GMB provides a method for ensuring that each of a given port's outbound traffic priority queues has a specified minimum consideration for sending traffic out on the link to another device. This can prevent a condition where applications generating lower-priority traffic in the network are frequently or continually "starved" by high volumes of higher-priority traffic. You can configure GMB per-port.
The switch services per-port outbound traffic in a descending order of priority; that is, from the highest priority to the lowest priority. By default, each port offers eight prioritized, outbound traffic queues. Tagged VLAN traffic is prioritized according to the 802.1p priority the traffic carries. Untagged VLAN traffic is assigned a priority of 0 (normal.)
Per-port outbound priority queues
| 802.1p Priority settings in tagged VLAN packets[a] | Outbound priority queue for a given port |
|---|---|
| 1 (low) | 1 |
| 2 (low) | 2 |
| 0 (normal) | 3 |
| 3 (normal) | 4 |
| 4 (medium) | 5 |
| 5 (medium) | 6 |
| 6 (high) | 7 |
| 7 (high) | 8 |
|
[a] The switch processes outbound traffic from an untagged port at the "0" (normal) priority level. |
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You can use GMB to reserve a specific percentage of each port's available outbound bandwidth for each of the eight priority queues. This means that regardless of the amount of high-priority outbound traffic on a port, you can ensure that there will always be bandwidth reserved for lower-priority traffic.
Since the switch services outbound traffic according to priority (highest to lowest), the highest-priority outbound traffic on a given port automatically receives the first priority in servicing. Thus, in most applications, it is necessary only to specify the minimum bandwidth you want to allocate to the lower priority queues. In this case, the high-priority traffic automatically receives all unassigned bandwidth without starving the lower-priority queues.
Conversely, configuring a bandwidth minimum on only the high-priority outbound queue of a port (and not providing a bandwidth minimum for the lower-priority queues) is not recommended, because it may "starve" the lower-priority queues.
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The section on Configuring Guaranteed Minimum Bandwidth (GMB) for outbound traffic assumes the ports on the switch offer eight prioritized, outbound traffic queues. This may not always be the case, however, because the switch supports aQoS queue configuration feature that allows you to reduce the number of outbound queues from eight (the default) to four queues, or two.
Changing the number of queues affects the GMB commands (interface bandwidth-min and show bandwidth output) such that they operate only on the number of queues currently configured. If the queues are reconfigured, the guaranteed minimum bandwidth per queue is automatically re-allocated according to the following percentages:
Default GMB percentage allocations per QoS queue configuration
| 802.1p priority | 8 queues (default) | 4 queues | 2 queues |
|---|---|---|---|
| 1 (lowest) | 2% | 10% | 90% |
| 2 | 3% | ||
| 0 (normal) | 30% | 70% | |
| 3 | 10% | ||
| 4 | 10% | 10% | 10% |
| 5 | 10% | ||
| 6 | 15% | 10% | |
| 7 (highest) | 20% |
For more information on queue configuration and the associated default minimum bandwidth settings, (see the Advanced Traffic Management Guide.)
Changing the number of queues causes the GMB commands (interface bandwidth-min and show bandwidth output) to operate only on the number of queues currently configured. In addition, when the qos queue-config command is executed, any previously configured bandwidth-min output settings are removed from the startup configuration.