Product guide

Quality of Service (QoS): Managing Bandwidth More Effectively
Introduction
3400cl/6400cl Packet Classifiers and Evaluation Order
The 3400cl/6400cl switches provide six QoS classifiers (packet criteria) you
can use to configure QoS priority.
Table 8-5. 3400cl/6400cl Classifier Search Order and Precedence
Search Order Precedence QoS Classifier
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)
A 3400cl or 6400cl switch uses the lowest-to-highest search order shown in
table 8-5 to identify the highest-precedence classifier to apply to any given
packet. (Note that this is the opposite of the order used in the 5300xl or 4200vl
switches.) 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 config-
ured 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 prece-
dence 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.
Note On the 3400cl/6400cl switches, intermixing lower-precedence classifiers con-
figured 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 prece-
dence 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.
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