Exhibit 5.1



                                    May 1, 1998



Excel Switching Corporation
255 Independence Drive
Hyannis, Massachusetts  02601

     Re:  Registration Statement on Form S-8 Relating to each of the Stock
          Option Program, the 1997 Stock Option Plan, the 1997 Non-Employee
          Director Stock Option Plan and the 1997 Employee Stock Purchase Plan
          of Excel Switching Corporation  (all of the foregoing are
          hereinafter referred to collectively as the "Plans")

Dear Sir or Madam:

          Reference is made to the above-captioned Registration Statement on
Form S-8 (the "Registration Statement") to be filed by Excel Switching
Corporation (the "Company") with the Securities and Exchange Commission under
the Securities Act of 1933 relating to an aggregate of 14,139,480 shares of
Common Stock, par value $.01 per share, of the Company issuable pursuant to the
Plans (the "Shares").

          We have examined, and are familiar with, and have relied as to factual
matters solely upon, copies of the Plans, the Restated Articles of Organization
and the Restated By-Laws of the Company, the minute books and stock records of
the Company and originals of such other documents, certificates and proceedings
as we have deemed necessary for the purpose of rendering this opinion.

          Based on the foregoing, we are of the opinion that the Shares have
been duly authorized and, when issued and paid for in accordance with the terms
of the related Plan and the terms of any agreement relating to any of the
options granted thereunder, will be validly issued, fully paid and
nonassessable.

          We hereby consent to the filing of this opinion as Exhibit 5.1 to the
Registration Statement.

                                    Very truly yours,
 
                                    /s/ Testa, Hurwitz & Thibeault, LLP

                                    TESTA, HURWITZ & THIBEAULT, LLP