WRR (Weighted Round Robin) vs SRR (Shared / Shaped Round Robin)

WRR (Weighted Round Robin), but in practice, SRR has been gradually more and more of the Switch platforms in use, including WRR and SRR is the biggest difference is that they both use the weights but Scheduling in different ways.
The following map was concerned (aside consideration of Strict Priority Queue), if it is to use WRR, then, Queue 3 the weight of 4, so every one can also send four packets; Queue 2 the weight of 2, so every one can simultaneously send two packets; Queue 1 weight 1, so each time can send a packet.
So Q3, Q2, Q1 out of a packet each, followed by Q3, Q2 out of a packet each, followed by Q3 the two packets, like this be considered as one.

Therefore, the benefits of SRR is very mean to each Queue interlude before sending the packet into the FIFO Queue (Hardware Queue), rather than as a clearing WRR weight for each share of Queue number of packets, so the results will let down the different sub- Queue packet scheduling in a more smooth, not crowding each other.

SRR is a scheduling service for specifying the rate at which packets are dequeued. With SRR there are two modes, Shaped and Shared (default).

Shaped mode is only available on the egress queues. Shaped egress queues reserve a set of port bandwidth and then send evenly spaced packets as per the reservation.

 Shared egress queues are also guaranteed a configured share of bandwidth, but do not reserve the bandwidth. That is, in Shared mode, if a higher priority queue is empty, instead of the servicer waiting for that reserved bandwidth to expire, the lower priority queue can take the unused bandwidth.

Neither Shaped SRR nor Shared SRR is better than the other. Shared SRR is used when one wants to get the maximum efficiency out of a queuing system, because unused queue slots can be used by queues with excess traffic. This is not possible in a standard Weighted Round Robin (WRR). Shaped SRR is used when one wants to shape a queue or set a hard limit on how much bandwidth a queue can use. When one uses Shaped SRR one can shape queues within a ports overall shaped rate.

As stated earlier, SRR differs from WRR. With WRR, queues are serviced based on the weight. Q1 is serviced for Weight 1 period of time, Q2 is served for Weight 2 period of time, and so forth. The servicing mechanism works by moving from queue to queue and services them for the weighted amount of time.

With SRR weights are still followed; however, SRR services the Q1, moves to Q2, then Q3 and Q4 in a different way. It doesn't wait at and service each queue for a weighted amount of time before moving on to the next queue . Instead, SRR makes several rapid passes at the queues, in each pass, each queue may or may not be serviced. For each given pass, the more highly weighted queues are more likely to be serviced than the lower priority queues. Over a given time, the number of packets serviced from each queue is the same for SRR and WRR. However, the ordering is different.

With SRR, traffic has a more evenly distributed ordering. With WRR one sees a bunch of packets from Q1 and then a bunch of packets from Q2, etc. With SRR one sees a weighted interleaving of packets.

SRR is an evolution of WRR that protects against overwhelming buffers with huge bursts of traffic by using a smoother round-robin mechanism.

Reference URL:
http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps5023/prod_white_paper09186a00801b096a.html