Overview - HP ProCurve 5300xl Series Management Manual

Quality of Service (QoS): Managing Bandwidth More Effectively
Introduction
Term Use in This Document
outbound port For any port, a buffer that holds outbound traffic until it can leave the switch through that port. There
queue are four outbound queues for each port in the switch: high, medium, normal, and low. Traffic in a port's
high priority queue leaves the switch before any traffic in the port's medium priority queue, and so-on.
re-marking Assigns a new QoS policy to an outbound packet by changing the DSCP bit settings in the ToS byte.
(DSCP re-
marking)
tagged port Identifies a port as belonging to a specific VLAN and enables VLAN-tagged packets belonging to that
membership VLAN to carry an 802.1p priority setting when outbound from that port. Where a port is an untagged
member of a VLAN, outbound packets belonging to that VLAN do not carry an 802.1p priority setting.
Type-of-Service Comprised of a three-bit (high-order) precedence field and a five-bit (low-order) Type-of-Service field.
(ToS) byte Later implementations may use this byte as a six-bit (high-order) Differentiated Services field and a
two-bit (low-order) reserved field. See also "IP-precedence bits" and DSCP elsewhere in this table.
upstream A device linked directly or indirectly to an inbound switch port. That is, the switch receives traffic from
device upstream devices.
8-6

Overview

QoS settings operate on two levels:
■ Controlling the priority of outbound packets moving through the
switch: Each switch port has four outbound traffic queues; "low", "nor­
mal", "medium", and "high" priority. Packets leave the switch port on the
basis of their queue assignment and whether any higher queues are empty:
Table 8-1.Port Queue Exit Priorities
Port Queue and
802.1p Priority Values
Low (1 - 2)
Normal (0, 3)
Medium (4 - 5)
High (6 - 7)
A QoS configuration enables you to set the outbound priority queue to
which a packet is sent. (In an 802.1Q VLAN environment with VLAN-
tagged ports, if QoS is not configured on the switch, but is configured on
an upstream device, the priorities carried in the packets determine the
forwarding queues in the switch.)
Priority for Exiting
From the Port
Fourth
Third
Second
First