The buffer size is the amount of time in milliseconds of port bandwidth that a queue can use to continue to transmit packets during periods of congestion, before the buffer runs out and packets begin to drop.
On EX Series switches except EX4300 switches, the default scheduler transmission rate and buffer size percentages for queues 0 through 7 are 95, 0, 0, 0, 0, 0, 0, and 5 percent, respectively. On EX4300 switches, the default scheduler transmission rate and buffer size for queues 0 through 11 are 75, 0, 0, 5, 0, 0, 0, 0, 15, 0, 0 and 5 percent, respectively, of the total available buffer.
To deduct packet drop on a specific queue, you can check which queue is getting packet bursts and dropping more packets. Depending on that, you can specify the buffer-size in the scheduler configuration.
Use a scheduler map to map the scheduler to the forwarding class that is mapped to the queue to which you want to apply the buffer size.
For example, suppose that you want to change the dedicated buffer allocation for FCoE traffic. FCoE traffic is mapped to the fcoe forwarding class, and the fcoe forwarding class is mapped to queue 3 (this is the default configuration). To use default FCoE traffic mapping, in the scheduler map configuration, map the scheduler to the fcoe forwarding class.
The buffer capacity usually depends on the type of the line card. By default the buffer capacity is 100ms for ports 1g and higher for an Ethernet port which is distributed to the queues configured on that port. Few of the newer line card also support 250ms as well.
Just to give you an instance:
Calculation for a single 10gig port with default config is 125MB 100ms for interface (125MByte)=(100/1000)*10*1000000000/8
I tried finding it but couldn't find a public facing document for it though.
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