Enterprise QoS Solution Reference Network Design Guide

Catalyst 2970/3560/3750—QoS Considerations and Design
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Enterprise QoS Solution Reference Network Design Guide
Chapter 2 Campus QoS Design
Note The Catalyst 2970/3560/3750 also supports two configurable ingress queues (normal and expedite).
Ingress scheduling, however, is rarely—if ever—required, as it only becomes enabled if the combined
input rates from any/all switch ports exceed the switch fabric’s capacity. Such cases are extremely
difficult to achieve, even in controlled lab environments. In the extreme case where such a scenario
develops in a production environment, the default settings of the ingress queues are acceptable to
maintain VoIP quality and network availability.
The three remaining egress queues on the Catalyst 2970/3560/3750 are scheduled by a Shaped
Round-Robin (SRR) algorithm, which can be configured to operate in shaped mode or in shared mode.
In shaped mode, assigned bandwidth is limited to the defined amount; in shared mode, any unused
bandwidth is shared among other classes (as needed).
Shaped or Shared bandwidth weights can be assigned to a queue via the srr-queue bandwidth shape
and srr-queue bandwidth share interface commands. Shaped mode weights override shared mode
weights and use an inverse ratio (1/weight) to determine the shaping bandwidth for the queue.
Furthermore, if shaped weights are set to 0, then the queue is operating in shared mode. For example,
the following interface command srr-queue bandwidth shape 3 0 0 0 would shape Q1 to 1/3 of the
available bandwidth and set all other queues to operate in sharing mode.
To make the queuing structure consistent with examples provided for previously discussed platforms,
Queues 2 through 4 should be set to operate in shared mode (which is the default mode of operation on
Queues 2 through 4). The ratio of the shared weights determines the relative bandwidth allocations (the
absolute values are meaningless). The ratio of the shared weights determines the relative bandwidth
allocations (the absolute values are meaningless). Since the PQ of the Catalyst 2970/3560/3750 is Q1
(not Q4 as in the Catalyst 3550), the entire queuing model can be flipped upside down, with Q2
representing the Critical Data queue, Q3 representing the Best Effort queue, and Q4 representing the
Scavenger/Bulk queue. Therefore, shared weights of 70, 25, and 5 can be assigned to Queues 2, 3, and
4, respectively.
Additionally, the Catalyst 2970/3560/3750 supports three Weighted Tail Drop (WTD) thresholds per
queue. Two of these thresholds are configurable (explicit); the third is non-configurable (implicit), as it
is set to the queue-full state (100%). These thresholds can be defined with the mls qos queue-set output
qset-id threshold global command. The only queues that these thresholds need defining (away from
defaults) are Queues 2 and 4. In Queue 2, it is recommended to set the first threshold to 70% and the
second to 80%, which leaves the third (implicit) threshold set at 100% (the tail of the queue). In Queue
4, it is recommended to set the first threshold to 40%, leaving the default values for both the second and
third thresholds at 100%.
Once the queues and thresholds have been defined, traffic can be assigned to queues and thresholds either
by CoS values or DSCP values, using the mls qos srr-queue output cos-map queue and mls qos
srr-queue output dscp-map queue global commands, respectively. While DSCP-to-Queue/Threshold
maps override CoS-to-Queue/Threshold maps, these mappings should be as consistent as possible to
ensure predictable behavior and simplify troubleshooting.
That being said, CoS 0/DSCP 0 (Best Effort traffic) should be mapped to Queue 3 Threshold 3 (the tail
of the queue), as no other traffic is to be assigned to Queue 3.
CoS 1 (Scavenger and Bulk) should be mapped to Queue 4 Threshold 3. Scavenger traffic can then be
further contained by a DSCP-to-Queue/Threshold mapping assigning DSCP CS1 to Queue 4 Threshold
1 (previously set at 40%); Bulk Data using DSCP values AF11, AF12, or AF13 (decimal values 10, 12,
and 14, respectively) can then use the remainder of the queue. Bulk Data can use either Threshold 2 or
Threshold 3 as its WTD limit (both of which are set to 100%).
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