<PAGE>

EXHIBIT 10.13

                                      
                          FIRST AMENDMENT AGREEMENT
                                     TO
         THIRD AMENDED AND RESTATED REVOLVING CREDIT LOAN AGREEMENT


                         Date:  As of April 30, 1998
                                      


NationsBank of Texas, N.A.
Attention:  Mr. Frank Izzo
901 Main Street, 7th Floor
Dallas, Texas 75201

Ladies and Gentlemen:

     The undersigned, ODS Networks, Inc. (formerly known as Optical Data 
Systems, Inc.), a Texas corporation ("Borrower"), entered into a Third 
Amended and Restated Revolving Credit Loan Agreement (the "Loan Agreement") 
dated December 31, 1997, with NationsBank of Texas, N.A., a national banking 
association ("Bank").  Pursuant to the Loan Agreement, Bank agreed, under 
certain terms and conditions, to extend a loan (the "Loan") to Borrower 
evidenced by a Ninth Renewal Promissory Note (the "Note") dated December 31, 
1997, executed by Borrower and payable to the order of Bank in the original 
principal amount of $15,000,000.00.

     Borrower has informed Bank that it desires, through its wholly-owned 
subsidiary ECC Acquisition Corp., to merge (such merger herein referred to as 
the "Essential Merger") with Essential Communication Corporation, a Delaware 
corporation, with Essential Communication Corporation being the surviving 
entity.  

     Borrower has also informed Bank that Borrower desires to purchase (such 
purchase referred to herein as the "Blue Ridge Investment") convertible 
preferred stock in Blue Ridge Networks, Inc. ("Blue Ridge") for a maximum 
consideration of $1,250,000, which shares are convertible into common voting 
stock of Blue Ridge in an amount, if converted, equal to a minimum of 25% of 
the then outstanding voting interests of Blue Ridge.  

     Borrower has requested that Bank consent to Borrower's departure from 
the covenants set forth in SECTIONS 7.5, 7.6, 7.9 and 7.11 of the Loan 
Agreement (collectively, the "Applicable Covenants") to allow the Essential 
Merger and the Blue Ridge Investment.  Upon the terms and subject to the 
conditions set forth in this First Amendment Agreement (the "Agreement", 
terms used in this Agreement shall have the same meaning as in the Loan 
Agreement, if defined therein, unless otherwise defined in this Agreement), 
Bank consents to Borrower's departure 

                                      
<PAGE>

from the Applicable Covenants for the sole purpose of allowing the Essential 
Merger and the Blue Ridge Investment.

     Borrower has also requested that Bank make certain modifications to the 
terms of the Loan Agreement.  Bank has advised Borrower that Bank is willing 
to do so upon the terms and subject to the conditions of this Agreement.  In 
consideration for the above premises and mutual promises and covenants herein 
contained, Borrower and Bank do hereby agree as follows:

     1.   CONSENT.  

          (a)  The Bank hereby consents to Borrower's departure from the
          Applicable Covenants of the Loan Agreement to the extent such
          covenants would be violated by either the Essential Merger or the Blue
          Ridge Investment.  
     
          (b)  The consent specifically described in clause 1(a) above shall not
          constitute and shall not be deemed a waiver of any Event of Default or
          a consent to the departure from any provisions to the Loan Agreement
          other than the Applicable Covenants, or a waiver of any rights or
          remedies arising as a result of any Event of Default.  The failure to
          comply with the covenants described in clause 1(a) above for any
          activity other than the Essential Merger or the Blue Ridge Investment
          shall constitute an Event of Default.
     
     2.   AMENDMENTS TO THE LOAN AGREEMENT.  Effective as of the date hereof,
          the Loan Agreement is hereby amended as follows:
     
          (a)  The first sentence in SECTION 2.4 is amended and restated to read
          in its entirety as follows:  "Borrower agrees to pay Bank a commitment
          fee of one-fourth of  one percent (0.25%) per annum on the daily
          unused portion of the Revolving Credit Commitment."
     
          (b)  SECTION 7.12 is amended and restated to read in its entirety as
          follows:
     
               7.12 CONSOLIDATED TANGIBLE NET WORTH. Borrower will not
          permit, as of the last day of any calendar quarter, the
          Consolidated Tangible Net Worth for such day to be less than the
          amount opposite the applicable date in the following chart:
     
<TABLE>
              --------------------------------------------------------
                                             Minimum Consolidated
                          Date                Tangible Net Worth
              --------------------------------------------------------
<S>                                          <C>
                    December 31, 1997            $66,000,000
                     March 31, 1998              $63,000,000
                      June 30, 1998              $55,100,000
                   September 30, 1998            $55,050,000
              --------------------------------------------------------
                December 31, 1998 and the
                 last day of each fiscal         
                   quarter thereafter            $55,500,000
              --------------------------------------------------------
</TABLE>
<PAGE>

          (c)  The first sentence in SECTION 7.13 is amended and restated to
          read in its entirety as follows:  "Borrower will at all times maintain
          a Current Ratio of not less than 1.35 to 1.00."
     
          (d)  Exhibit "C" to the Loan Agreement is amended by adding the 
          following location to the list of U.S. Offices:
     
                    4374 Alexander Blvd., N.E., Suite T
                    Albuquerque, NM  87107
     
     3.   CONDITIONS PRECEDENT.  Bank's willingness to enter into this Agreement
          is subject to the conditions precedent that, as of the date hereof:
     
          (a)  Bank shall receive the following (the "Amendment Documents"), 
          duly executed by each party thereto, other than Bank, each in form 
          and substance satisfactory to Bank:
     
                    (i)    this Agreement;
                    
                    (ii)   a Subsidiary Joinder Agreement executed by
                    Essential Communication Corporation;
                    
                    (iii)  a certificate from the appropriate
                    governmental authority evidencing Essential
                    Communication Corporation's existence and good
                    standing in the state of its incorporation;
                    
                    (iv)   final, executed copies of each of the
                    agreements, bills of sale and other documents
                    effecting the Essential Merger and the Blue Ridge
                    Investment; and
                    
                    (v)    all other documents that Bank may request
                    with respect to any matter relevant to this
                    Agreement or the transactions contemplated hereby.
                    
          (b)  The representations and warranties of Borrower
          contained herein shall be true and correct in all material
          respects on and as of the date hereof, and Borrower shall have 
          complied with all of the terms and conditions herein.  The 
          execution by Borrower of this Agreement is hereby deemed to 
          constitute a 
<PAGE>

     representation and warranty by Borrower to the foregoing effect.

4.   REPRESENTATIONS AND WARRANTIES.  In order to induce Bank to enter into
     this Agreement, Borrower hereby represents and warrants to Bank that:

     (a)  The Loan Papers are the legal, valid and binding obligations of
     Borrower, enforceable in accordance with their respective terms,
     except as limited by bankruptcy, insolvency or other laws of general
     application relating to the enforcement of creditors' rights;

     (b)  Neither the execution and delivery of this Agreement nor the
     consummation of any of the transactions herein contemplated, nor
     compliance with the terms and provisions hereof will contravene or
     conflict with any provision of law, statute or regulation to which
     Borrower is subject or any judgment, license, order or permit
     applicable to Borrower or any indenture, mortgage, deed of trust or
     other instrument to which Borrower may be subject; no consent,
     approval, authorization or order of any court, governmental authority
     or third party is required in connection with the execution, delivery,
     or performance by Borrower of this Agreement or to consummate the
     transactions contemplated herein;

     (c)  To the best of Borrower's knowledge, all financial statements
     delivered by Borrower to Bank prior to the date hereof are true and
     correct, fairly present the financial condition of Borrower and have
     been prepared in accordance with generally accepted accounting
     principles, consistently applied; as of the date hereof, there are no
     obligations, liabilities or indebtedness (including contingent and
     indirect liabilities) which are material to Borrower and not reflected
     in such financial statements;
               
     (d)  To the best of Borrower's knowledge, no litigation,
     investigation, or governmental proceeding is pending, or, to the
     knowledge of any of Borrower's officers, threatened against or
     affecting Borrower, which may result in any material adverse change in
     Borrower's business, properties or operations, except as has been
     previously disclosed by Borrower to Bank;

     (e)  To the best of Borrower's knowledge, Borrower owns all of the
     assets reflected on its most recent balance sheet free and clear of
     all liens, security interests or other encumbrances, other than those
     (if any) in favor of Bank;

     (f)  To the best of Borrower's knowledge, Borrower is not in violation
     of any law, ordinance, governmental rule or regulation to which it is
     subject, and is not in default under any material agreement, contract
     or understanding to which it is a party, except as has been previously
     disclosed by Borrower to Bank; and
<PAGE>

     (g)  All of the representations and warranties contained in SECTION 5
     of the Loan Agreement are true and correct on and as of the date
     hereof with the same force and effect as if made on and as of the date
     hereof, provided that the representations and warranties contained in
     SECTIONS 5.8 and 5.9 of the Loan Agreement are made with respect to
     the financial statements most recently submitted to Bank pursuant to
     SECTION 6.5 of the Loan Agreement.

     (h)  Borrower has (i) begun analyzing the operations of Borrower and
     its subsidiaries and affiliates that could be adversely affected by
     failure to become Year 2000 compliant (that is, that computer
     applications, imbedded microchips and other systems will be able to
     perform date-sensitive functions prior to and after December 31, 1999)
     and; (ii) developed a plan for becoming Year 2000 compliant in a
     timely manner, the implementation of which is on schedule in all
     material respects.  Borrower reasonably believes that it will become
     Year 2000 compliant for its operations and those of its subsidiaries
     and affiliates on a timely basis except to the extent that a failure
     to do so could not reasonably be expected to have a material adverse
     effect upon the financial condition of Borrower.

     (i)  Borrower reasonably believes any suppliers and vendors that are
     material to the operations of Borrower or its subsidiaries and
     affiliates will be Year 2000 compliant for their own computer
     applications except to the extent that a failure to do so could not
     reasonably be expected to have a material adverse effect upon the
     financial condition of Borrower.

     (j)  Borrower will promptly notify Bank in the event Borrower
     determines that any computer application which is material to the
     operations of Borrower, its subsidiaries or any of its material
     vendors or suppliers will not be fully Year 2000 compliant on a timely
     basis, except to the extent that such failure could not reasonably be
     expected to have a material adverse effect upon the financial
     condition of Borrower.

5.   MISCELLANEOUS.  

     (a)  WAIVER.  No failure to exercise, and no delay in exercising, on
the part of Bank, any right hereunder shall operate as a waiver thereof,
nor shall any single or partial exercise thereof preclude any other or
further exercise thereof or the exercise of any other right.  The rights of
Bank hereunder and under the other Loan Papers shall be in addition to all
other rights provided by law.  No notice or demand given in any case shall
constitute a waiver of the right to take other action in the same, similar
or other instances without such notice or demand.

     (b)  GOVERNING LAW.  THIS AGREEMENT IS BEING EXECUTED AND DELIVERED,
AND IS INTENDED TO BE PERFORMED, IN THE STATE OF TEXAS, AND THE SUBSTANTIVE
LAWS OF TEXAS SHALL GOVERN THE VALIDITY, CONSTRUCTION, ENFORCEMENT AND
INTERPRETATION OF 

<PAGE>

THIS AGREEMENT AND ALL OTHER LOAN PAPERS, EXCEPT TO THE EXTENT: (i) OTHERWISE 
SPECIFIED THEREIN; (ii) THE FEDERAL LAWS GOVERNING NATIONAL BANKS EXPRESSLY 
SUPERSEDE AND HAVE CONTRARY APPLICATION; OR (iii) FEDERAL LAWS GOVERNING 
MAXIMUM INTEREST RATES SHALL PROVIDE FOR RATES OF INTEREST HIGHER THAN THOSE 
PERMITTED UNDER THE LAWS OF THE STATE OF TEXAS.
                    
     (c)  INVALID PROVISIONS.  If any provision of this Agreement is held to 
be illegal, invalid or unenforceable under present or future laws effective 
during the term of this Agreement, such provision shall be fully severable 
and this Agreement shall be construed and enforced as if such illegal, 
invalid or unenforceable provision had never comprised a part of this 
Agreement, and the remaining provisions of this Agreement shall remain in 
full force and effect and shall not be affected by the illegal, invalid or 
unenforceable provision or by its severance from this Agreement.

     (d)  ENTIRETY AND AMENDMENTS.  The Loan Papers embody the entire 
agreement between the parties and supersede all prior agreements and 
understandings, if any, relating to the subject matter hereof and thereof, 
and this Agreement and the other Loan Papers may be amended only by an 
instrument in writing executed by the party, or an authorized officer of the 
party, against whom such amendment is sought to be enforced.

     (e)  PARTIES BOUND.  This Agreement shall be binding upon and inure to 
the benefit of the parties hereto and their respective successors, assigns 
and legal representatives; provided, however, that Borrower may not, without 
the prior written consent of Bank, assign any rights, powers, duties or 
obligations hereunder.

     (f)  PAYMENT OF EXPENSES.  Borrower agrees to pay all costs and expenses 
of Bank (including, without limitation, the reasonable attorneys' fees of 
Bank's legal counsel) incurred by Bank in connection with the preparation of 
this Agreement and the preservation and enforcement of Bank's rights under 
this Agreement and the other Loan Papers.

     (g)  HEADINGS.  Section headings are for convenience of reference only 
and shall in no way affect the interpretation of this Agreement.

     (h)  COUNTERPARTS. This Agreement may be executed in any number of 
counterparts, each of which shall for all purposes be deemed an original and 
all of which are identical.

6.   NO ORAL AGREEMENTS.  THIS AGREEMENT, TOGETHER WITH THE OTHER LOAN
PAPERS AS WRITTEN, REPRESENTS THE FINAL 

<PAGE>

AGREEMENT BETWEEN THE PARTIES AND MAY NOT BE CONTRADICTED BY EVIDENCE OF 
PRIOR, CONTEMPORANEOUS OR SUBSEQUENT ORAL AGREEMENTS OF THE PARTIES.  THERE 
ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN THE PARTIES.

If Bank agrees to the foregoing, Bank should execute this Agreement in the
space indicated below.
     
                                   BORROWER:
                              
                              ODS NETWORKS, INC.
                              (formerly known as Optical Data Systems, Inc.)
     
     
                              By:     /s/ Timothy Kinnear  
                                  ---------------------------------

                              Name:   Timothy Kinnear
                              Title:  Vice President and CFO
     
     ACCEPTED:
     
     NATIONSBANK OF TEXAS, N.A.
     
     
     By:  /s/  Frank Izzo               
         ---------------------------------
     Name:  Frank Izzo              
     Title: Senior Vice President     

<PAGE>

                         SUBSIDIARY JOINDER AGREEMENT
                                      
     This SUBSIDIARY JOINDER AGREEMENT (the "Agreement") dated as of April 
30, 1998, is executed by the undersigned (the "Debtor") for the benefit of 
NATIONSBANK OF TEXAS, N.A., (the "Bank") in connection with that certain 
Third Amended and Restated Revolving Credit Loan Agreement dated as of 
December 31, 1997, between the Bank and Optical Data Systems, Inc., now known 
as ODS Networks, Inc. (the "Borrower) (as modified from time to time, the 
"Credit Agreement", and capitalized terms not otherwise defined herein being 
used herein as defined in the Credit Agreement).

     The Debtor is a newly formed or newly acquired Granting Subsidiary and 
is required to execute this Agreement pursuant to the Credit Agreement.

     NOW THEREFORE, in consideration of the premises and other good and 
valuable consideration, the receipt and sufficiency of which are hereby 
acknowledged, the Debtor hereby agrees as follows:

     1.   The Debtor assumes all the obligations of a "Debtor" under the 
Subsidiaries Security Agreement and agrees that it is a "Debtor" and bound as 
a "Debtor" under the terms of the Subsidiaries Security Agreement as if it 
had been an original signatory thereto.  In furtherance of the foregoing, the 
Debtor hereby assigns, pledges and grants to Bank a security interest in all 
of its right, title and interest in and to Debtor's Collateral (as defined in 
the Subsidiaries Security Agreement) to secure the Obligations (as defined in 
the Subsidiaries Security Agreement) under the terms of the Subsidiaries 
Security Agreement.

     2.   Schedules 3.1 and 3.2 of the Subsidiaries Security Agreement are 
hereby amended to add the information relating to Debtor set out on Schedules 
3.1 and 3.2 hereof.  The Debtor hereby confirms that the representations and 
warranties set forth in Article 3 of the Subsidiaries Security Agreement 
applicable to it and its Collateral and the representations and warranties 
set forth in the Credit Agreement are true and correct after giving effect to 
such amendment to the Schedules.

     3.   In furtherance of its obligations under Section 4.2 of the 
Subsidiaries Security Agreement, Debtor agrees to execute and deliver such 
UCC (as defined in the Subsidiaries Security Agreement) financing statements 
naming the Debtor as debtor, the Bank as secured party and describing its 
Collateral and such other documentation as the Bank may require to evidence, 
protect and perfect the Liens created by the Subsidiaries Security Agreement 
as modified hereby.

     4.   This Agreement shall be deemed to be part of, and a modification 
to, the Subsidiaries Security Agreement and shall be governed by all the 
terms and provisions 

<PAGE>

of the Subsidiaries Security Agreement, which terms are incorporated herein 
by reference, and which is ratified and confirmed and shall continue in full 
force and effect as a valid and binding agreement of Debtor enforceable 
against Debtor.  The Debtor hereby waives notice of Bank's acceptance of this 
Agreement. 

     IN WITNESS WHEREOF, the Debtor  has executed this Agreement as of the 
day and year first written above.

                              DEBTOR:

                              ESSENTIAL COMMUNICATION CORPORATION


                              By:    /s/ Timothy W. Kinnear
                                  ----------------------------------
                              Name:  Timothy W. Kinnear    
                              Title: CFO and Secretary        


<PAGE>

                                    SCHEDULE 3.1
                                         TO
                            SUBSIDIARY JOINDER AGREEMENT

                                     LOCATIONS



                          I.  PRINCIPAL PLACE OF BUSINESS

     4374 Alexander Blvd. N.E., Suite T
     Albuquerque, NM  87107



                                II.  OTHER LOCATIONS

     None

<PAGE>

                                    SCHEDULE 3.2
                                         TO
                            SUBSIDIARY JOINDER AGREEMENT

                        TRADE AND OTHER NAMES; TAX ID NUMBER



     Trade Name:  Essential Communications Corporation

     Tax ID#:  85-0401328