Enterprise QoS Solution Reference Network Design Guide

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