Egress Scheduling

Posted: May 9, 2006 by sankar in QOS

Depending on the switch model or linecard (in case of 6500), different types of queueing may be supported.

a. WRR (Weighted Round Robin) – WRR uses round robin scheduling for sending packets out of a queue. Some linecards do support strict priority queueing. If there is a strict priority queue, all WRR queueing stops and the priority queue is serviced first. When priority queue is empty, it will go back to WRR for the rest of the queues.

b. DWRR – DWRR is much like WRR, but it keeps track of how much bandwidth each queue is using. If queue 2 was allowed to transfer only 3000 bytes and a packet with 1500 bytes, 500 byte and another 1500 byte is already stored in the queue. If the port was configured for WRR, it would have sent 3500 bytes while servicing the queue. An extra 500 bytes would have been sent. DWRR tracks how many extra bytes were sent during each time it services the queue and in the next run, it subtracts that 500 bytes from queue 2. Next time, queue2 will only be able to send 2500 bytes.

c. SRR – SRR stands for Shaped Round Robin. It enables shaping per queue to reduce the rate at which packets are sent out. It has the same effect as Policing, but doesnt do packet drops unlike Policing does. It buffers packets and send them out at a reduced rate.

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Comments
  1. Schumacher says:

    The links to the pictures are not working!

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