HP ProCurve 2810-24G, Management And Configuration Manual

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6-10
Quality of Service (QoS): Managing Bandwidth More Effectively
Introduction
Packet Types and Evaluation Order
The switches covered by this chapter provide five QoS types (packet-matching
criteria) you can use to configure QoS priority.
Table 6-4. Switch Type Search Order and Precedence
The switches use the lowest-to-highest search order shown in table 6-4 to
identify the highest-precedence type to apply to any given packet. (Note that
this is the opposite of the order used in the 5300xl switches.) If there is only
one configured type that matches a given packet, then the switch applies the
QoS policy specified in that type. If multiple configured types match a given
packet, the switch applies each one in turn to the packet and concludes with
the QoS policy for the highest-precedence type. Note that if the highest
precedence type is configured to apply a DSCP policy, then both the DSCP in
the packet and the 802.1p priority applied to the packet can be changed.
However, if the highest precedence type is configured to apply an 802.1p
priority rule, only the 802.1p priority in the final QoS match for the packet is
changed.
N o t e
Intermixing lower-precedence types configured with DSCP policies and
higher-precedence types configured with 802.1p priority rules is not
recommended, as this can result in a packet with an 802.1p priority assigned
by one type and a DSCP policy by another type. This is because the search
order would allow a lower precedence type configured with a DSCP policy to
change both the DSCP and the 802.1p setting in a packet, and then would allow
a subsequent, higher precedence type configured with an 802.1p priority rule
to change only the 802.1p setting.
To avoid this problem, a DSCP policy option
should be applied only on the highest-precedence type in use on the switch
or applied to all QoS types in use on the switch
.
In general, the precedence of QoS types should be considered when
configuring QoS policies. For example, suppose that a system administrator
has used an 802.1p priority rule to assign a high priority for packets received
on VLAN 100, but has also used another 802.1p priority rule to assign a normal
priority for TCP port 80 packets received on the switch. Since TCP/UDP port
Search Order Precedence QoS type
1 5 Interface - Incoming source-port on the switch
2 4 VLAN Priority
3 3 IP Type of Service (ToS) field (IP packets only)
4 2 Device Priority (destination or source IP address)
5 1 (highest) UDP/TCP Application Type (port)