In the following post you can find details about egress Queue Buffer, Memory and Threshold Allocation on Cisco 3560-E & 3750-E. Hope this clarifies you the strange Cisco concept.

Buffer allocation is interface bandwidth allocation control how much data can be buffered and sent. Bandwidth is allocated as percentage of interface bandwidth (ratio weight of the frequency in which the SRR scheduler sends packets from each queue).

The buffer space (whole blue outline on the diagram) is divided between the Common Pool (yellow field) and the Reserved Pool (green, blue, orange, red areas are reserved pools for each queue). Cisco defined Common Pool as 400% of interface resources, so each queue is defined as 100% of interface resources. Each queue has reserved pool of interface resources, unreserved space is allocated to the Common Pool and can be used by remaining queues.

In case traffic in target queue has to consumed whole queue’s interface resources (100%) and even more over 100% then switch can allocate buffer space from the Common Pool (if it is not empty, what it means is that remaining queues don’t use all queues’ resources). If there are no free buffers in the Common Pool the switch drops the frames.

Let’s take a look on the following diawing example.

In this case Reserved Pool (Reserved Treshold) for queue 1 has been defined at the level of 80%, additionaly remaining unreserved pools from Q1, Q2, Q3 and Q4 tottaly 200% is is going to the Common Pool.

In case WTD Drop Threshold 2 that is on the level of 180% is exceeded the packets are dropped. Maximum Treshold for the Q1 is set on the level of 220% so up to this level traffic can be used from the Common Pool, but in this case it not a make sense because frames will be droped sooner due to WTD Drop Threshold 2.

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