31-13
Catalyst 6500 Series Switch Cisco IOS Software Configuration Guide—Release 12.1 E
78-14099-04
Chapter 31 Configuring PFC QoS
Understanding How PFC QoS Works
Marking at Untrusted Ingress LAN Ports
PFC QoS marks all frames received through untrusted ingress LAN ports with the ingress port CoS value
(the default is zero). PFC QoS does not implement ingress port congestion avoidance on untrusted
ingress LAN ports.
Note • To use the ingress port CoS value applied to untrusted traffic as the basis of egress DSCP, configure
a trust-CoS policy map that matches the ingress traffic.
• The ingress port CoS value is configurable for each ingress LAN port (see the “Configuring the
Ingress LAN Port CoS Value” section on page 31-54).
Marking at Trusted Ingress LAN Ports
When an ISL frame enters the Catalyst 6500 series switch through a trusted ingress LAN port, PFC QoS
accepts the three least significant bits in the User field as a CoS value. When an 802.1Q frame enters the
switch through a trusted ingress LAN port, PFC QoS accepts the User Priority bits as a CoS value. PFC
QoS marks all traffic received in untagged frames with the ingress port CoS value.
Note • PFC QoS uses the received CoS value in trusted tagged traffic as the basis of egress DSCP, unless
there is a policy map that changes the trust state of the traffic.
• PFC QoS uses the ingress port CoS value applied to trusted untagged traffic as the basis of egress
DSCP, unless there is a policy map that changes the trust state of the traffic.
• The ingress port CoS value is configurable for each ingress LAN port (see the “Configuring the
Ingress LAN Port CoS Value” section on page 31-54).
Ingress LAN Port Scheduling and Congestion Avoidance
On ingress LAN ports configured to trust CoS, PFC QoS uses Layer 2 CoS-value based receive-queue
drop thresholds to avoid congestion (see the “Configuring the Trust State of Ethernet LAN and OSM
Ingress Ports” section on page 31-53).

Receive Queues

Enter the show queueing interface {ethernet | fastethernet | gigabitethernet | tengigabitethernet}
slot/port | include type command to see the queue structure of a LAN port.
• 1q2t indicates one standard queue with one configurable tail-drop threshold and one
nonconfigurable tail-drop threshold.
• 1q4t indicates one standard queue with four configurable tail-drop thresholds (usable only on
Gigabit Ethernet ports).
• 1p1q4t indicates one strict-priority queue and one standard queue with four configurable tail-drop
thresholds.