The packets in a queue are transmitted based on their transmission priority, transmit rate, and the available bandwidth.
By default, each queue can exceed the assigned bandwidth if additional bandwidth is available from other queues. When a forwarding class does not fully use the allocated transmission bandwidth, the remaining bandwidth can be used by other forwarding classes if they receive a larger amount of offered load than the bandwidth allocated. A queue receiving traffic within its bandwidth configuration is considered to have positive bandwidth credit, and a queue receiving traffic in excess of its bandwidth allocation is considered to have negative bandwidth credit.
A queue with positive credit does not need to use leftover bandwidth, because it can use its own allocation. For such queues, packets are transmitted based on the priority of the queue, with packets from higher-priority queues transmitting first. The transmit rate is not considered during transmission. In contrast, a queue with negative credit needs a share of the available leftover bandwidth.
On J-series Services Routers, the leftover bandwidth is allocated to queues with negative credit in proportion to the configured transmit rate of the queues within a given priority set. The queues for an interface are divided into sets based on their priority. For more information, see Scheduling Priority. If no transmit rate is configured, each queue in the set receives an equal percentage of the leftover bandwidth. However, if a transmit rate is configured, each queue in the set receives the configured percentage of the leftover bandwidth.
Table 115 shows a sample configuration of priority and transmit rate on six queues. The total available bandwidth on the interface is 100 Mbps.
Table 115: Sample Transmission Scheduling
In this example, queues are divided into three sets based on their priority: