Troubleshooting Egress Bandwidth That Exceeds the Configured Maximum Bandwidth
Problem
Description
The maximum bandwidth of a queue when measured
at the egress port exceeds the maximum bandwidth rate shaper (shaping-rate statement on QFX5200, QFX5100, EX4600, QFX3500,
QFX3600, and OCX1100 switches, and on QFabric systems, and transmit-rate
(rate | percentage percent exact statement
on QFX10000 switches) configured for the queue.
Cause
When you configure bandwidth for a queue (forwarding class) or a priority group (forwarding class set), the switch accounts for the configured bandwidth as data only. The switch does not rate-shape 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 maximum bandwidth calculations.
The measured egress bandwidth can exceed the configured maximum 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.
Solution
When you calculate the bandwidth requirements for queues on which you expect a significant amount of traffic with small packet sizes, consider the shaping rate as the maximum 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 maximum data rate (shaping rate) and the preamble and IFG.
If the maximum bandwidth measured at the egress port exceeds the amount of bandwidth that you want to allocate to the queue, reduce the shaping rate for that queue.