** Ciena CIEN.N shares up 8.3% to $293.88 as networking gear maker joins S&P 500 index .SPX on Mon

** CIEN elevated from S&P MidCap 400 .IDX to replace Dayforce after software provider's acquisition by private equity firm Thoma Bravo, per S&P Dow Jones Indices announcement late last Weds

** CIEN shares on the session surpass prior 52-week high of $285.97 hit last week and at highest levels since 2001

** With move on the session, Hanover, Maryland-based CIEN's stock up ~26% YTD and has roughly tripled over the past six months for about $41.4 bln market cap

** Stock recently traded at approx 48x forward earnings, suggesting it may be overvalued compared to its 5-yr avg of 13x, per LSEG Datastream

** Of 18 analysts covering the stock, ratings breakdown is 4 "strong buy", 7 "buy" and 7 "hold"

** Median PT of $255 unchanged from a month ago but up from $175 on Dec 9 and $138 three months ago

(Lance Tupper is a Reuters market analyst. The views expressed are his own)

((lance.tupper@tr.com 1-646-279-6380))