Ethernet Priority - IBM Midrange System DS4000 Series Hardware Manual

In the DS5000 storage subsystems that are connected using iSCSI, you can use an option
that enables VLAN support and provides a valid VLAN ID, as shown in Figure B-4. If it is not
possible to segregate an iSCSI storage subsystem onto a physically separate LAN, use a
VLAN to maximize potential performance.
Figure B-4 Ethernet priority and VLAN support for iSCSI port

Ethernet priority

Ethernet priority, sometimes referred to as quality of service or class of service, is a relatively
recent addition to the Ethernet specification. The 802.1 Ethernet standards working group
has defined an extension to the Media Access Control (MAC) layer that can take into account
a user-defined class of service. The 802.1p specification is a standard for traffic prioritization
where network frames are tagged with one of eight priority levels using a 3-bit value added to
the Tag Control Info (TCI) inside of a standard Ethernet frame, where 7 is high and 0 is low.
Switches and routers that are 802.1p compliant can give traffic that is time-sensitive
preferential treatment if the priority tag has been set to a higher value than other traffic.
Generically, the seven levels are defined as shown in Figure B-4. The seven levels are:
0
1
2
3
4
5
6
7
For the DS5000 storage subsystem, you can modify the Ethernet priority of the target iSCSI
interfaces to increase the class of service received within the network itself. Use Ethernet
priority on isolated networks, both LANs and VLANs, only where multiple hosts and devices
exist. Modifying this value can and will impact the performance of other devices located on
the network. Avoid using a priority setting of 7, which is the highest priority.
Appendix B. Deploying iSCSI host interface controllers on the IBM System Storage DS5000 series
Routine (default)
Priority
Immediate
Flash
Flash Override
Critical
Internetwork control
Network Control
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