On the 7210 SAS-Dxp, two slopes can be used per queue: a high-priority RED slope and a low-priority RED slope. The slope policy is only used for TCP traffic. Non-TCP traffic is always tail-dropped if the queues are full.
The high-priority RED slope manages access to the shared portion of the buffer pool for high-priority or in-profile TCP packets. The low-priority RED slope manages access to the shared portion of the buffer pool for low-priority or out-of-profile TCP packets. By default, the high-priority and low-priority RED slopes are disabled.
When queue depth exceeds the queue CBS, packets received on that queue must contend with other queues exceeding their CBS for shared buffers. To resolve this contention, two RED slopes are used to determine buffer availability on a packet-by-packet basis. A TCP packet that is classified as high-priority or considered in-profile is handled by the high-priority RED slope. This slope should be configured with RED parameters that prioritize buffer availability over packets associated with the low-priority RED slope. Packets that are classified as low-priority or out-of-profile are handled by the low-priority RED slope. Non-TCP packets are tail-dropped if the queue is full.