314 Chapter 8: QoS Support on the Catalyst 6500
Congestion Management and Congestion Avoidance
After a frame has been classified, processed through all the applicable QoS policies, and a
forwarding decision has been made, the frame is then forwarded to the appropriate egress
queue. At the transmit interface, the Catalyst 6500 employs congestion management and
congestion avoidance features to ensure priority traffic has precedence when accessing the
network during periods of link oversubscription. Deeper buffers and more complex queuing
mechanisms are normally found at egress interfaces. This is because during peak traffic
periods congestion is more common at these points. Congestion within the campus LAN is
attributed to link-speed mismatches and to aggregation points within the network. Although
input scheduling ensures certain critical traffic is given priority to the switch bus or fabric,
congestion upon ingress is not a common occurrence and does not provide guarantees to
traffic exiting the switch. Because it is more common to have either a single Gigabit
Ethernet connection flowing downstream to a single Fast Ethernet connection, or 24 to 48
Fast Ethernet connections flowing upstream to a single Gigabit Ethernet connection, it is
imperative to ensure mission-critical applications and voice traffic are guaranteed access to
the network at these points, minimizing the end-to-end latency and jitter. For these
examples, the switch backplane does not serve as a bottleneck for the network traffic;
instead, the bottleneck is the outbound interface. As a result, it is more efficient and critical
to properly deploy congestion management and congestion avoidance techniques at the
egress interface. Table 8-16 summarizes some of the egress congestion management and
congestion avoidance port capabilities.
QoS is enabled globally
Microflow policing is enabled globally
(text omitted)
----- Module [1] -----
QoS global counters:
Total packets: 291876
IP shortcut packets: 0
Packets dropped by policing: 297504
IP packets with TOS changed by policing: 151
IP packets with COS changed by policing: 282786
Non-IP packets with COS changed by policing: 0
native# show mls qos ip fastEthernet 6/10
[In] Policy map is HTTP-police [Out] Default.
QoS Summary [IP]: (* - shared aggregates, Mod - switch module)
Int Mod Dir Cl-map DSCP AgId Trust FlId AgForward-Pk AgPoliced-Pk
--------------------------------------------------------------------------------
Fa6/10 1 I HTTP-traf 0 2 dscp 0 153203 297504
Example 8-34 Configuring an Aggregate Two-Rate Policer in Native Mode (Continued)
Congestion Management and Congestion Avoidance 315
Congestion management on the 6500 involves associating different CoS levels with the
available transmit queues on the egress port. It also pertains to how the various transmit
queues are scheduled and the frequency they are serviced. Figure 8-2 depicts the various
congestion management and congestion avoidance features available at the output port. The
Catalyst 6500 utilizes weighted round-robin (WRR) as an output scheduling mechanism
between the various queues. Congestion avoidance allows the switch to monitor and
manage the buffer utilization within the queue. The Catalyst 6500 implements two
congestion avoidance mechanisms, tail drop and WRED.
The purpose of this section is to cover the various congestion management and congestion
avoidance techniques available within the Catalyst 6500. Congestion management focuses
on mapping CoS values to transmit queues and thresholds, configuring output queue sched-
uling WRR weighting factors, and configuring the transmit queue size ratio. The section
concludes with congestion avoidance strategies, including configuring tail-drop thresholds
for transmit queues and configuring WRED thresholds for transmit queues. Throughout the
discussion, examples are provided demonstrating the necessary configuration steps and
behavior for Hybrid and Native modes.
Table 8-16 Congestion Management and Avoidance Mechanisms per Module
Module Transmit Ports
Supports
WRED
Priority
Queue
Number of WRR
Queues
WS-X6024 2q2t No No 2
WS-X6148 2q2t No No 2
WS-X6224 2q2t No No 2
WS-X6248 2q2t No No 2
WS-X6316 1p2q2t Yes Yes 2
WS-X6324 2q2t No No 2
WS-X6348 2q2t No No 2
WS-X6408 2q2t No No 2
WS-X6408A 1p2q2t Yes Yes 2
WS-X6416 1p2q2t Yes Yes 2
WS-X6501 1p2q1t Yes Yes 2
WS-X6502 1p2q1t Yes Yes 2
WS-X6516 1p2q2t Yes Yes 2
WS-X6524 1p3q1t Yes Yes 3
WS-X6548 1p3q1t Yes Yes 3
WS-X6816 1p2q2t Yes Yes 2
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
Congestion Management and Congestion Avoidance 317
As mentioned, the CoS values mapped to the various queues are derived from the internal
DSCP values used as the frame traverses the switch. The DSCP value, in turn, originates
from the classification policy assigned to the ingress port. As a result, it is imperative to
carefully consider any ingress QoS policies, because they ultimately impact how the frame
is processed upon egress. To modify which queue a particular CoS is mapped to and verify
those changes, use the following commands:
(Hybrid)
set qos map {
port-type
} tx {
queue#
} {
thr #
} cos {
cos-list
}
show qos info config {
port-type
} tx
(Native)
wrr-queue cos-map {
queue#
} {
thr #
} {
CoS1
[
CoS2
] [
CoS3
] [
CoS4
] [
CoS5
] [
CoS6
] [
CoS7
]
[
CoS8
]}
show queueing interface {
type num
} [ | begin queue thresh]
NOTE As you may recall from the input scheduling section, modifying the ingress CoS mapping
characteristics for 1q4t port types affects the CoS mappings for 2q2t port types. The
opposite also holds true when modifying the 2q2t port types.
4–5 2 Threshold 1
6–7 2 Threshold 2
Default Queue Assignments:
1p2q2t and 1p2q1t Port Types
0–1 1 Threshold 1
2–3 1 Threshold 2
4,6 2 Threshold 1
7 2 Threshold 2
53
Default Queue Assignments:
1p3q1t Port Types
0–1 1
2–4 2
6–7 3
54
Table 8-17 Default CoS Queue Assignments for Egress Port Types (Continued)
318 Chapter 8: QoS Support on the Catalyst 6500
In Hybrid, when modifying the ingress and egress queue mappings, the configuration
changes apply to all ports associated with the designated port type. Therefore, if you modify
the CoS assignments for 1p2q2t port types, all modules with this transmit capability inherit
the specified parameters. Examples 8-35 and 8-36 demonstrate altering the default CoS
assignments and then validating the changes made.
The preceding example represents the recommended configuration for ports without a
priority queue in a voice-deployed environment. With no strict-priority queue, frames with
CoS 5 remain mapped to the second queue and first threshold. In this instance, frames
marked with CoS 3 are mapped to the same queue and threshold as frames marked with
CoS 4 and 5. Depending on the traffic levels, this configuration affords frames marked with
CoS 5 the least amount of potential delay, jitter, and drop probability (because traffic in the
second, or high, queue is scheduled more frequently than traffic in the lower queue). This
ensures that frames with low delay tolerances are expedited, whereas traffic in the lower
queue is buffered. Also as depicted in the example, the CoS mapping parameters for 2q2t
port types are duplicated for 1q4t port types on the receive side.
Example 8-35 Mapping CoS Value to Transmit Queue and Threshold in Hybrid Mode
hybrid (enable) set qos map 2q2t tx 2 1 cos 3
QoS tx priority queue and threshold mapped to cos successfully.
hybrid (enable) show qos info config 2q2t tx
QoS setting in NVRAM for 2q2t transmit:
QoS is enabled
Queue and Threshold Mapping for 2q2t (tx):
Queue Threshold CoS
----- --------- ---------------
1 1 0 1
1 2 2
2 1 3 4 5
2 2 6 7
(text omitted)
**Note: Receive queue configuration matches transmit queue.
hybrid (enable) show qos info config 1q4t rx
QoS setting in NVRAM for 1q4t receive:
QoS is enabled
Queue and Threshold Mapping for 1q4t (rx):
Queue Threshold CoS
----- --------- ---------------
1 1 0 1
1 2 2
1 3 3 4 5
1 4 6 7
(text omitted)
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