Known Issue: XS 6.5 has known issue with bnx2 drivers.
http://support.citrix.com/servlet/KbServlet/download/38334-102-714582/XenServer-6.5.0-releasenotes.pdf
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Networking • Jumbo frames cannot be enabled on QLogic 10 Gigabit Ethernet adapters (10GbE) as the QLogic driver uses contiguous network buffers. Contiguous network buffers cause fragmentation of dom0 memory and leads to memory allocation failures and performance issues in dom0.
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Jumbo Frames was enabled for bnx2 driver 10G Qlogic , which is not supported.