HP TopTools for Hubs and Switches - Manual (Page 168)

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Managing Switches
Configuring Switch Features
■ Override “illegal” packet priorities set by upstream devices or applications
that use 802.1Q VLAN tagging with 802.1p priority tags.
■ Avoid or delay the need to add higher-cost NICs (network interface cards)
to implement prioritizing. Instead, control priority through network policy
administered by HP TopTools for Hubs & Switches.
Definitions
The following terms are used frequently with policy configuration.
■ Differentiated services bits: The upper 6 bits of the Type of Service (ToS)
field (the DS field) of an IP packet
■ Downstream device: A device linked directly or indirectly to an outbound
switch port. The switch sends traffic to downstream devices.
■ Inbound port: Any port on a switch through which traffic enters the switch
■ Outbound port: Any port on a switch through which traffic leaves the
switch
■ Outbound port queue: For any port, a buffer that holds outbound traffic
until it can leave the switch through that port. There is a “high priority”
queue and a “normal priority” queue for each port in the HP Procurve
switches supporting this feature. Traffic in a port’s high priority queue
leaves the switch before any traffic in the port’s normal priority queue.
■ Precedence bits: The upper three bits in the Type of Service (ToS) field of
an IP packet
■ Upstream device: A device linked directly or indirectly to an inbound
switch port. The switch receives traffic from upstream devices.
■ 802.1p priority: A traffic priority setting carried only in packets in 802.1Q
tagged VLANs. This setting can be from 0 to 7.
■ 802.1Q tagged VLAN: A virtual LAN (VLAN) that complies with the 802.1Q
standard and is configured as “tagged”. (For more on VLANS, see the
Management and Configuration Guide you received with your HP
Procurve switch.)
Basic Operation
Configuring a CoS policy in the switch affects switch internal traffic priorities
at the outbound port and, if 802.1Q VLANs are configured in your network,
the priority settings in traffic leaving the switch. This enables control over
traffic movement within the switch as well as control over the priority settings
in packets going to downstream devices and applications that can use those
settings.
Policy Option for Controlling Traffic Priorities at the Outbound Port.
Each port in the switch has two outbound traffic queues; “normal” priority
and “high” priority. (High-priority packets leave the switch port first. Normal-
priority packets leave the switch port after the port’s high-priority queue is
emptied.) With no CoS control, all traffic (with two exceptions) goes through