Packet classifiers and evaluation order

The switches covered in this guide provide six types of globally-configured QoS classifiers (match criteria) to select packets for QoS traffic marking.

The switches covered in this guide provide six QoS classifiers (packet criteria) you can use to configure QoS priority.

Classifier search order and precedence

Search order

Precedence

Global QoS classifier

1

1 (highest)

UDP/TCP application type (port)

2

2

Device priority (destination or source IP address)

3

3

IP type of service (ToS): precedence and DSCP bit sets (IP packets only)

4

4

IP protocol (IP, IPX, ARP, AppleTalk, SNA, and NetBeui)

5

5

VLAN ID

6

6

Incoming source-port on the switch

Default

7 (lowest)

The incoming 802.1p priority (present in tagged VLAN environments) is preserved if no global QoS classifier with a higher precedence matches.

The switch uses the lowest-to-highest search order shown in the table to identify the highest-precedence classifier 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.

  • 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.

  • If the highest-precedence classifier is configured to apply an 802.1p priority rule, then only the 802.1 priority in the final QoS match for the packet can be 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 apply to all QoS types in use on the switch.