Administrator Guide

574 iSCSI Optimization
What Does iSCSI Optimization Do?
In networks containing iSCSI initiators and targets, iSCSI Optimization
helps to monitor iSCSI sessions or give iSCSI traffic preferential QoS
treatment. Dynamically-generated classifier rules generated by snooping
iSCSI traffic are used to direct iSCSI data traffic to queues that can be given
the desired preference characteristics over other data traveling through the
switch. This may help to avoid session interruptions during times of
congestion that would otherwise cause iSCSI packets to be dropped.
However, in systems where a large proportion of traffic is iSCSI, it may also
interfere with other network control-plane traffic, such as ARP or LACP.
The preferential treatment of iSCSI traffic needs to be balanced against the
needs of other critical data in the network.
What Occurs When iSCSI Optimization Is Enabled or Disabled?
When iSCSI is enabled on the switch, the following actions occur:
• Flow control is globally enabled, if it is not already enabled.
• iSCSI session snooping is enabled
• iSCSI LLDP monitoring starts to automatically detect Dell EqualLogic
arrays.
If the iSCSI feature is disabled on the switch, iSCSI resources are released
and the detection of Dell EqualLogic arrays by using LLDP is disabled.
Disabling iSCSI does not remove the MTU, flow control, portfast or storm
control configuration applied as a result of enabling iSCSI. iSCSI
Optimization is enabled by default.
How Does the Switch Detect iSCSI Traffic Flows?
The switch snoops iSCSI session establishment (target login) and
termination (target logout) packets by installing classifier rules that trap
iSCSI protocol packets to the CPU for examination. Devices that initiate
iSCSI sessions generally use well-known TCP ports 3260 or 860 to contact
targets. When iSCSI optimization is enabled, by default the switch identifies
IP packets to or from these ports as iSCSI session traffic. In addition, the
switch separately tracks connections associated with a login session (ISID)
(dynamically allocated source/destination TCP port numbers). The switch
can be configured to monitor traffic for additional port numbers or port
number-target IP address combinations, and the well-known port numbers