Troubleshooting Egress Bandwidth That Exceeds the Configured Minimum Bandwidth
Problem
Description
The guaranteed minimum bandwidth of a queue (forwarding class) or a priority group (forwarding class set) when measured at the egress port exceeds the guaranteed minimum bandwidth configured for the queue (transmit-rate) or for the priority group (guaranteed-rate).
On switches that support enhanced transmission selection (ETS) hierarchical scheduling, the switch allocates guaranteed minimum bandwidth first to a priority group using the guaranteed rate setting in the traffic control profile, and then allocates priority group minimum guaranteed bandwidth to forwarding classes in the priority group using the transmit rate setting in the queue scheduler.
On switches that support direct port scheduling, there is no scheduling hierarchy. The switch allocates port bandwidth to forwarding classes directly, using the transmit rate setting in the queue scheduler.
In this topic, if you are using direct port scheduling on your switch, ignore the references to priority groups and forwarding class sets (priority groups and forwarding class sets are only used for ETS hierarchical port scheduling). For direct port scheduling, only the transmit rate queue scheduler setting can cause the issue described in this topic.
Cause
When you configure bandwidth for a queue or a priority group, the switch accounts for the configured bandwidth as data only. The switch does not include the preamble and the interframe gap (IFG) associated with frames, so the switch does not account for the bandwidth consumed by the preamble and the IFG in its minimum bandwidth calculations.
The measured egress bandwidth can exceed the configured minimum bandwidth when small packet sizes (64 or 128 bytes) are transmitted because the preamble and the IFG are a larger percentage of the total traffic. For larger packet sizes, the preamble and IFG overhead are a small portion of the total traffic, and the effect on egress bandwidth is minor.
For ETS, the sum of the queue transmit rates in a priority group should not exceed the guaranteed rate for the priority group. (You cannot guarantee a minimum bandwidth for the queues that is greater than the minimum bandwidth guaranteed for the entire set of queues.)
For port scheduling, the sum of the queue transmit rates should not exceed the port bandwidth.
Solution
When you calculate the bandwidth requirements for queues and priority groups on which you expect a significant amount of traffic with small packet sizes, consider the transmit rate and the guaranteed rate as the minimum bandwidth for the data only. Add sufficient bandwidth to your calculations to account for the preamble and IFG so that the port bandwidth is sufficient to handle the combined minimum data rate and the preamble and IFG.
If the minimum bandwidth measured at the egress port exceeds the amount of bandwidth that you want to allocate to a queue or to a priority group, reduce the transmit rate for that queue and reduce the guaranteed rate of the priority group that contains the queue.