Sheppard replaced Lee Shorten at the helm in August last year, and vowed to take a "back to basics" approach to the firm's dealings with the networking channel.
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His immediate brief was to help Avaya's UK operation get a better line of sight into its internal development and priorities, as well as understanding where it could take steps to improve.
Avaya gave no clue as to the circumstances surrounding Shepperd's sudden departure, although in a statement it hinted the parting was an amicable one.
The firm praised Shepperd for "driving and progressing positive changes within the UK and Ireland sales organisation.... We wish him well in his future career."
Shepperd is the third major Avaya exec to take his leave so far in 2012, following the departure of Jeremy Butt for a new role in distribution at Westcon, and UK channel director Niall Anderson
who left in January to work at Avaya partner G3.
EMEA veep Mario DiMascio will pitch in to run the UK while a full-time replacement is sought.