Reference Guide

Application of Quality of Service to iSCSI Traffic Flows
You can configure iSCSI CoS mode. This mode controls whether CoS (dot1p priority) queue assignment and/or packet
marking is performed on iSCSI traffic.
When you enable iSCSI CoS mode, the CoS policy is applied to iSCSI traffic. When you disable iSCSI CoS mode, iSCSI
sessions and connections are still detected and displayed in the status tables, but no CoS policy is applied to iSCSI
traffic.
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 dot1p priority 4. To map incoming iSCSI traffic on an interface
to a dot1p priority-queue other than 4, use the CoS dot1p-priority command (refer to QoS dot1p Traffic
Classification and Queue Assignment). Dell Networking recommends setting the CoS dot1p priority-queue to 0 (zero).
You can configure whether iSCSI frames are re-marked to contain the configured VLAN priority tag or IP DSCP when
forwarded through the switch.
NOTE: On a switch in which a large proportion of traffic is iSCSI, CoS queue assignments may interfere with other
network control-plane traffic, such as ARP or LACP. Balance preferential treatment of iSCSI traffic against the
needs of other critical data in the network.
Information Monitored in iSCSI Traffic Flows
iSCSI optimization examines the following data in packets and uses the data to track the session and create the
classifier entries that enable QoS treatment.
• Initiator’s IP Address
• Target’s IP Address
• ISID (Initiator defined session identifier)
• Initiator’s IQN (iSCSI qualified name)
• Target’s IQN
• Initiator’s TCP Port
• Target’s TCP Port
• Connection ID
• Aging
• Up Time
If no iSCSI traffic is detected for a session during a user-configurable aging period, the session data is cleared.
If more than 256 simultaneous sessions are logged continuously, the following message displays indicating the queue
rate limit has been reached:
%STKUNIT2-M:CP %iSCSI-5-ISCSI_OPT_MAX_SESS_EXCEEDED: New iSCSI Session Ignored:
ISID -
400001370000 InitiatorName - iqn.1991-05.com.microsoft:dt-brcd-cna-2 TargetName
-
iqn.2001-05.com.equallogic:4-52aed6-b90d9446c-162466364804fa49-wj-v1 TSIH - 0"
NOTE: If you are using EqualLogic or Compellent storage arrays, more than 256 simultaneous iSCSI sessions are
possible. However, iSCSI session monitoring is not capable of monitoring more than 256 simultaneous iSCSI
sessions. If this number is exceeded, sessions may display as unknown in session monitoring output. Dell
Networking recommends that you disable iSCSI session monitoring for EqualLogic and Compellent storage arrays
or for installations with more than 256 simultaneous iSCSI sessions.
Only sessions the switch observes are learned; sessions flowing through an adjacent switch are not learned. Session
monitoring learns sessions that actually flow through the switch, it does not learn all sessions in the entire topology.
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