED FINANCIAL STATEMENTS.


                                       4
<PAGE>   5
                               MMC NETWORKS, INC.
                 CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS
                                 (IN THOUSANDS)
                                   (UNAUDITED)


<TABLE>
<CAPTION>
                                                                                   NINE MONTHS ENDED
                                                                                      SEPTEMBER 30,
                                                                                 -----------------------
                                                                                   1999           1998
                                                                                 --------       --------
<S>                                                                              <C>            <C>
Cash flows from operating activities:
     Net income ...........................................................      $ 12,264       $  5,530
     Adjustments to reconcile net income to net cash
        provided by operating activities:
        Depreciation and amortization .....................................         2,300          1,562
        Tax benefit from the exercise of stock options ....................         4,798          2,434
        Changes in assets and liabilities:
           Accounts receivable ............................................        (1,602)        (3,178)
           Inventories ....................................................        (1,658)          (155)
           Prepaid expenses and other assets ..............................          (630)        (1,712)
           Accounts payable ...............................................         1,832            744
           Accrued expenses ...............................................        (1,471)         1,477
                                                                                 --------       --------
               Net cash provided by operating activities ..................        15,833          6,702
                                                                                 --------       --------
Cash flows from investing activities:
     Purchase of short-term investments ...................................       (21,439)        (9,140)
     Acquisition of property and equipment ................................        (3,780)        (3,155)
                                                                                 --------       --------
               Net cash used in investing activities ......................       (25,219)       (12,295)
                                                                                 --------       --------
Cash flows from financing activities:
     Proceeds from exercise of stock options and other ....................         4,931          1,339
     Proceeds from the repayment of notes receivable from stockholders ....            --             65
     Principal payments on capital lease obligations ......................          (234)          (269)
                                                                                 --------       --------
               Net cash provided by financing activities ..................         4,697          1,135
                                                                                 --------       --------
Net decrease in cash and cash equivalents .................................        (4,689)        (4,458)
Cash and cash equivalents at beginning of period ..........................        31,452         45,401
                                                                                 --------       --------
Cash and cash equivalents at end of period ................................      $ 26,763       $ 40,943
                                                                                 ========       ========
SUPPLEMENTAL DISCLOSURE:
     Cash paid for interest ...............................................      $     29       $     46
     Cash paid for income taxes ...........................................      $  3,377       $     --
</TABLE>

                 THE ACCOMPANYING NOTES ARE AN INTEGRAL PART OF
               THESE CONDENSED CONSOLIDATED FINANCIAL STATEMENTS.


                                       5
<PAGE>   6
                               MMC NETWORKS, INC.

            NOTES TO THE CONDENSED CONSOLIDATED FINANCIAL STATEMENTS


NOTE 1 - BASIS OF PRESENTATION

The accompanying condensed consolidated financial statements and notes to the
condensed consolidated financial statements of MMC Networks, Inc. ("MMC") are
unaudited. In the opinion of management, these statements and notes have been
prepared on the same basis as the audited consolidated financial statements and
include all adjustments, consisting only of normal recurring adjustments,
necessary for the fair presentation of the financial position, results of
operations and cash flows for MMC for the periods presented. Results for the
interim periods presented are not necessarily indicative of results for the
entire year. The words "we", "us" and "our" used in this report refer to MMC
Networks, Inc.

The accompanying condensed consolidated financial statements and notes do not
include certain information and footnote disclosures normally required under
generally accepted accounting principles. Therefore, these condensed
consolidated financial statements and notes should be read in conjunction with
the audited consolidated financial statements and notes included in our Annual
Report on Form 10-K filed with the Securities and Exchange Commission for the
year ended December 31, 1998.

NOTE 2 - EARNINGS PER SHARE

The computations of basic and diluted earnings per share for the periods
presented are as follows:

<TABLE>
<CAPTION>
                                                 THREE MONTHS ENDED           NINE MONTHS ENDED
                                                    SEPTEMBER 30,                SEPTEMBER 30,
                                                ----------------------      ----------------------
                                                  1999          1998          1999          1998
                                                --------      --------      --------      --------
                                                       (IN THOUSANDS, EXCEPT PER SHARE DATA)
<S>                                             <C>           <C>           <C>           <C>
Net income available to
     common stockholders ...................    $  4,884      $  2,982      $ 12,264      $  5,530
                                                ========      ========      ========      ========
Shares used to compute basic income
     per share .............................      31,417        29,854        30,821        29,561

Effect of dilutive securities:
     Warrants ..............................          --            31             2            31
     Stock options .........................       3,866         3,838         3,633         4,153
                                                --------      --------      --------      --------
Shares used to compute diluted income
     per share .............................      35,283        33,723        34,456        33,745
                                                ========      ========      ========      ========
Basic income per share .....................    $   0.16      $   0.10      $   0.40      $   0.19
                                                ========      ========      ========      ========
Diluted income per share ...................    $   0.14      $   0.09      $   0.36      $   0.16
                                                ========      ========      ========      ========
</TABLE>


                                       6
<PAGE>   7

NOTE 3 - COMPOSITION OF INVENTORIES

<TABLE>
<CAPTION>
                                                     SEPTEMBER 30,  DECEMBER 31,
                                                         1999           1998
                                                     -------------  ------------
                                                          (IN THOUSANDS)
<S>                                                  <C>               <C>
Inventories:
         Work in process ............................    $1,308        $ 244
         Finished goods .............................       980          386
                                                         ------        -----
                                                         $2,288        $ 630
                                                         ======        =====
</TABLE>

NOTE 4 - EQUITY

On January 15, 1999 a warrant to purchase 33,963 shares of Common Stock at an
exercise price of $1.77 was exercised. As payment for the exercise, 3,277 shares
of the 33,963 shares available for exercise under the warrant were surrendered
to MMC. On the date of the exercise, the fair market value of the shares
surrendered was $60,000. The transaction resulted in no net proceeds to MMC.

NOTE 5 - FINANCING AGREEMENTS AND OTHER COMMITMENTS

During the first half of 1998, MMC established two credit facilities with a
bank: (1) a loan agreement and (2) a non-recourse receivables purchase
agreement. The loan agreement allows the Company to borrow up to $8.0 million.
Borrowings bear interest at the bank's prime rate. The agreement requires that
MMC comply with certain financial covenants. In the event of default, all
outstanding borrowings will accrue interest at a rate of five percentage points
above the rate effective immediately prior to a default. The loan agreement
expired in May 1999 and was renewed for one year on the same terms. The
non-recourse receivables purchase agreement expired in February 1999 and was not
renewed. To date, MMC has not utilized either credit facility.

In the second quarter of 1999, MMC entered into an agreement with a software
vendor. Under this agreement, MMC has committed to pay $2.9 million over 3 years
beginning April 1999 in exchange for the use of certain design software. During
the third quarter of 1999, MMC increased its total commitment from $2.9 million
to $3.8 million in exchange for the use of additional design software. As of
September 30, 1999, approximately $3.4 million remains outstanding under this
commitment.


                                       7
<PAGE>   8
ITEM 2. MANAGEMENT'S DISCUSSION AND ANALYSIS OF FINANCIAL CONDITION AND RESULTS
OF OPERATIONS

The following information should be read in conjunction with: (1) the interim
condensed consolidated financial statements and the notes included in Part I,
Item 1 of this Quarterly Report on Form 10-Q and (2) the consolidated financial
statements and notes contained in our Annual Report on Form 10-K for the year
ended December 31, 1998.

This Management's Discussion and Analysis of Financial Condition and Results of
Operations contains forward-looking statements which reflect our current views
with respect to future events which may impact our results of operations and
financial condition. In this report, the words "anticipates", "believes",
"expects", "intends" and similar expressions identify forward-looking
statements. These forward-looking statements are subject to risks and
uncertainties and other factors, including those described below under the
caption "Factors Affecting Future Results", which could cause our actual future
results to differ materially from historical results or from those described in
the forward-looking statements. Readers are urged not to place undue reliance on
these forward-looking statements, which reflect our outlook only as of the date
of this report.

BACKGROUND

MMC Networks, Inc. ("MMC") is a leading developer and supplier of Network
Processors, which are high-performance, open-architecture, software-programmable
processors optimized for network applications. MMC's Network Processors form the
core silicon "engines" of LAN and WAN switches and routers and are designed to
allow network equipment vendors to rapidly develop high-performance,
feature-rich, cost-effective products supporting a broad range of networking
functions.

MMC Networks Processors consist of switch engines, including the AF5400, AF5500,
ATMS2000 and PS1000 Network Processors, and packet processors, including the
EPIF4-L3, EPIF-105, EPIF-200, XPIF-300 and GPIF-207 Network Processors. In
varying configurations these Network Processors provide the core functionality
of high-performance Gigabit Ethernet, Fast Ethernet and Asynchronous Transfer
Mode ("ATM") networking equipment. We believe that network equipment vendors are
able to reduce design and development costs and accelerate product development
cycles for high-performance routers and switches by using our products. All of
MMC's products are based on our proprietary ViXTM architecture, which enables
network equipment vendors to easily and cost-effectively implement scaleable,
high-performance, value-added features in their switch and router products. Our
customers employ MMC's Network Processors to develop and market multi-gigabit,
wire-speed switches, routers, access concentrators, firewalls and gateways with
advanced features such as Layer 3 switching, converged voice and data
networking, class of service, quality of service, virtual private networking and
network management.

MMC was incorporated in California in September 1992 and reincorporated in
Delaware in October 1997. Our principal executive offices are located at 1134
East Arques Avenue, Sunnyvale, California 94086, and our telephone number is
(408) 731-1600.


                                       8
<PAGE>   9
RESULTS OF OPERATIONS

The following table shows certain statement of operations data expressed as a
percentage of our revenues.

<TABLE>
<CAPTION>
                                                   THREE MONTHS ENDED         NINE MONTHS ENDED
                                                      SEPTEMBER 30,             SEPTEMBER 30,
                                                   -------------------       -------------------
                                                    1999         1998         1999         1998
                                                   ------       ------       ------       ------
<S>                                                <C>          <C>          <C>          <C>
STATEMENT OF OPERATIONS DATA:
Revenues ....................................       100.0%       100.0%       100.0%       100.0%
Cost of revenues ............................        29.7%        28.8%        29.6%        29.3%
                                                   ------       ------       ------       ------
         Gross profit .......................        70.3%        71.2%        70.4%        70.7%
                                                   ------       ------       ------       ------
Operating expenses:
     Research and development, net ..........        27.5%        27.6%        27.4%        29.9%
     Selling, general and administrative ....        13.5%        17.2%        14.0%        19.0%
     Litigation settlement ..................         0.0%         0.0%         0.0%         3.5%
                                                   ------       ------       ------       ------
         Total operating expenses ...........        41.0%        44.8%        41.4%        52.4%
                                                   ------       ------       ------       ------
Operating income ............................        29.3%        26.4%        29.0%        18.3%
     Interest income, net ...................         3.6%         4.0%         3.7%         4.3%
                                                   ------       ------       ------       ------
Income before income taxes ..................        32.9%        30.4%        32.7%        22.6%
Provision for income taxes ..................        11.8%         9.1%        11.8%         7.1%
                                                   ------       ------       ------       ------
Net income ..................................        21.1%        21.3%        20.9%        15.5%
                                                   ======       ======       ======       ======
</TABLE>

REVENUES

Revenues primarily consist of sales of our Network Processors. Revenues
increased by 19.8% to $23.1 million in the third quarter of 1999 from $19.3
million in the second quarter of 1999 and increased by 65.2% from $14.0 million
for the same quarter of the previous year. The revenue growth from the second
quarter to the third quarter of 1999 was due to increased unit shipments across
all of our products as the number and run rate of our customer design wins in
production increased from period to period.

Revenues for the nine months ended September 30, 1999 increased by 64.3% to
$58.6 million as compared to $35.7 million for the same period in the previous
year. This increase in revenues from period to period is primarily due to
increased sales resulting from the shift into volume production of the AF5500
product.

In the third quarter of 1999, International Business Machines Corporation
("IBM") accounted for 22% of our revenues. During the quarter, IBM announced it
is closing down most of its network equipment business. IBM informed MMC that it
will cease the manufacture and sale of its network equipment products that use
MMC products, and it will cease future purchases of those MMC products. In
reaction to these events, MMC announced on August 31, 1999 that we believed
revenues in the fourth quarter of 1999 would decrease sequentially between 5-10%
from the third quarter due to the loss of IBM as a major customer. After further
discussions with IBM, we announced on October 14, 1999 that we believe revenues
in the fourth quarter will decrease more than the 5-10% estimate on August 31,
1999.

COST OF REVENUES; GROSS PROFIT

Cost of revenues primarily includes the cost of our Network Processors and the
costs of personnel and equipment associated with manufacturing support. The cost
of our Network Processors consists of either (1) the cost of fully assembled and
tested units which we buy for resale to our customers, or (2) the cost of
purchasing finished silicon wafers and subcontracting the assembly and test of
finished units. Gross profit represents revenues less the cost of revenues.


                                       9
<PAGE>   10
Cost of revenues increased to $6.9 million in the third quarter of 1999 from
$5.7 million in the second quarter of 1999 and from $4.0 million in the same
quarter of the previous year. The increase in cost of revenues compared to prior
periods reflects the increase in unit shipments from period to period. Gross
profit as a percentage of total revenues stayed relatively constant: 70.3% for
the third quarter of 1999, 70.7% for the second quarter of 1999 and 71.2% for
the third quarter of 1998.

The cost of revenues for the nine months ended September 30, 1999 was $17.3
million as compared to $10.5 million for the same period in the previous year.
Gross margins remained relatively constant from period to period: 70.4% for the
nine months ended September 30, 1999 and 70.7% for the same period in the
previous year.

RESEARCH AND DEVELOPMENT EXPENSES, NET

Research and development expenses, net, consist primarily of salaries and
related expenses of employees engaged in research, design and development
activities, depreciation and amortization of computing equipment and software
design tools and subcontracted engineering. On occasion, we receive partial or
complete reimbursement for expenses incurred under certain contracts with
customers. These reimbursements are recorded as an offset against research and
development expenses. Research and development expenses, net increased by 21.0%
to $6.4 million in the third quarter of 1999 from $5.3 million in the second
quarter of 1999 and increased by 64.3% from $3.9 million for the same quarter of
the previous year. The increase in research and development expenses, net from
period to period was due to the addition of personnel, increased subcontracting
expenses and investment in design tools for the development of new products and
enhancement of existing products. Research and development expenses as a
percentage of total revenues remained relatively constant: 27.5% for the third
quarter of 1999 from 27.2% for the second quarter of 1999 and 27.6% for the
third quarter of 1998.

Research and development expenses, net for the nine months ended September 30,
1999 were $16.1 million or 27.4% of revenues, as compared to $10.7 million or
29.9% of revenues for the nine months ended September 30, 1998. The increase in
research and development expenses, net from period to period was due to the
addition of personnel, increased subcontracting expenses and investment in
design tools for the development of new products and enhancement of existing
products. We expect research and development expenses to continue to increase in
absolute dollars for the foreseeable future.

SELLING, GENERAL AND ADMINISTRATIVE EXPENSES

Selling, general and administrative expenses consist primarily of salaries and
related expenses of employees performing sales, marketing and administrative
functions and various professional fees. Selling, general and administrative
expenses increased by 16.9% to $3.1 million in the third quarter of 1999 from
$2.7 million in the second quarter of 1999 and increased by 30.3% from $2.4
million for the same quarter of the previous year. The increase in selling,
general and administrative expenses was due to additional personnel hired to
support new customer design wins and the increased level of activity across all
MMC functions. Selling, general and administrative expenses decreased as a
percentage of revenues to 13.5% in the third quarter of 1999 compared to 13.9%
in the second quarter of 1999 and 17.2% in the same quarter of the previous year
due to the pace of revenue growth.

Selling, general and administrative expenses for the nine months ended September
30, 1999 were $8.2 million or 14.0% of revenues as compared to $6.8 million or
19.0% of revenues for the nine months ended September 30, 1998. Selling, general
and administrative expenses decreased as a percentage of revenue from period to
period as revenue growth outpaced the increase in selling, general and
administrative expenses required to support that growth. We expect selling,
general and administrative expenses to increase in absolute dollars for the
foreseeable future.

INTEREST INCOME, NET

Interest income, net reflects interest earned on average cash, cash equivalents
and short-term investment balances, less interest expense on our capital lease
obligations. Interest income, net increased for the three and nine months ended
September 30, 1999 as compared to the same periods of the previous year due to


                                       10
<PAGE>   11
increased cash balances available to invest resulting primarily from the cash
flow from operations.

PROVISION FOR INCOME TAXES

The provision for income taxes increased to $2.7 million in the third quarter of
1999 from $2.3 million in the second quarter of 1999 due to the increase in
pre-tax income from period to period. The effective tax rate for the first three
quarters of 1999 was 36.0%. We expect the effective tax rate to stay at
approximately this rate for the remainder of 1999.

LIQUIDITY AND CAPITAL RESOURCES

As of September 30, 1999, our cash, cash equivalents and short-term investments
totaled $69.9 million, up from $53.2 million at December 31, 1998. At September
30, 1999, our working capital was $77.7 million, up from $57.3 million at
December 31, 1998. Cash and cash equivalents totaled $26.8 million at September
30, 1999, down $4.7 million from $31.5 million at December 31, 1998.

Operating activities for the nine months ended September 30, 1999 provided cash
flows of $15.8 million. The majority of this increase was due to net income for
the period of $12.3 million and a tax benefit of $4.8 million resulting from the
exercise of stock options during the period. Investing activities for the nine
months ended September 30, 1999 used $25.2 million in cash primarily for the
purchase of short-term investments of $21.4 million. The remaining $3.8 million
was used for the purchase of property and equipment. Financing activities for
the nine months ended September 30, 1999 provided cash flows of $4.7 million
primarily due to proceeds resulting from the exercise of stock options during
the period.

During the first half of 1998, MMC established two credit facilities with a
bank: (1) a loan agreement and (2) a non-recourse receivables purchase
agreement. The loan agreement allows the Company to borrow up to $8.0 million.
Borrowings bear interest at the bank's prime rate. The agreement requires that
MMC comply with certain financial covenants. In the event of default, all
outstanding borrowings will accrue interest at a rate of five percentage points
above the rate effective immediately prior to a default. The loan agreement
expired in May 1999 and was renewed for one year on the same terms. The
non-recourse receivables purchase agreement expired in February 1999 and was not
renewed. To date, MMC has not utilized either credit facility.

We believe that our existing cash, cash equivalents and investment balances,
together with the borrowing capacity under our loan agreement and the cash flow
we expect to generate from future operations, will be sufficient to meet our
capital needs through the next twelve months. However, our future capital
requirements may vary materially from those now planned. We may decide to raise
additional funds in future periods. The amount of capital that we will need in
the future will depend on many factors, including:

    -   our future revenues

    -   the timing and extent of spending to support product development efforts
        and the expansion of sales and marketing efforts

    -   the timing and size of any business or technology acquisitions

    -   the timing of introductions of new products and enhancements to existing
        products

    -   the market acceptance of our products

If we need to raise additional funds, we may not be able to obtain additional
equity or debt financing on favorable terms, or at all.

IMPACT OF THE YEAR 2000 ISSUE

The Year 2000 Issue is the result of computer programs being written using two
digits rather than four to define the applicable year. Any computer programs
that have date-sensitive software may recognize a date using "00" as the year
1900 rather than the year 2000. This could result in a system failure or


                                       11
<PAGE>   12
miscalculations causing disruptions of operations, including among other things
a temporary inability to process transactions, send invoices or engage in
similar normal business activities.

YEAR 2000 PLAN

We have largely completed our business risk assessment of the impact that the
Year 2000 Issue may have on our operations. As business conditions warrant, we
may revise our assessment as new information is made available to us. To date,
we have identified and continue to assess four key areas of our business that
the Year 2000 Issue may affect. These four areas are our products, internal
information systems, third party suppliers and non-information technology
systems. MMC's Year 2000 plan aims at identifying and addressing Year 2000
issues based on a four phase approach applied to each of the key areas mentioned
above, as applicable.

    Phase 1: Identify potential business disruption problems, including those
    with products and systems, as well as potential disruption from suppliers
    and other third parties.

    Phase 2: Prioritize potential problems and allocate resources at the
    appropriate level to implement specific programs aimed at solving or
    mitigating any potential Year 2000 Issues identified.

    Phase 3: Test systems considered to be Year 2000 ready, if applicable.

    Phase 4: Develop contingency plans to address those Year 2000 issues that
    may pose a significant risk to our on-going operations, if applicable.

MMC PRODUCTS

We have completed testing our products, including semiconductors, software, and
reference design systems, and we believe them to be Year 2000 compliant.
However, it is possible that errors which we are not aware of at this time will
be discovered in the future. In addition, we cannot determine whether all of our
customers' products into which our products are incorporated will be Year 2000
compliant because we have little or no control over the design, production and
testing of our customers' products.

INTERNAL INFORMATION SYSTEMS

We believe that we have identified substantially all of the major systems,
software applications and related equipment used in connection with our internal
operations and have largely completed the implementation of specific programs
addressing the most critical areas. We have substantially completed the testing
of our existing systems for Year 2000 compliance, and we expect to complete the
remaining testing in November 1999. For purchases of new systems, we have a
procedure in place to test new systems for Year 2000 compliance prior to putting
them into production. Although we do not anticipate any significant issues to be
identified during this testing, the failure of any of our critical internal
information systems to achieve Year 2000 readiness could cause a significant
disruption in our operations. We have substantially completed contingency plans
to address issues that may pose a significant risk to our on-going operations.
These plans include temporary use of back-up software and or systems or
implementation of manual procedures to compensate for system deficiencies.
Remaining contingency plans are expected to be completed by November 1999.

THIRD PARTY SUPPLIERS

We rely directly and indirectly on external systems utilized by our third party
suppliers for the fabrication, assembly and testing of substantially all of our
products. In addition to our foundries and third party subcontractors, we rely
on systems utilized by other critical third party service providers such as
banks. We have identified critical third party service providers and surveyed
these suppliers regarding their Year 2000 preparedness. We have completed our
assessment of the survey of critical suppliers and continue to monitor secondary
suppliers as they complete their Year 2000 effort. Based on our assessment this
far, we do not expect to uncover any significant Year 2000 issues. However, we
view the risks associated with third party suppliers to be higher than other
risks we face because we have little or no control over the ability of our third
party suppliers to adequately resolve potential Year 2000 issues on a timely
basis and adequately test the compliance of their systems. To the extent
possible, we have completed contingency plans to address


                                       12
<PAGE>   13
critical third party suppliers. These plans include the use of certain back-up
suppliers, on-site and off-site buffer inventories for certain products and the
implementation of manual procedures to compensate for system deficiencies.

NON-INFORMATION TECHNOLOGY SYSTEMS

Non-information technology systems such as heating, security, test equipment and
electricity at our facilities may also be affected by Year 2000 issues. We have
identified and assessed the risks associated with our existing non-information
technology systems and have completed making all significant systems Year 2000
compliant. We are identifying and assessing risks for several less critical new
facilities and systems as they are being added. We expect to complete making
these systems Year 2000 compliant in November 1999. We have completed all
related contingency plans which include the temporary use of back-up equipment
and manual procedures to compensate for system deficiencies.

We presently estimate that the cost of addressing our Year 2000 issues will not
exceed $50,000, excluding expenses related to our own internal information
technology staff. This estimate was derived using numerous assumptions,
including the assumption that we have already identified our most significant
Year 2000 issues and that the plans of our third party suppliers will be
fulfilled in a timely manner without cost to us. However, these assumptions may
not be accurate. Actual results could differ significantly from those
anticipated after completion of all phases of our Year 2000 Plan. In addition,
contingency plans we implement may not succeed or may not be adequate to meet
our needs without significantly impacting our operations. Also, delays and
inefficiencies inherent in conducting operations in an alternative manner could
negatively impact our results of operations.


                                       13
<PAGE>   14
FACTORS AFFECTING FUTURE RESULTS

This report contains forward-looking statements (including statements about
expected future expenses, future tax rates, sufficiency of our existing cash,
cash equivalents and investments, and expected fourth quarter financial
results). These statements are subject to many risks and uncertainties,
including those discussed below and in MMC's Annual Report on Form 10-K for the
year ended December 31, 1998. Any of these risks could significantly impact our
business, financial condition or results of operations.

FLUCTUATIONS IN OPERATING RESULTS

Our quarterly revenues and expenses have fluctuated significantly in the past
and may continue to do so in the future. For example, as discussed above (see
"Results of Operations - Revenue"), MMC announced on August 31 and October 14,
1999, that it expected revenues for its fourth quarter to decrease from third
quarter revenues. Such fluctuations can be caused by a variety of factors, many
of which are outside our control. Factors that could affect our future operating
results include:

    -   the loss of or a reduction in sales to major customers

    -   changes in demand for the network equipment products of our customers

    -   variations, delays or cancellations of orders and/or shipments of our
        products

    -   reduction in the selling prices of our products

    -   changes in the mix of products being sold

    -   fluctuations in our manufacturing yields and other potential problems or
        delays in the fabrication, assembly, testing or delivery of our products

    -   our failure to manufacture and ship products on time

    -   increases in the costs of products from our suppliers

    -   availability of semiconductor foundry capacity

    -   fluctuations in product life cycles

    -   delays in introducing new products

    -   introduction of new products by our competitors

    -   variances in the timing and amount of non-recurring engineering funding
        and operating expenses

    -   intellectual property disputes

A large portion of our operating expenses including salaries, rent and capital
lease expenses is fixed and difficult to reduce or change on short notice.
Accordingly, if our total revenues do not meet our expectations, we may not be
able to adjust our expenses quickly enough to compensate for the shortfall in
revenues. This situation could have a negative effect on our business, financial
condition and results of operations.

CUSTOMER CONCENTRATION

A relatively small number of major network equipment vendors dominate the
network equipment market. Accordingly, a small number of customers has accounted
for a large portion of our revenues to date. The following table lists our
significant customers (i.e., those accounting for more than 10% of our total
revenues) for the periods presented:


                                       14
<PAGE>   15

<TABLE>
<CAPTION>
                                                                       THREE                  NINE
                                                                    MONTHS ENDED         MONTHS ENDED
                                                                    SEPTEMBER 30,        SEPTEMBER 30,
                                                                  -----------------    -----------------
                                                                  1999         1998    1999         1998
                                                                  ----         ----    ----         ----
<S>                                                               <C>          <C>     <C>          <C>
Cisco Systems, Inc ("Cisco") .................................     54%          56%     53%          48%
International Business Machines Corporation ("IBM") ..........     22%           *      22%           *
Mitsui Comtek Corp, a non-stocking sales representative
     for Japan ("Mitsui") ....................................      *            *       *           14%
</TABLE>

----------

* less than 10%

We expect the Company to continue to be dependent on a small number of customers
for the majority of its sales. This subjects the Company and its financial
results to particular risks, including the loss of or reduction in sales to
these customers. For example, during the quarter, IBM announced it is closing
down most of its network equipment business. IBM informed MMC that it will cease
the manufacture and sale of its network equipment products that use MMC
products, and it will cease future purchases of those MMC products. In reaction
to these events, MMC announced that revenues for the fourth quarter would
decrease from third quarter levels (see "Results of Operations - Revenue")."

If MMC loses other significant customers or if sales to these customers
decrease, future financial results would likely suffer. It is important to note
that our ability to maintain or increase sales to key customers and/or attract
new significant customers are subject to a variety of factors, including:

    -   our network equipment vendor customers may stop incorporating our
        products into their own products with limited notice to us and suffer
        little or no penalty

    -   the introduction of one of our customer's significant new products may
        be late or less successful in the market than planned

    -   a significant customer's product line using our products may rapidly
        decline or be phased out

    -   our significant customers may not incorporate our Network Processors in
        their future product designs

    -   design wins with customers may not result in significant sales to such
        customers

    -   our agreements with our customers typically do not require them to
        purchase a minimum amount of our products

    -   many of our customers have pre-existing relationships with our current
        or potential competitors that may cause them to switch from our products
        to competing products

    -   we may not be able to successfully develop relationships with additional
        significant network equipment vendors

    -   our relationship with some of our larger customers may deter other
        potential customers (who compete with these customers) from buying our
        products

Any one of the factors above could have a material adverse effect on our
business, financial condition and results of operations.

NEW PRODUCT DEVELOPMENT AND TECHNOLOGICAL CHANGE

The data networking and semiconductor industries are characterized by rapidly
changing technology, frequent product introductions and evolving industry
standards. The technical innovations required for MMC to remain competitive must
be completed before developments in networking technologies or standards make
them obsolete. These innovations must also be compelling enough to make network
equipment vendors want to use them over other technologies. Accordingly, our
future performance depends on a number of factors, including our ability to:

    -   properly identify target markets


                                       15
<PAGE>   16
    -   identify emerging technological trends in our target markets

    -   develop and maintain competitive products

    -   enhance our products by adding innovative features that differentiate
        MMC products from competitor products

    -   bring products to market on a timely basis at competitive prices

    -   respond effectively to new technological changes or new product
        announcements by others

It is possible that our design and introduction schedules for future products
and additions and enhancements to our existing products will not be met. It is
possible that these products will not achieve market acceptance or that we will
not be able to sell these products at prices that are favorable to us. We must
design new products and sometimes redesign existing products to be manufactured
on newer wafer process technologies that provide smaller circuit dimensions. It
is possible that either (1) the suppliers of these new manufacturing
technologies will not deliver them to us or our vendors on schedule or (2) we
will not be able to complete our designs on the new processes on schedule. Our
success will also depend on the ability of our customers to develop new products
and enhance existing products within our market segment and to successfully
introduce and promote those products.

To keep pace with the rapid technological changes, we must incur substantial
research and development costs before we can determine the likelihood that the
technology for a product is sound and that our network equipment vendors will
accept the product. It is possible that revenues from future products or product
enhancements will not be sufficient to recover the development costs associated
with those products or enhancements. It is possible that we may not be able to
obtain additional financing necessary to fund future development efforts. Our
failure to adequately address any of the factors above could have a negative
impact on our business, financial condition and results of operations.

DEPENDENCE ON INDEPENDENT MANUFACTURERS

We depend on independent foundries to manufacture all of our products. We
purchase both (1) fully assembled and tested products which we can immediately
resell to our customers and (2) finished silicon wafers which we then consign to
third party subcontractors who perform assembly and test functions. Because we
rely on independent foundries and third party subcontractors, we face several
significant risks including:

    -   a lack of committed manufacturing, assembly or test capacity

    -   limited control over delivery schedules, quality assurance and control,
        manufacturing yields and production costs

    -   the unavailability of, or potential delays in, obtaining access to key
        process technologies

Although we work closely with our suppliers to minimize the likelihood of
reduced manufacturing yields, in the past our suppliers have occasionally
experienced lower than anticipated manufacturing yields. This often occurs
during the production of new products or the installation and start-up of new
wafer process technologies which provide for smaller circuit dimensions.

We do not have long-term volume purchase agreements or a guaranteed level of
production capacity with any of our suppliers. We depend on our suppliers to
deliver sufficient quantities of finished product to us in a timely manner.
Since we place our orders on a purchase order basis, these suppliers can
allocate, and in the past have allocated, capacity to the production of other
companies' products while reducing deliveries to us on short notice. Only one of
our products is currently manufactured by more than one supplier.

In order to meet our customer demand, currently we must place most orders
approximately 10 to 12 weeks in advance of expected delivery. Some orders
require up to 18 weeks. If capacity becomes less available in the future, we may
be required to place orders earlier than 10 to 18 weeks in advance of expected
delivery, making it more difficult to meet customer demand. Any sudden increase
in customer demand that we did not anticipate in advance could result in our
inability to deliver product on a timely basis. This inability to deliver
product may reduce our product revenues or increase our cost of revenues and
negatively impact our


                                       16
<PAGE>   17
business, financial condition and results of operations.

In addition, we must place orders based on forecast demand to ensure enough lead
time to be able to meet anticipated customer orders. Any sudden decrease in
customer demand could result in excess inventory which could reduce our profit
margins and have a negative effect on our business, financial condition and
results of operations.

Processes used to manufacture our products are complex, customized to our
specification and can only be performed by a limited number of manufacturing
facilities. If our current manufacturing suppliers are unable to provide us with
adequate manufacturing capacity, we would have to identify and qualify one or
more substitute suppliers for a substantial majority of our products. Our
manufacturers may experience unanticipated events, like the September 1999
Taiwan earthquake, that could inhibit their abilities to provide us with
adequate manufacturing capacity on a timely basis, or at all. Introducing new
products or transferring existing products to a new third party manufacturer
would require significant development time to adapt our designs to their
manufacturing processes and could cause product shipment delays. In addition,
the costs associated with manufacturing our products may increase if we are
required to use a new third party manufacturer. If we fail to satisfy our
manufacturing requirements, our business would be materially harmed.

COMPETITION

The data networking and semiconductor industries are intensely competitive. This
competition results in constant evolution to better and more complex
technologies and shorter life cycles for our products. In addition, competition
continues to drive down the average selling price of our products. Currently,
our main competition is from ASIC solutions that our customers develop
internally. In the future we expect competition to significantly increase.
Companies such as Broadcom Corporation, Galileo Technology Ltd, IBM Corporation
and Intel Corporation have announced new Network Processor products that will
directly compete with our products. In addition to these well established
companies, several start-up companies such as Agere, Inc., C-Port and SiTera
have also announced Network Processors that will compete with our products.

Other major semiconductor companies have expressed an intent to enter the
Network Processor market. Companies such as PMC Sierra, Inc., Texas Instruments,
Motorola and others may either introduce competitive products or acquire a
smaller company, such as the startups mentioned above, and therefore abruptly
alter the competitive environment.

We also face competition from suppliers of products based on new or emerging
technologies. Many of our existing and potential customers also internally
develop integrated circuits which attempt to perform all or a portion of the
functions performed by our products. Many of our current and prospective
competitors offer broader product lines and have significantly greater
financial, technical, manufacturing and marketing resources than we do. To the
extent we fail to overcome these challenges, there would be a negative effect on
our business and operating results.

PROTECTION OF INTELLECTUAL PROPERTY

We rely on nondisclosure agreements and other contractual provisions as well as
patent, trademark, trade secret and copyright law to protect our proprietary
rights. Although we have an active patent application program, we can not be
sure that any patents will be issued from our current or future patent
applications. It is possible that patents issued from such applications may be
invalidated, circumvented, challenged or licensed to others. In addition, we can
not be sure that the rights granted under any patents we receive will provide us
with competitive advantages or be adequate to safeguard and maintain our
proprietary rights. Any failure to enforce and protect our intellectual property
rights could have a negative effect on our business, financial condition and
results of operations.

From time to time, third parties, including our competitors, may assert patent,
copyright and other intellectual property rights to technologies that are
important to us. In the future third parties may assert infringement claims
against us, and these assertions may result in costly litigation. We may not
prevail in


                                       17
<PAGE>   18
any such litigation or be able to license any valid and infringed patents from
third parties on commercially reasonable terms, if at all.

RISKS ASSOCIATED WITH EXPANSION OF INTERNATIONAL BUSINESS ACTIVITIES

Substantially all of our sales to date have been to customers located in the
United States, including sales to U.S.-based affiliates of non-U.S. network
equipment vendors. We also ship products to our domestic customers'
international manufacturing divisions and subcontractors. We currently obtain
substantially all of our manufacturing, assembly and testing services from
suppliers located outside of the United States. In 1998 we established a wholly
owned subsidiary in Israel which functions as a design center. In the future, we
intend to expand these international business activities. International
operations are subject to many inherent risks, including:

    -   political, social and economic instability

    -   trade restrictions

    -   changes in tariffs

    -   the imposition of governmental controls

    -   import and export license requirements and restrictions

    -   restrictions on the export of critical technology

    -   burdens of complying with a variety of foreign laws

    -   difficulties in staffing and managing international operations

    -   currency exchange fluctuations

In particular, certain Asian countries have recently experienced significant
economic difficulties. These difficulties include currency devaluation and
instability, business failures and a generally depressed business climate. As a
percentage of total revenues, sales to Mitsui have declined to less than 10% for
the nine months ended September 30, 1999 as compared to 14% for the nine months
ended September 30, 1998. Sales to Mitsui may remain at this lower level or
decline further due to the economic downturn in Japan and Asia and could have an
adverse effect on our revenues in the future.

General economic or industry-specific conditions in our primary overseas markets
may negatively impact the demand for our products abroad. All of our
international sales to date have been denominated in U.S. dollars. Accordingly,
an increase in the value of the U.S. dollar relative to foreign currencies could
make our products less competitive in international markets. Any one or more of
the factors mentioned above could have a negative effect on our business,
financial condition and results of operations or require us to modify our
current business practices significantly. These factors are anticipated to
impact our business to a greater degree in the event that we expand our
international business activities.

RISKS RELATED TO ACQUISITIONS

The data networking and semiconductor industries are intensely competitive and
MMC's growth is dependent our ability to introduce new products and enhancements
to existing products on a timely basis. It is possible that one way we may
expand our business is through the acquisition of businesses, products or
technologies that complement our existing products, expand our market coverage
or enhance our technological capabilities. Acquisitions involve numerous risks,
including:

    -   the risk of diverting management's attention from normal daily
        operations of the business

    -   difficulties in integration of the operations, technologies, products
        and information systems of the acquired company

    -   potential difficulties in completing projects associated with purchased
        in-process research and development projects

    -   risks of entering markets in which MMC has limited or no direct prior
        experience and where competitors have stronger market positions

    -   the potential loss of key employees of the acquired company


                                       18
<PAGE>   19
In addition to the risks above, acquisitions may negatively affect our results
of operations because they may require large one-time write-offs, increased debt
and contingent liabilities, substantial depreciation or deferred compensation
charges or the amortization of expenses related to goodwill and other intangible
assets. We may seek to account for acquisitions under the pooling of interest
method, but that method may not be available. It is possible that any of these
events could cause the price of our stock to decline. In the future we may not
be able to find suitable acquisition opportunities. Even if we do find such
opportunities, it is possible that the acquisitions may be unsuccessful.
Unsuccessful acquisitions could have a negative effect on our business,
financial condition and results of operations.

EXPECTED VOLATILITY OF STOCK PRICE

In recent years the stock market in general, and the market for shares of high
technology, data networking and semiconductor companies in particular, have
experienced extreme price fluctuations. Our stock price has fluctuated
substantially in the past and is likely to continue to be highly volatile and
subject to wide fluctuations in the future in response to various factors, many
of which we can not control. These factors include:

    -   quarterly variations in operating results

    -   announcements of technological innovations or new products by our
        competitors, our customers or ourselves

    -   the gain or loss of significant network equipment vendor customers

    -   changes in earnings estimates or investment recommendations of analysts

    -   the presence or absence of short-selling of our stock

    -   changes in investor perceptions

    -   changes in expectations relating to our products, plans and strategic
        position or those of our competitors or customers

In addition, market prices of technology stocks have been especially volatile.
This volatility has significantly affected the market prices of securities of
many technology companies for reasons frequently unrelated to the operating
performance of the specific companies. Accordingly, investors may not be able to
resell shares of our Common Stock at or above the price originally paid for the
stock. In the past, companies that have experienced volatility in the market
price of their securities have been the subject of securities class action
litigation. If we were the object of securities class action litigation, it
could result in substantial losses and divert management's attention and
resources from other matters.


                                       19
<PAGE>   20
PART II. OTHER INFORMATION

ITEM 1. LEGAL PROCEEDINGS

Not Applicable.

ITEM 2. CHANGES IN SECURITIES

USE OF PROCEEDS FROM THE SALE OF REGISTERED SECURITIES

The effective date of our first registration statement (the "Registration
Statement") was October 29, 1997. We filed the Registration Statement on Form
S-1 under the Securities Act of 1933, No. 333-34005. The class of securities
registered was Common Stock. The offering commenced on October 29, 1997 and all
securities were sold in the offering.

MMC registered a total of 4,025,000 shares under the Registration Statement. MMC
sold all the shares for its own account, for an aggregate offering price of
$44.3 million. We incurred expenses of approximately $4.2 million for the
offering, of which $3.1 million represented underwriting discounts and
commissions and $1.1 million represented other expenses. All these expenses were
direct or indirect payments to others. The net offering proceeds to MMC after
total expenses were approximately $40.1 million. As of September 30, 1999, MMC
had used the net proceeds from the offering as follows:

<TABLE>
<CAPTION>
         Amount            Purpose/Use
         ------            -----------
<S>                        <C>
     $ 7.1 million         purchase and installation of capital equipment and purchased software
     $ 0.5 million         plant, buildings and facilities
     $ 0.6 million         repayment of indebtedness and lease obligations
     $31.9 million         temporary investments
</TABLE>

MMC did not pay any of the proceeds directly or indirectly to any director or
officer of MMC or to their associates, or to any person owning ten percent or
more of any class of equity securities of MMC or to any affiliate of MMC. The
actual use of the proceeds from the offering does not represent a material
change in the use of proceeds as described in the prospectus contained in the
Registration Statement.

ITEM 3. DEFAULTS UPON SENIOR SECURITIES

Not Applicable.

ITEM 4. SUBMISSION OF MATTERS TO A VOTE OF SECURITY HOLDERS

Not Applicable.

ITEM 5. OTHER INFORMATION

Not Applicable.

ITEM 6. EXHIBITS AND REPORTS ON FORM 8-K

        (a) Exhibits

<TABLE>
<CAPTION>
           Exhibit No.      Description of Exhibit
           -----------      ----------------------
<S>                         <C>
              27.1          Financial Data Schedule as of September 30, 1999 and for the 9 months then ended.
</TABLE>


                                       20
<PAGE>   21

        (b) Reports on Form 8-K

            The Company filed no reports on Form 8-K during the three months
            ended September 30, 1999.


                                       21
<PAGE>   22
                                   SIGNATURES

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

Dated: November 12, 1999        MMC NETWORKS, INC.

                                By: /s/ Douglas C. Spreng
                                    --------------------------------------------
                                    Douglas C. Spreng
                                    President, Chief Executive Officer and
                                    Director (Principal Executive Officer)
                                    Acting Chief Financial Officer,
                                    (Principal Financial and Accounting Officer)


                                       22
<PAGE>   23
                                  EXHIBIT INDEX

<TABLE>
<CAPTION>
Exhibit #           Description
---------           -----------
<S>                 <C>
  27.1              Financial Data Schedule
</TABLE>