HP PROCURVE 2610 Advanced Traffic Management Manual page 200

Quality of Service (QoS): Managing Bandwidth More Effectively
Introduction
Table 6-4. Switch Classifier Search Order and Precedence
Search Order Precedence QoS Classifier
The switches use the lowest-to-highest search order shown in table 6-4 to
identify the highest-precedence classifier to apply to any given packet. If there
is only one configured classifier that matches a given packet, then the switch
applies the QoS policy specified in that classifier. If multiple configured
classifiers match a given packet, the switch applies each one in turn to the
packet and concludes with the QoS policy for the highest-precedence
classifier. Note that if the highest precedence classifier 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
classifier 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
On switches covered in this guide, intermixing lower-precedence classifiers
configured with DSCP policies and higher-precedence classifiers configured
with 802.1p priority rules is not recommended, as this can result in a packet
with an 802.1p priority assigned by one classifier and a DSCP policy by another
classifier. This is because the search order would allow a lower precedence
classifier 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 classifier 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 classifier in use on the switch or
applied to all QoS classifiers in use on the switch.
In general, the precedence of QoS classifiers 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
precedence supersedes VLAN precedence, all TCP port 80 packets on VLAN
100 will be set to normal priority. For a classifier precedence listing, see table
6-4, ''Switch Classifier Search Order and Precedence'', on page 6-10.
6-10
1 6 (lowest) Incoming 802.1p Priority (present in tagged VLAN environments)
2 5 Incoming source-port on the switch
3 4 VLAN Priority
4 3 IP Type of Service (ToS) field (IP packets only)
5 2 Device Priority (destination or source IP address)
6 1 (highest) UDP/TCP Application Type (port)