Enterprise QoS Solution Reference Network Design Guide
Enterprise QoS Solution Reference Network Design Guide
Enterprise QoS Solution Reference Network Design Guide
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Chapter 2 Campus <strong>QoS</strong> <strong>Design</strong><br />
Version 3.3<br />
Catalyst 4500 Supervisor II+/III/IV/V—<strong>QoS</strong> Considerations and <strong>Design</strong><br />
globally with the qos dbl global command, as well as on a per-class basis within a policy-map with the<br />
dbl policy command. A default DBL policy can be applied to all transmit queues, as is shown in the<br />
example below.<br />
By default, all queues are scheduled in a round robin manner. The third transmit queue can be designated<br />
as an optional strict-priority queue. This can be enabled with the tx-queue 3 interface command<br />
followed by the priority high interface transmit-queue sub-command. This queue can be defined to be<br />
shaped to a peak limit, such as 30%, to allow bandwidth to be available to non-voice applications. This<br />
would be valuable in the event that a trust boundary has been compromised and a DoS/worm attack is<br />
saturating voice queues.<br />
Bandwidth allocations can also be assigned to queues (for certain interfaces) using the tx-queue<br />
interface command followed by the bandwidth sub-command. Bandwidth allocations to queues can<br />
only be assigned on the following interface types:<br />
Uplink ports on supervisor engines<br />
Ports on the WS-X4306-GB linecard<br />
The 2 1000BASE-X ports on the WS-X4232-GB-RJ linecard<br />
The first 2 ports on the WS-X4418-GB linecard<br />
The two 1000BASE-X ports on the WS-X4412-2GB-TX linecard<br />
The Catalyst 4500 does not support CoS-to-Queue mappings, only DSCP-to-Queue mappings. These can<br />
be defined with the qos map dscp to tx-queue global command.<br />
Given these features and the objective to make queuing consistent across platforms, it is recommended<br />
to enable DBL globally on the Catalyst 4500, as well as enable Q3 as the strict-priority queue on all<br />
interfaces (such that the switch operates in 1P3Q1T mode). This queue can be shaped to 30% of the link’s<br />
capacity. Furthermore, Q1 can then be used as the Scavenger/Bulk queue, Q2 as the Best-Effort queu,e<br />
and Q4 as the preferential queue.<br />
On interfaces that support bandwidth allocation, 5% could be assigned to Q1, 25% to Q2, and 40% to<br />
Q3. Unlike bandwidth-weights that are used on other platforms, these bandwidth allocations are defined<br />
in absolute bps or as relative percentages of the link’s bandwidth. In either case, they should not total in<br />
excess of the link’s bandwidth-limit (1 Gbps or 100%), including the priority-bandwidth allocation for<br />
Q3.<br />
By default, the DSCP-to-Queue assignments are as follows:<br />
DSCP 0-15 Queue 1<br />
DSCP 16-31 Queue 2<br />
DSCP 32-47 Queue 3<br />
DSCP 48-63 Queue 4<br />
The recommended DSCP-to-Queue assignments for the Catalyst 4500 are as follows:<br />
DSCP 0 should be assigned to Q2<br />
DSCP CS1 (Scavenger) and DSCP AF11/AF12/AF13 (Bulk Data) should be assigned to Q1<br />
DSCP CS2 (<strong>Network</strong> Management) as well as AF21/AF22/AF23 (Transactional Data) should be<br />
assigned to Q4<br />
DSCP CS3 and AF31 (Call-Signaling) should be assigned to Q4<br />
DSCP 25 (temporary marking for Mission-Critical Data) should be assigned to Q4<br />
DSCP CS4 (Streaming Video) and AF41/AF42/AF43 (Interactive Video) should be assigned to Q4<br />
DSCP EF (Voice) should be assigned to Q3 (the strict priority queue)<br />
<strong>Enterprise</strong> <strong>QoS</strong> <strong>Solution</strong> <strong>Reference</strong> <strong>Network</strong> <strong>Design</strong> <strong>Guide</strong><br />
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