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The California-based firm has appointed CFO Bob Corey as acting CEO in the meantime.
Canepa's departure came just days before Extreme, which competes with firms such as Cisco and Juniper, lifted the lid on disappointing results for the first quarter of its fiscal 2010, which closed on 27 September.
The company swung to a net loss of $5.5m (£3.3m), down from a profit of $1.6m this time last year.
Sales were down 25.9% year-on-year to $66.3m, with EMEA down 32%, accounting for revenues of $28.1m.
"Our supply chain was constrained during Q1 impacting our ability to deliver product," said a candid Bob Corey. "We are disappointed with our performance and are actively improving availability from our supply chain to meet customer demand in Q2."
Last week, Extreme completed a restructuring plan through which it hopes to save over $2m per quarter.
As part of the programme, Extreme has taken the axe to 9% of its global workforce, with 70 positions cut.
It will book a one-off charge of $4.2m during Q2 relating to the process.