Why Traffic Rates Cannot Be Limited Accurately After CAR Is Configured
A switch counts lengths of the inter-frame gaps and VLAN tags when limiting the traffic rate based on CAR. It is recommended that you use packets of over 1000 bytes in CAR tests to minimize the impact of inter-frame gaps and VLAN tags.
For example, a 64-byte packet usually has a 20-byte inter-frame gap and a 4-byte VLAN tag. Therefore, the total packet length is 88 bytes (64 bytes + 20 bytes + 4 bytes = 88 bytes). During CAR rate limiting, the switch calculates the traffic rate based on the packet length of 88 bytes, so the rate limiting result is inaccurate. If the switch uses large packets, the lengths of inter-frame gap and the VLAN tag account for a small proportion of the total packet length and cause a little impact on the packet rate. Therefore, the rate limiting result is more accurate.