System information

6-9
Quality of Service (QoS): Managing Bandwidth More Effectively
Introduction
Packet Types and Evaluation Order
The switches covered by this chapter provide two 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. 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.
Note 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.
Search Order Precedence QoS type
1 2 Interface - Incoming source-port on the switch
2 1 (highest) IP Type of Service (ToS) field (IP packets only)