Exhibit 5.1


                  [TESTA, HURWITZ & THIBEAULT, LLP LETTERHEAD]


                                                              August 27, 1999


Excel Switching Corporation
255 Independence Drive
Hyannis, MA 02601

         Re:      REGISTRATION STATEMENT ON FORM S-3 RELATING TO
                  THE REGISTRATION OF 924,458 SHARES OF COMMON STOCK

Dear Sir or Madam:

         Reference is made to the above-captioned Registration Statement on Form
S-3 (the "Registration Statement") filed by Excel Switching Corporation (the
"Company") on the date hereof with the Securities and Exchange Commission under
the Securities Act of 1933, as amended, relating to an aggregate of 924,458
shares of Common Stock, par value $.01 per share, of the Company (the "Shares").

         We have reviewed the corporate proceedings taken by the Board of
Directors of the Company with respect to the authorization and issuance of the
Shares. We have also examined and relied upon originals or copies, certified or
otherwise authenticated to our satisfaction, of all corporate records,
documents, agreements or other instruments of the Company and have made all
investigations of law and have discussed with the Company's officers all
questions of fact that we have deemed necessary or appropriate.

         We are only members of the bar of the Commonwealth of Massachusetts and
are not expert in, and express no opinion regarding, the laws of any
jurisdiction other than the Commonwealth of Massachusetts, the General
Corporation Law of the State of Delaware and the United States of America.

         Based on the foregoing, we are of the opinion that the Shares are
validly issued, fully paid and non-assessable.

         We hereby consent to the filing of this opinion as Exhibit 5.1 to the
Registration Statement and further consent to the use of our name wherever
appearing in the Registration Statement and any amendments thereto.

                                            Very truly yours,

                                            /s/ Testa, Hurwitz & Thibeault, LLP

                                            TESTA, HURWITZ & THIBEAULT, LLP