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                             SECOND LEASE AMENDMENT

                  This Second Lease Amendment (the "Second Amendment") is made
this 8th day of February, 1999, between TA/WESTERN, LLC ("Landlord"), and VISUAL
NETWORKS OPERATIONS, INC., a Delaware corporation ("Tenant").

                  WHEREAS, The Equitable Life Assurance Society of the United
States ("Equitable"), as landlord, and Tenant entered into a Lease Agreement
dated December 12, 1996 (the "Lease"), for premises containing approximately
23,026 rentable square feet of space (the "Initial Premises") in the office
building located at 2092 Gaither Road, Rockville, Maryland (the "Building"); and

                  WHEREAS, the Lease Agreement was amended by a Lease Amendment
(the "Amendment") dated September 2, 1997, under which Tenant expanded its
presence in the Building by leasing an additional two hundred thirty-one (231)
rentable square feet (the "Expansion Premises',) from Equitable (the Lease
Agreement and the Amendment are hereafter referred to collectively as the
"Lease"), and

                  WHEREAS, the Building was thereafter transferred and assigned
to Landlord, and

                  WHEREAS, the Lease is scheduled to expire on November 30,
2001, and

                  WHEREAS, Landlord .and Tenant wish, among other matters, to
amend the Lease to further expand the Premises leased by Landlord to Tenant in
the Building, all on the terms hereinafter contained.

                  NOW, THEREFORE, in consideration of the foregoing, and other
good and valuable consideration, the receipt and sufficiency of which are
acknowledged by the parties, the parties agree as follows;

                  1. SECOND EXPANSION PREMISES. Effective as of the Second
Expansion Premises Commencement Date, Landlord demises and leases to Tenant and
Tenant leases and accepts from Landlord, for a term and upon the conditions
hereinafter provided, an additional five thousand three hundred thirty-seven
(5,337) rentable square feet of space on the first (lst) floor of the Building
known as Suite 150 (the "Second Expansion Premises"), outlined on EXHIBIT A
attached to this Amendment. The initial Premises, the Expansion Premises and the
Second Expansion Premises will comprise approximately 28,594 rentable square
feet in the Building.

                  2. SECOND EXPANSION PREMISES TERM. The Term of the Lease for
the Second Expansion Premises shall commence on March 1, 1999 (the "Second
Expansion Premises Commencement Date), and will expire on the Lease termination
date.

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                  3. BASIC RENTAL FOR SECOND EXPANSION PREMISES. Tenant shall
pay Landlord Basic Rental for the Second Expansion Premises at the initial rate
of $19.50 per rentable square foot, in accordance with the terms of Sections 3
and 4 of the Lease, in legal tender, at Landlord's office, in care of McShea
Management, Inc., or as directed from time to time by Landlord's notice, payable
at the initial monthly sum of Eight Thousand Six Hundred Seventy-two and 63/100
Dollars ($8,672.63), in advance, promptly on the first day of each calendar
month of the Second Expansion Premises Term, without notice or demand, the same
being hereby waived, and without any setoff, deduction, or recoupment
whatsoever. The first installment of Basic Rental for the Second Expansion
Premises shall be paid upon execution of this Second Amendment; provided,
however, that Basic Rental for the second calendar month shall be prorated based
on one-three hundred sixtieth (l/360th) of the current annual Basic Rental for
each day of the first partial month, if any, this Second Amendment is in effect
for the Second Expansion Premises, and shall be due and payable as aforesaid.
The Basic Rental provided in this paragraph shall be in addition to any other
rent due to the Landlord under the Lease with respect to the Initial Premises
and the Expansion Premises.

                  4. ESCALATION IN BASIC RENTAL FOR SECOND EXPANSION PREMISES.
The Basic Rental for the Second Expansion Premises shall increase each year, at
a rate of three percent (3%) per annum, effective on the anniversary of the
Second Expansion Premises Commencement Date.

                  5. OPERATING EXPENSES FOR SECOND EXPANSION PREMISES.
Commencing on the anniversary of the Second Expansion Premises Commencement
Date, and continuing during each calendar year or portion thereof included in
the Lease Term, Tenant shall pay as additional rent Tenant's Proportionate Share
for Basic Cost for the Second Expansion Premises and Tenant's Proportionate
Share for Real Estate Taxes for the Second Expansion Premises over the Base
Year, all as more fully calculated in accordance with the provisions of Section
8 of the Lease. The Base Year for the Second Expansion Premises during the Lease
Term will be the 1999 calendar year. Tenant's Proportionate Share for Basic Cost
and Tenant's Proportionate Share for Real Estate Taxes for the Second Expansion
Premises shall be 9.94%. The additional rent paid by Tenant pursuant to this
Paragraph shall be in addition to any additional rent Tenant may be paying for
Tenant's Proportionate Share for Basic Cost and Tenant's Proportionate Share for
Real Estate Taxes for the Initial Premises and the Expansion Premises under the
Lease.

                  6. SECOND EXPANSION PREMISES IMPROVEMENT.

                  Tenant agrees to accept the Second Expansion Premises in its
"as is" condition, as of the Second Expansion Premises Commencement Date subject
to the following:

                  a. Tenant acknowledges it has inspected and is fully familiar
with the Second Expansion Premises and is taking possession of such in its "as
is" condition as of the Second Expansion Premises Commencement Date.


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                  b. Landlord shall provide Tenant a cash allowance (the "Cash
Allowance") in an amount equal to $7.00 per rentable square foot for the Second
Expansion Premises, for a total of $37,359.00. The Cash Allowance shall be used
by Tenant to pay for costs actually incurred by Tenant in renovating and
remodeling the leasehold improvements in the Second Expansion Premises, in
accordance with the plans and specifications (the "Tenant Plans"). All Tenant
Plans shall be subject to the written approval of Landlord, which approval shall
not be unreasonably withheld or delayed. Landlord shall pay Tenant the Cash
Allowance, or the portion thereof, in accordance with the provisions of this
Section. Any renovation of the Second Expansion Premises after their initial
build-out shall constitute an Alteration and shall, in addition to the
provisions of this Section, be subject to the terms and conditions of Section 10
of the Lease. Any costs in excess of the Cash Allowance shall be paid solely by
Tenant.

                  c. Provided that no Event of Default has occurred and remains
uncured, Landlord shall reimburse Tenant From the Cash Allowance for the cost of
improvements made to the Second Expansion Premises, provided that such
reimbursement shall not exceed, in the aggregate, the Cash Allowance. The Cash
Allowance shall be available to Tenant in monthly installments upon timely
submission of Tenant's statement with all required lien waivers and certificates
as provided below as construction of the improvements to the Second Expansion
Premises progresses and Tenant incurs expenses toward which the Cash Allowance
may be applied. Each statement delivered by Tenant shall show, in reasonable
detail, all costs incurred and shall be accompanied by invoices of each
contractor, subcontractor, supplier or vendor for which reimbursement is sought,
and a lien waiver and a certificate, from each contractor and subcontractor
whose contract has an aggregate value equal to or greater than $2,500,
certifying that all payments then due such contractor or subcontractor and to
laborers, materialmen and subcontractors under it have been made, except the
amounts then being requisitioned. All contract documents and requisitions
submitted by Tenant for reimbursement from the Cash Allowance relating to design
and construction shall be in the then current AIA format. Disbursement shall be
made from the Cash Allowance on or before thirty (30) days after Landlord
receives Tenant's complete and correct statements with all required supporting
documentation. In addition to the foregoing, Tenant shall deliver lien waivers
from all contractors and subcontractors providing services or material to the
Tenant within thirty (30) days of the Commencement Date.

                  d. Tenant shall make all improvements to The Second Expansion
Premises in accordance with its obligations set forth under the Lease and in
accordance with the requirements of the Standard Rules and Regulations for
Contractors attached to this Second Amendment as EXHIBIT B and incorporated
herein by reference. Tenant shall select the general contractor from a list that
previously has been approved by Landlord.

                     Once approved by Landlord, no material deviation from the
Tenant Plans which shall affect the base Building or any mechanical, electrical
or plumbing systems or equipment or which would alter the appearance of the
Building from the exterior as determined by Landlord in Landlord's reasonable
judgment, shall be made by 

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Tenant without Landlord's prior written consent. Upon completion of all
improvements, Tenant shall at no cost or expense to Landlord furnish to Landlord
a complete set of final, as-built drawings related to such work showing all
changes and deviations from the Tenant Plans. Approval of the Tenant Plans by
Landlord shall not constitute the assumption of any responsibility by Landlord
for their accuracy or sufficiency, or that they comply in any way with
applicable federal, state or local law, and Tenant shall be solely responsible
for such accuracy, sufficiency or compliance. All work performed by Tenant shall
be in accordance with: good construction practices; all applicable laws, orders,
regulations and requirements of federal, state and local authorities having
jurisdiction ("Jurisdictional Authorities"); and all insurance requirements. It
is further understood and agreed that Landlord shall have no responsibility or
liability whatsoever for any loss of, or damage to, any fixtures, installed or
left in the Second Expansion Premises. Tenant shall obtain at its sole cost and
immediately thereafter furnish to Landlord all certificates and approvals with
respect to work done and installations made pursuant to this Section that may be
required from any of the Jurisdictional Authorities for the issuance of a
certificate of occupancy for the Second Expansion Premises. Upon the issuance of
the certificate of occupancy, a copy thereof shall be immediately delivered to
Landlord.

                  e. All improvements constructed in the Second Expansion
Premises by Tenant, whether at Landlord's or Tenant's expense, or the joint
expense of Landlord and Tenant, shall become and remain the property of
Landlord. In addition to the foregoing, in the event Tenant decides to demolish
any of the existing improvements in the Second Expansion Premises, Landlord
shall have the right to recover and remove any items which Tenant intends to
demolish or remove. Notwithstanding the foregoing, however, Tenant's trade
fixtures, equipment and moveable wall systems, furniture and furnishings shall
be and remain the property of Tenant. In the event Tenant removes any of the
foregoing, Tenant agrees to repair any damage to the Second Expansion Premises
or the Building. Any replacement of any property, fixtures or improvements of
Landlord, whether made at Tenant's expense or otherwise, shall be and remain the
property of Landlord.

                  f. Tenant shall use the services of an architect selected by
Tenant for the preparation of all space planning and construction documents with
respect to the renovation of the Second Expansion Premises. Tenant's architect,
who must be fully licensed in the jurisdiction in which the Building is located
and insured in accordance with industry standards, shall be subject to the
approval of Landlord. The cost associated with the preparation of such
documents, including any services of an engineering consultant approved by
Landlord, which approval shall not be unreasonably withheld or delayed, shall be
paid by Tenant and reimbursed by Landlord, to the extent available, from the
Cash Allowance. Tenant shall be responsible for all bidding and construction of
the Tenant Work in the Second Expansion Premises. Landlord's agent, McShea
Management, Inc. shall be notified of, and shall have the right to attend, all
architectural and construction meetings, and shall have the right to enter and
inspect the Second Expansion Premises, without notice to Tenant, during the
construction and build-out.

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                  7. PARKING. Tenant is entitled to parking in Building at a
ratio of 3.5 spaces per 1,000 rentable square feet, for an additional 19
unreserved parking spaces for the Second Expansion Premises.

                  8. BROKERAGE. Tenant warrants that it has had no dealings with
any broker or agent other than McShea Management, Inc. in connection with the
negotiation or execution of this Second Amendment, and Tenant agrees to
indemnify Landlord against all costs, expenses, attorneys' fees or other
liability for commissions or other compensation or charges claimed by any other
broker or agent claiming the same by, through or under Tenant.

                  9. DEFINED TERMS. Except as otherwise expressly provided
herein. All defined terms shall have the same meanings as provided in the Lease.

                  10. HEADINGS. Headings contained in this Second Amendment are
for convenience only and are not substantive to the provisions of this Second
Amendment.

                  11. LEASE TERMS RATIFIED. Except as otherwise expressly
provided herein, and unless inconsistent with the terms hereof, all other terms,
conditions and covenants of the Lease are hereby ratified and confirmed.

                  IN WITNESS WHEREOF, the parties have executed this Second
                  Amendment by affixing their hands and seals as of the date
                  noted above.

                                            TENANT:

ATTEST:                                     VISUAL NETWORKS OPERATIONS, INC.



/s/ Sharon L. Skolaski                      By: /s/ Richard H. Deily (Seal)
    ------------------                      Name: Richard H. Deily
                                                  ------------------------------
                                            Title: Director, Treasury Operations
                                                 -------------------------------

                                            LANDLORD:
                                                  
                                            TA/WESTERN, LLC, a Delaware limited
                                            liability company
                                             
                                            By:  Realty Associates Advisors LLC,
                                                 a Delaware limited liability 
                                                 company, manager

                                            By:  Realty Associates Advisors 
                                                 Trust, a Massachusetts 
                                                 business trust, sole member

                                            By:  /s/ Henry G. Brauer 
                                                     ---------------

                                            Its:  Henry G. Brauer
                                                  -----------------
                                                  Regional Director
                                            


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