316 Chapter 8: QoS Support on the Catalyst 6500
Congestion Management
As was the case with the input queues and scheduling, output scheduling and congestion
management is accomplished using individual port ASICs. Also similar to input sched-
uling, the Catalyst 6500 utilizes CoS values to determine which transmit queue a departing
frame is assigned. As demonstrated in the mapping section, the egress CoS value is derived
from the internal DSCP value. In addition, at the same time the DSCP is mapped to the CoS, the
ToS field in the IP header is rewritten with the internal DSCP value. As a result, upon egress the
QoS setting is sustained in the Layer 2 trunk header and the Layer 3 IP packet header.
Refer back to Table 8-2, which depicts the transmit queue capabilities of the different
linecards available on the Catalyst 6500. Many of the earlier 10/100 and Gigabit modules
have two transmit queues, with two configurable thresholds assigned to each queue. This
configuration is denoted as 2q2t. More recent Gigabit linecards incorporate an additional
strict-priority queue. This queue preemptively services all frames marked with CoS 5 by
default. As long as the priority queue is void of packets, the lower queues are serviced.
Therefore, by default all voice traffic is sent to the priority queue and given preference over
traffic in the other queues. The addition of the priority queue, in this instance, changes the
transmit port type to 1p2q2t. For newer 10/100 and 100-Mb modules, the queue structure
differs slightly. These cards have four queues, one strict-priority queue and three normal
queues. In this instance, however, each queue only utilizes one WRED threshold. This port
type is represented by 1p3q1t. Finally, the 10 Gigabit linecards offer yet another transmit
port type. Specified as 1p2q1t, these port types support strict-priority queuing and two
normal queues, each with one WRED threshold.
Mapping CoS Values to Transmit Queues and Thresholds
After the linecard queuing structure has been determined, it is possible to modify how the
CoS values are mapped to the various queues. Table 8-17 displays the default CoS distri-
bution across the various transmit queues.
NOTE To verify the queuing capabilities of a specific port or interface, issue the show port
capabilities {mod [/port]} in Hybrid or show interface capabilities [module {mod#}] in
Native mode.
Table 8-17 Default CoS Queue Assignments for Egress Port Types
CoS Values Transmit Queue
Default Queue Assignments:
2q2t Port Types
0–1 1 Threshold 1
2–3 1 Threshold 2