When Should iSCSI Optimization Be Enabled?
Use this feature in networks containing iSCSI initiators and targets where you
want to protect this traffic from interruption by giving it preferential QoS
treatment. The dynamically-generated classifier rules are used to direct the
iSCSI data traffic to queues that can be given the desired preference
characteristics over other data traveling through the switch. This can avoid
session interruptions during times of congestion that would otherwise cause
iSCSI packets to be dropped.
The preferential treatment of iSCSI traffic needs to be balanced against the
needs of other critical data in the network, such as Voice over IP (VoIP)
How Does the Switch Detect iSCSI Traffic Flows?
The switch detects iSCSI session establishment and termination 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. You can configure the switch to monitor traffic for
additional port numbers or port number-target IP address combinations, and
you can remove the well-known port numbers from monitoring. You can also
associate a target name with a configured target TCP port entry.
How Is Quality of Service Applied to iSCSI Traffic Flows?
The iSCSI CoS mode is configurable and controls whether CoS queue
assignment and/or packet marking is performed on iSCSI traffic. When the
iSCSI CoS mode is enabled, the CoS policy is applied to packets in detected
iSCSI sessions. When the iSCSI CoS mode is disabled, iSCSI sessions and
connections are detected and shown in the status tables, but no CoS policy is
applied to packets.
You can configure whether the iSCSI optimization feature uses the VLAN
priority or IP DSCP mapping to determine the traffic class queue. By default,
iSCSI flows are assigned to the highest VLAN priority tag or DSCP value
mapped to the highest queue not used for stack management or voice VLAN.
Use the classofservice dot1p-mapping command or the Quality of Service →
Configuring iSCSI Optimization