Dell EMC SmartFabric OS10 User Guide Release 10.5.0

Rate adjustment

QoS features such as policing and shaping do not include overhead fields such as Preamble, smart frame delimiter (SFD), inter-frame gap (IFG), and so on. For rate calculations, these feature only include the frame length between the destination MAC address (DMAC) and the CRC field.

You can optionally include the following overhead fields in rate calculations by enabling rate adjustment:

• Preamble—7 bytes
• Start frame delimiter—1 byte
• Destination MAC address—6 bytes
• Source MAC address—6 bytes
• Ethernet type/length—2 bytes
• Payload—variable
• Cyclic redundancy check—4 bytes
• Inter-frame gap—variable

The rate adjustment feature is disabled by default. To enable rate adjustment, use the qos-rate-adjust value_of_rate_adjust command. For example:

qos-rate-adjust 8

If you have configured WDRR and shaping on a particular queue, the queue can become congested. You should configure the QoS rate adjust value considering the overhead field size to avoid traffic drops on uncongested queues.

If you have multiple streams within a queue, you must find the overhead size for the different streams and the QoS rate adjust value should be the highest overhead size from among the various streams within that queue.

Consider the example where you have configured WDRR and shaping on a queue that has two different traffic streams, TS1 and TS2, that uses preamble, SFD, and IFG overhead fields:

• If the IFG in TS1 uses 16 bytes, QoS rate adjust value should be 24 (preamble + SFD requires 8 bytes and IFG 16 bytes).

• If the IFG in TS2 uses 12 bytes, QoS rate adjust value should be 20 (preamble + SFD requires 8 bytes and IFG 12 bytes).

In this case, the highest QoS rate adjust value between the two streams is 24 bytes. Hence, you must configure the QoS rate adjust value as 24.