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Cisco Catalyst 3750 and 2970 Switches: Using Switches with a PS Series Group

enable Cisco’s PortFast option on each switch port. PortFast will reduce network interruptions that

occur when devices restart.

Note: The use of Spanning-Tree for a single-cable connection between switches or the use of

trunking for multiple-cable connections between switches is encouraged.

Spanning-Tree Protocol (IEEE 802.1d) is a link management protocol that prevents loops in an

Ethernet network by ensuring that only one active path exists between switches. Upon link up, a

switch performs a 30 to 50 second STP calculation to transition ports into forwarding or blocking

state. During this time, no user data passes through the ports. The result is that links will take

longer to come online. This can cause long failover times when recovering from a network switch

or PS Series storage array control module failover.

Rapid Spanning Tree Protocol (RSTP; IEEE 802.1w) allows a switch port to bypass the Spanning-

Tree listening and learning states and quickly enter the STP forwarding state. In order to achieve

fast convergence on a port, RSTP relies upon two variables: edge ports and link type. Switch ports

connecting end nodes (iSCSI initiators and storage array network interfaces) should be configured

as edge ports to take advantage of RSTP features. However, do not connect other hubs, switches,

concentrators, or bridges to edge ports, because this may cause infinite packet loops.

Some switches have port settings that allow the immediate transition of the port into STP

forwarding state upon link up. If you are using STP or RSTP, these settings should be enabled on

ports that connect end nodes (iSCSI initiators and storage array network interfaces). For example,

Cisco enhanced the original 802.1d STP specification with a proprietary feature called PortFast,

which immediately transitions a port into STP forwarding state upon link up. If you are using STP

or RSTP, switch ports that are connected to end nodes should be configured as edge ports using the

PortFast command. However, do not connect other hubs, switches, concentrators, or bridges to

these ports, because this may cause infinite packet loops.

See your switch documentation for information on Spanning-Tree, in addition to Error! Reference

source not found. in this document.

Flow Control

EqualLogic recommends that you 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, you must also enable Flow Control on the NICs to obtain the performance benefit.

On many networks, there can be an imbalance in the network traffic between the devices that send

network traffic and the devices that receive the traffic. This is often the case in SAN configurations

in which many servers (initiators) are communicating with storage devices. If senders transmit

data simultaneously, they may exceed the throughput capacity of the receiver. When this occurs,

the receiver may drop packets, forcing senders to retransmit the data after a delay. Although this

will not result in any loss of data, latency will increase because of the retransmissions, and I/O

performance will degrade.

Flow Control allows the receiver to instruct the sender to “throttle back" when the receiver senses

that it is being overwhelmed by packets. The receiver does this by sending "pause frames" to the

sender, which causes the sender to slow packet transmission for a short period of time. This allows

the receiver to process its backlog so it can later resume accepting input. However, the amount of

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