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                                                                  EXHIBIT 10.5.1

           SECOND AMENDMENT TO NATIONAL RESELLER/INTEGRATOR AGREEMENT

       This second Amendment to the National Reseller Agreement is made and
entered into this 4th day of November, 1998 (the "Effective Date") by and
between MCI Telecommunications Corporation, A Delaware Corporation, having its
principle place of business at Six Concourse Parkway, Atlanta GA 30328
(hereinafter called "MCI") and Visual Networks, Inc., having its principle place
of business at 2092 Gaither Road, Rockville, MD 20850.

       WHEREAS, MCI and Visual Networks, Inc. previously entered into a
nationwide Reseller/Integrator Agreement dated August 29, 1997 (the
"Agreement"); and

       WHEREAS, MCI and Visual Networks, Inc. now desire to amend the Agreement
as hereinafter more particularly set forth;

       NOW THEREFORE, in consideration of the foregoing premises, and of mutual
covenants and conditions hereinafter set forth, the parties hereto agree as
follows:

1.     As of the effective date, the following definitions will be added:

       Visual UpTime as 1.22 - The system developed by Visual Networks Inc. that
       delivers service level management functions to the system provider and
       the end-user of system providers' data communication services. The system
       consists of hardware that is deployed on the customer's premise, a server
       which communicates to the hardware components, and a software client
       application which provides the user interface to the system.

       PAC as 1.23 - The Performance Applicable Client (PAC) is the name of the
       client software module of the Visual UpTime system.

2.     As of the effective date, Section 3 is amended to add the following:

       "3.7     MCI intends to incorporate Visual Uptime into one or more MCI
                Service offerings (e.g.: Service Level Agreement (SLA)
                validation products, customer network management reporting
                products, and Smart Circuits products). Systems Providers DSU
                and CSU/DSUs are to be required whenever a customer purchases
                one of these service offerings. Further, the Systems Providers
                system components are to be deployed within the network
                operations environment such that MCI will derive operational
                benefit from the ASEs that are deployed".

       "3.8     MCI intends to productize one or more of these service
                offerings in the *** beginning *** with an expectation to 
                terminate greater than *** of new MCI frame relay ports per 
                month with Systems



       ***Confidential information has been omitted and filed separately with 
          the Securities and Exchange Commission.
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                Provider ASE's. The *** per month, or better, run rate will be
                achieved within ***.

3.   As of the effective date, Section 17.1 is replaced with the following:

                (a)  Systems Provider grants to MCI a non-transferable and
                     non-exclusive license (the "License") to use the Software,
                     created and supplied by Systems Provider, internally as a
                     means to provide a managed service to MCI's customers;
                     including such Software supplied to MCI on disks,
                     diskettes, on-line, and/or as part of the equipment
                     supplied by Systems Provider; any modifications,
                     enhancements to and/or replacements of such Software; and
                     any documentation provided in connection therewith. The
                     License imposes certain restrictions on the use of the
                     Software. If the Software is an upgrade, MCI may now use
                     that upgraded product only in accordance with this License.

                (b)  MCI may install copies of the Performance Archive Manager
                     ("PAM") and the Platform Applicable Client ("PAC")
                     Software on multiple platforms for its internal use in
                     providing managed service to its clients. MCI may
                     distribute to its customers copies of the Software,
                     provided the Software is only to be used in conjunction
                     with Analysis Service Elements ("ASE's") purchased from
                     Systems Provider under the Agreement. MCI may not
                     distribute copies of the Software for use in conjunction
                     with ASE units that have not been purchased by MCI under
                     this Agreement without the payment of an additional fee
                     to Systems Provider.

                (c)  Systems Provider retains ownership of the Software and no
                     rights are granted to MCI other than a license to use the
                     Software on the terms expressly set forth in the License.

                (d)  With respect to Software that is distributed to end
                     users, Systems Provider grants directly to the end user a
                     nonexclusive and non-transferable license, as set forth
                     in the End User Software License Agreement which
                     accompanies the Software and an example of which is
                     attached hereto as Exhibit F. If MCI is a subcontractor
                     to a prime contractor for an Integrated Business Solution
                     or to a prime contractor for the purpose of distributing
                     to an end user, Systems Provider grants directly to the
                     end user a nonexclusive and non-transferable license.

                (e)  Without the express written consent of Systems Provider,
                     MCI may not, except as set forth above:


                ***Confidential information has been omitted and filed 
                   separately with the Securities and Exchange Commission.
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                (1)     use, copy, modify, distribute, reverse engineer,
                               reverse assemble, or reverse compile the Software
                               or accompanying documentation, or

                (2)     transfer, assign, sublicense or otherwise convey
                               MCI's rights to use the Software or the
                               accompanying documentation except upon a transfer
                               of any equipment supplied by Systems Provider
                               with which or for which it was supplied, and then
                               only if Systems Provider receives written
                               agreement of the transferee to be bound by all of
                               the terms of this License.

4.       As of the effective date, Exhibit B is replaced with a new and updated 
         Exhibit B(v2):

5.       In the event that MCI is no longer offering and deploying Visual
         products as listed in Section 3.7 above, then Visual Networks has the
         option to adjust the applicable product discounts to the level they
         were prior to this Agreement.

ALL OTHER TERMS AND CONDITIONS WILL REMAIN THE SAME.

         VISUAL NETWORKS, INC.              MCI TELECOMMUNICATIONS
                                            CORPORATION

         By:/s/Peter J. Minihane            /s/ Eddie Lewis
            --------------------            -----------------------
         Name:                              Name

         Exec VP/CFO/Tres                   Sr. Contract Administrator II
         -----------------------            -----------------------------
         Title                              Title

         Nov. 2, 1998                       November 4, 1998
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         Date                               Date