Document:

<PAGE>

                                                                 Exhibit (10)(y)

                     FOURTH AMENDMENT TO CREDIT AGREEMENT
                     ------------------------------------

          THIS FOURTH AMENDMENT TO CREDIT AGREEMENT (this "Agreement") is made
as of this 5th day of June, 2001 between C-COR.NET CORP., a Pennsylvania
corporation ("C-Cor"), BROADBAND CAPITAL CORPORATION, a Delaware corporation
("Broadband," and collectively with C-Cor, the "Borrowers"), THE UNDERSIGNED
BANK PARTIES (individually a "Bank" and collectively, the "Banks") and MELLON
BANK, N.A., a national banking association, as agent for the Banks (the
"Agent").

                             W I T N E S S E T H :

          WHEREAS, the Borrowers, the Banks and the Agent are parties to a
Credit Agreement dated as of August 9, 1999, as amended by that certain First
Amendment to Credit Agreement dated as of December 29, 1999, that certain Second
Amendment to Credit Agreement dated as of November 24, 2000 and that certain
Third Amendment to Credit Agreement dated as of May 17, 2001 (as so amended, the
"Credit Agreement"), pursuant to which the Banks agreed to extend to the
Borrowers a Twenty Million Dollar ($20,000,000.00) revolving credit facility, a
Fifty Million Dollar ($50,000,000.00) standby acquisition facility and a Two
Million Five Hundred Thousand Dollar ($2,500,000.00) term loan (Capitalized
terms used herein but not defined in this Agreement shall have the meanings
ascribed to them in the Credit Agreement.);

          WHEREAS, the Borrowers have repaid the Term Loan in full;

          WHEREAS, the Borrowers have requested that the Credit Agreement be
amended in order to, among other things, amend certain financial covenants; and

          WHEREAS, the Agent and the Banks are willing to grant such request,
subject to the terms and conditions of this Agreement.

          NOW, THEREFORE, in consideration of the mutual promises contained
herein and intending to be legally bound, the Borrowers, the Agent and the Banks
hereby covenant and agree as follows:

          1.  Amendments.  Upon the execution and delivery by the Borrowers and
              ----------
the Banks of this Agreement, the Credit Agreement shall be amended as follows:

               (a)  The following defined term shall be added to Section 1.01,
Certain Definitions, in alphabetical order:

          "Adjusted EBITDA" shall mean EBITDA excluding (i) the restructuring
     expense in the maximum amount of $7,241,000 (after tax adjustments), plus
                                                                          ----
     (ii) non-recurring charges in the maximum amount of $4,100,000
<PAGE>

               (b)  Subsection 2.01(e) shall be deleted in its entirety and
replaced with the following:

          (e)  Maturity. To the extent not due and payable earlier, the
     Revolving Credit Loans shall be due and payable on the Revolving Credit
     Expiration Date. If, for any reason, the Revolving Credit Commitment is not
     extended or renewed beyond the Revolving Credit Expiration Date, all
     amounts owing by the Borrowers hereunder or under the Notes (including,
     without limitation, all Letter of Credit Reimbursement Obligations) shall
     be immediately due and payable and the Agent and/or the Banks shall be
     entitled to exercise the remedies set forth in Article VIII hereof.

               (c)  The pricing grid set forth subsection (a) of Section 2.05,
Interest Rates, shall be deleted in its entirety and replaced with the
following:

<TABLE>
<CAPTION>
--------------------------------------------------------------------------------------------------------------
                                               Applicable Margin
--------------------------------------------------------------------------------------------------------------
     Funded              Prime Rate     LIBOR Rate Options and              Swingline Loan
  Indebtedness            Options        Letter of Credit Fee            Fed Funds Rate Option
   to Adjusted
  EBITDA/EBITDA
      Ratio
--------------------------------------------------------------------------------------------------------------
<S>                       <C>           <C>                              <C>
Less than 1.50             (0.50%)              1.25%                            1.25%
--------------------------------------------------------------------------------------------------------------
Greater than or            (0.25%)              1.50%                            1.50%
equal to 1.50 and
less than 2.25
--------------------------------------------------------------------------------------------------------------
Greater than or            (0.25%)              1.75%                            1.75%
equal to 2.25 and
less than 2.75
--------------------------------------------------------------------------------------------------------------
Greater than or              0.0                 2.0%                            2.0%
equal to 2.75 and
less than 3.75
--------------------------------------------------------------------------------------------------------------
</TABLE>

               (d)  Subsection 3.01(d) shall be deleted in its entirety and
replaced with the following:

          (d)  Letter of Credit Fee.  On the date of issuance (and quarterly
     thereafter), the Borrowers shall pay to the Agent for the account of each
     Bank a fee (the "Letter of Credit Fee") for each Letter of Credit equal to
     (i) the face amount of each issued and outstanding Letter of Credit times
                                                                         -----
     (ii) the Applicable Margin.

                                      -2-
<PAGE>

               (e)  Subsection (a) of Section 7.01, Financial Covenants, shall
be deleted in its entirety and replaced with the following:

          (a)  Funded Indebtedness to Adjusted EBITDA/EBITDA Ratio.  As of the
     last day of the fiscal quarters set forth below, the Funded Indebtedness to
     Adjusted EBITDA Ratio, as measured on a rolling four quarter basis of the
     four most recent quarters, shall not exceed the following ratios:

               Period Ending        Maximum Ratio
               -------------        -------------

               June 30, 2001        2.25:1.00

               September 30, 2001   3.75:1.00

     As of the last day of each fiscal quarter thereafter, the Funded
     Indebtedness to EBITDA Ratio, as measured on a rolling four quarter basis
     of the four most recent quarters, shall not exceed 2.25:1.00.

               (f)  Subsection (c) of Section 7.01, Financial Covenants, shall
be deleted in its entirety and replaced with the following:

          (c)  Current Ratio.  As of the last day of the fiscal quarters ending
     June 30, 2001 and September 30, 2001, the Current Ratio shall not at any
     time be less than 2.50:1.00.  As of the last day of each fiscal quarter
     thereafter, the Current Ratio shall not at any time be less than 1.25:1.00.

          2.  Representations and Warranties.  (a)  The Borrowers hereby
              ------------------------------
represent and warrant to the Agent and the Banks that there is no default, Event
of Default or Potential Default under the Credit Agreement, the Loan Documents,
or any other document executed in connection therewith.

               (b)  Each of the representations and warranties by the Borrowers
in or pursuant to the Credit Agreement or any Loan Document are true and correct
in all material respects on and as of the date of this Agreement as though made
on and as of such date.

          3.  Other Terms Confirmed.  All other terms and conditions of the
              ---------------------
Credit Agreement, including, without limitation, the right of the Agent and the
Banks to CONFESS JUDGMENT, are hereby confirmed and shall remain in full force
and effect without modification. From and after the effectiveness of the
amendments set forth in Section 1 hereof, all references in any document or
instrument to the Credit Agreement shall mean the Credit Agreement as amended by
this Agreement.

          4.  Amendment Fee.  Upon the execution of this Agreement, the
              -------------
Borrowers shall pay to the Agent for the sole account of the Agent an amendment
fee in an amount equal to 0.05% of the Commitments.

                                      -3-
<PAGE>

          5.  No New Indebtedness.  The Borrowers specifically acknowledge and
              -------------------
agree that this Agreement shall not represent in any way the extension of any
additional credit by the Banks to the Borrowers, or the satisfaction of any
indebtedness evidenced by Loan Documents or the Credit Agreement as amended
hereby.

          6.  Counterparts.  This Agreement may be executed in any number of
              ------------
counterparts, each of which shall be deemed an original, but all such
counterparts together shall constitute but one and the same instrument.

          7.  Headings.  The descriptive headings which are used in this
              --------
Agreement are for convenience only and shall not affect the meaning of any
provision of this Agreement.

          8.  Governing Law.  This Agreement shall be governed by and construed
              -------------
under the laws of the Commonwealth of Pennsylvania, without regard to conflict
of laws principles.

          IN WITNESS WHEREOF, the parties hereto have caused this Agreement to
be executed by their duly authorized officers as of the day and year first
written above.

ATTEST:                                           C-COR.NET CORP.

/s/ Joseph E. Zavacky                             By /s/ W. T. Hanelly
---------------------                                -----------------
By: Joseph E. Zavacky                             Name: W.T. Hanelly
Title: Controller                                  Title: VP, Finance
[CORPORATE SEAL]

ATTEST:                                           BROADBAND CAPITAL CORPORATION

/s/ Joseph E. Zavacky                             By /s/ George M. Savereno
---------------------                                ----------------------
By: Joseph E. Zavacky                             Name: George M. Savereno
Title: Director                                    Title: President
[CORPORATE SEAL]

                                      -4-
<PAGE>

                                    MELLON BANK, N.A., as Issuing Bank and as
                                    Agent for the Banks

                                    By /s/ Joseph N. Butto
                                       -------------------
                                       Name:  Joseph N. Butto
                                       Title: Vice President

                                    MELLON BANK, N.A., individually as a Bank

                                    By /s/ Joseph N. Butto
                                       -------------------
                                       Name:  Joseph N. Butto
                                       Title: Vice President

                                    FIRST UNION NATIONAL BANK, individually as a
                                    Bank

                                    By /s/ Frank Kulp
                                       --------------
                                       Name:  Frank Kulp
                                       Title: Vice President

                                    PNC BANK, NATIONAL ASSOCIATION, individually
                                    as a Bank

                                    By /s/ Thomas J. Fowlston
                                       ----------------------
                                       Name:  Thomas J. Fowlston
                                       Title: Vice President

                                      -5-<PAGE>

                                                                 Exhibit (10)(z)

                             C-COR.net CORPORATION
                             FISCAL YEAR 2001/2002
                       INCENTIVE & RETENTION PLAN (IRP)
                         (replaces the FY01 PIP plan)

1.  Conceptual Basis of the Plan

The Incentive & Retention Plan (IRP) is a two-fold program focusing on employee
retention and the achievement of specific financial goals. The plan supports the
strategic objectives of retaining the appropriate personnel needed to achieve
the fiscal year 2001 and 2002 goals and providing an incentive to those
employees to meet or exceed the fiscal year 2002 financial plan.

The retention aspect of the plan is based upon an individual being employed by
C-COR for a specific period of time prior to and on the date of the payments.
Payments will be made on a quarterly basis.

The incentive portion of the program is based upon the achievement of specific
financial goals for the Corporation as established by the Board of Directors.
The incentive portion of the plan provides employees with the opportunity to
receive an additional one-time payment at the end of fiscal year 2002 for
accomplishment of these specific financial goals.

The basic payment formula is as follows:

Key Executives (as defined in Attachment A):

Retention Payment =   Flat dollar amount per key executive*

Employees (excluding key executives):

Retention Payment =   % of employee's base wages during defined measurement
                      period

Incentive Payment =   % of employees base wages during FY02 measurement period
                      (based upon achievement of the financial goals)

*The CEO/Chairman of the Board receives an amount equal to three times the key
executive IRP payment.

                                       1
<PAGE>

2.   Eligibility

Full-time, active employees and part-time, active employees (working a minimum
of 20 hours per week/1040 hours per year) who are not provided with a
                                                  -------------------
specifically identified, alternative incentive bonus plan are eligible.
-----------------------------------------------------------------------

Employees of the following companies are eligible if they meet the criteria
defined above:

  .  C-COR.net Corp.
  .  Broadband Management Solutions, LLC
  .  Convergence.com Corporation
  .  Silicon Valley Communications, Inc.

Employees in the following entities/categories are not eligible:
                                               ----------------

  .  C-COR Europe, B.V.
  .  C-COR de Mexico, S.A. de C.V.
  .  Employees on Sales or Marketing Commission or Incentive Plans
  .  Employees of Worldbridge Broadband Services, Inc.
  .  Temporary Agency Employees, Independent Contractors, Co-op and Intern
     (part-time) Employees

* Employees of companies acquired during FY01 and FY02 will not be eligible for
the retention payment but may be eligible for the incentive payment based upon
time after acquisition.  Determination of eligibility will be made for each
acquisition on a case by case basis.

3.   Measurement Periods

Quarterly Retention Payments:

In order to be eligible for a quarterly retention payment, an employee must have
worked the full fiscal quarter identified below and be on the active rolls at
                                                ---
the time of the quarterly payment.

Quarterly measurement periods are defined as:

July 2001 payment:            July 1, 2000 through September 29, 2000
October 2001 payment:         September 30, 2000 through December 29, 2000
January 2002 payment:         December 30, 2000 through March 30, 2001
April 2002 payment:           March 31, 2001 through June 29, 2001

                                       2
<PAGE>

Fiscal Year 2002 Incentive Payment:

In order to be eligible for a payment at the end of fiscal year 2002, an
employee must have worked at least one full fiscal quarter during fiscal year
2002 and be on the active rolls at the time of payment following the end of
fiscal year 2002.

Note:  The formula for calculating a Incentive Payment takes into account the
pro-rationing of the payment amount to reflect the amount of time the individual
was actively employed during the fiscal year.

4.  Pro-rated Payments

Employees on Leave (Disability; Workers' Compensation; FMLA; Military Leave) -

An employee must be a full-time or part-time active employee in order to be
eligible for a payment. The individual would be eligible for a payment on a pro-
rata basis for the portion of FY 2001 (retention payment) or FY2002 (incentive
payment) in which he/she was a full-time or part-time active employee.

5.  Calculation of the IRP Payments for Employees (excluding key executive list)

Retention Payment =   Employee's base wages during defined measurement period
                      X 5%

Incentive Payment =   Employees base wages during FY02 measurement period
                      X 5% (based upon achievement of the FY02 financial goals)

Payment of the incentive payment will be determined by review of the Board of
Directors based upon achievement of the financial goals for the Corporation
established prior to the beginning of FY02.

6.  Key Executive IRP Payment

Retention Payment =   Flat dollar amount  per key executive*
                      Amount is based upon approximately 50% of the FY01 PIP
                      Plan at 100% of target level
Incentive Payment =   Flat dollar amount  per key executive*
                      Amount is based upon approximately 50% of the FY01 PIP
                      Plan at 100% of target level (based upon achievement of
                      the FY02 financial goals)

Payment of the incentive payment will be determined by review of the Board of
Directors based upon achievement of the financial goals for the Corporation
established prior to the beginning of FY02.

*The CEO/Chairman of the Board receives an amount equal to three times the key
executive IRP payment.

                                       3
<PAGE>

7.   Frequencies and Timing of Payments

Retention Payment:

At the end of each fiscal quarter, a review of employees meeting the criteria
for active employment for the defined measurement period will be completed.
Employees meeting the criteria as defined in section 3 above will receive a
payment as soon as practical following the review of the personnel data.
Normally this payment will occur on the first pay of the month shown in section
3 of this document.

Incentive Payment:

At the end of fiscal year 2002, the Board of Directors will review the year-end
financial records as they relate to the achievement of the fiscal goal
established for this incentive plan.  Based upon this review, should they
approve the plan, a one-time payment will be made as soon as practical following
the approval of the Board of Directors.

8.   Definition of base wages

Base wages are defined as the total base wages of an employee at the end of each
measurement period, not including any special compensation such as the
reimbursement of moving expenses, one-time bonuses, or the exercise of stock
options. Base wages do not include overtime, however shift differential will be
included in the calculation of base wages when applicable.

9.   Administration

(a)  The Compensation Committee of the Board of Directors oversees the Plan,
     which shall be administered by the Chief Executive Officer (CEO) and such
     other employees of the Company as may be appointed by the CEO.

(b)  Subject to the provisions of the Plan, the CEO shall have the sole
     authority and discretion:

     i)    to construe and interpret the Plan;

     ii)   to determine and recommend to the Board of Directors for approval,
           the amount of payments to be made under the Plan and the status and
           rights of any participant or beneficiary to payments under the Plan;

     iii)  to decide all questions concerning the Plan and to make all other
           determinations and to take all other steps necessary or advisable for
           the administration of the Plan.

     All decisions made by the CEO pursuant to the provisions of the Plan shall
     be made in his or her sole discretion and shall be final, conclusive, and
     binding upon all parties.

                                       4
<PAGE>

10.  Right to Withhold Taxes

The Company shall have the right to withhold such amounts from any payment under
this Plan as it determines necessary to fulfill any federal, state, or local
wage or compensation withholding requirements.

11.  Non-Transferability of Rights

A participant's rights and interests under the Plan may not be assigned or
transferred in whole or in part either directly or by operation of law or
otherwise (except in an event of the participant's death), including, but not
limited to, by way of execution, levy, garnishment, attachment, pledge,
bankruptcy or in any other manner, and no such rights or interests of any
participant under the Plan shall be subject to any obligation or liability of
such participant other than any obligations or liabilities owed by the
Participant to the Company.

12.  No Right to Continued Employment

Neither the Plan, nor any compensation payable under the Plan, shall confer upon
any participant any right to continuance of employment by the Company or any
affiliate of the Company nor shall they interfere in any way with the right of
the Company or any affiliate of the Company to terminate any participant's
employment at any time.

13.  No Claim Against Assets

Nothing in this Plan shall be construed as giving any participant or his or her
legal representative, or designated beneficiary, any claim against any specific
assets of the Company or any affiliate or as imposing any trustee relationship
upon the Company or any affiliate in respect of the participant.

The Company shall not be required to segregate any assets in order to provide
for the satisfaction of the obligations hereunder.  If and to the extent that
the participant or his or her legal representative or designated beneficiary
acquires a right to receive any payment pursuant to this Plan, such right shall
be no greater than the right of an unsecured general creditor of the Company or
any affiliate.

14.  Company's Books and Records Conclusive

The Company's books and records, and internal accounting procedures, will be
conclusive for all purposes under the Plan.

                                       5
<PAGE>

15.  Amendment or Termination

The Company may at any time, terminate or modify or amend the Plan in any
respect, at any time prior to payment for a fiscal quarter.

16.  No Other Agreements or Understandings

Except as expressly provided herein, this Plan represents the sole agreement
between the Company and participants concerning its subject matter and it
supersedes all prior agreements, arrangements, understandings, warranties,
representations, and statements between the parties concerning its subject
matter.

17.  Governing Law

The Plan and all actions taken pursuant thereto shall be governed by, and
construed in accordance with, the laws of the State of Pennsylvania applied
without regard to conflict of law principles.

                                       6
<PAGE>

                                 Attachment A
                                 ------------

                                 Mary G. Beahm

                                 David J. Eng

                              William T. Hanelly

                              William J. Milliron

                              Gerhard B. Nederlof

                               Carrie M. Packer

                               David A. Woodle*

                               Kenneth A. Wright

                               Joseph E. Zavacky

*CEO/Chairman of the Board receives a payment equal to 3 times the key executive
share

                                       7

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