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I found this post today regarding XenServer on HP Blades. its a straight copy and paste from About Roy found here but very very useful.
By default VC switch configured as default STP (Spanning Tree Protocol) . While uplinking virtual connect to the core switch , make sure that core switch port is configured for same STP . In our observation we found that core switch port were configured for MSPT (Multiple Spanning Tree Protocol) and link aggregation (LACP) were configured using 1GB port.
Due to STP configuration changes , convergence were talking some additional time. This results in RTO and packet drop which was causing XenApp session freeze.
Conclusion : If LACP is used to get 4GB thoughput on uplink , make sure that there are no protocol mismatch. Default protocol on VC can be changed to match core switch protocol.
Laters,
b@m