This mDNS query is sent by AP to permit it to auto discover SZ100 or vSZ-E (enterprise) controllers that are on the same L2 (VLAN) as the AP. This makes installation of a SZ100 or vSZ-E network in a simple single management network seamless and automatic.
It is only needed to be permitted in the L2 network where AP's and SZ are connected if auto discovery is required. mDNS is a layter 2 protocol which normally does not cross L3 boundaries.
I hope this answers your question,
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