Planning for Hyper-V: PowerVault™ MD3200 and MD3200i Series of Arrays
Spanning Tree Protocol
Disable the spanning-tree protocol (STP) on the switch ports that connect end nodes (iSCSI initiators
and SAN interfaces). If you still decide to enable STP on the switch ports, turn on the STP FastPort
feature on the ports to allow immediate transition of the ports into a forwarding state.
Note: FastPort immediately transitions the port into the STP Forwarding mode upon linkup.
Because the port still participates in STP and it is to be a part of the loop, it eventually
transitions into the STP Blocking mode.
Note: PowerConnect switches default to Rapid Spanning Tree Protocol (RSTP), which is an
evolution in STP that provides for faster spanning tree convergence and is preferable to
STP when available.
Note: Use STP for a single cable connection between switches or for trunking for multiple
cable connections between switches.
TCP Congestion Avoidance
TCP congestion avoidance is an end-to-end flow control protocol that limits the amount of data sent
between a TCP sender and a TCP transmitter. This protocol uses a sliding window to size the data being
sent to the TCP receiver. This protocol starts with a small segment size and keeps increasing with each
acknowledged segment sent until a segment is dropped. After a segment is dropped, TCP starts this
avoidance over again.
Ether Flow Control
It is best practice to enable flow control on the switch ports that handle iSCSI traffic. In addition, if a
server is using a software iSCSI initiator-and-NIC combination to handle iSCSI traffic, enable flow
control on the NICs to obtain a performance benefit. In SAN configurations in which many servers
(initiators) are communicating with storage devices, if senders transmit data simultaneously, the data
transmission might exceed the throughput capacity of the receiver. When this event occurs, the
receiver might drop packets, which forces the senders to retransmit the data after a delay. Although
this event does not result in any loss of data, latency increases because of the re-transmissions and I/O
performance degrades. By enabling flow control, you can avoid the imbalance in the network traffic
between the devices that send network traffic and the devices that receive the traffic.
Note: PowerConnect switches default to flow control being off. The Dell PowerVault MD3200i
storage array auto-configures to the switch when the flow control is turned on.
Unicast Storm Control
A traffic "storm" occurs when a large outpouring of IP packets creates excessive network traffic that
degrades network performance. Many switches have traffic storm control features that prevent ports
from being disrupted by broadcast, multicast, or unicast traffic storms on physical interfaces. These
features typically work by discarding network packets when the traffic on an interface reaches a
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