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A quick look at WRR.

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Weighted Round Robin our method of LAN QoS on the older switching platforms (2950’s/3550’s). Replaced by Shaped (or Shared) Round Robin on our newer platforms but WRR is still a big player out there so let’s take a quick look at it. Before we take a look at any configurations lets quickly take a look at how WRR functions from an overview perspective.

So if we start from top to bottom the first thing we notice is the fact we have 4 queues to work with. Remember these are egress/outbound queues so we are working with outgoing traffic. The switch first checks the fourth queue to see if to see if there are packets in the queue and then it transmits its configured amount of packets. Once the configured amount of packets have been transmitted for that queue it moves along to the next queue. It works this way to ensure queues do not get starved and the switch is not constantly transmitting one queue all the time. After it gets done with queue one it will then reset and start back at queue 4.

So from looking at the above flowchart you can probably guess a few things that need to get configured:

  1. What belongs in each queue
  2. How much from each should be transmitted

So in order to configure our queues we go into the interface we are looking to configure (Thank you Cisco for interface range command) and from there we configure the queues as followed:

So from the above we are configuring the following:

  • Queue 1 will contain packets marked with CoS 0 & 1
  • Queue 2 will contain packets marked with CoS 2 & 3
  • Queue 3 will contain packets marked with CoS 4 & 5
  • Queue 4 will contain packets marked with CoS 6 & 7

You don’t have to break it up so evenly you can put up 8 CoS markings in a single queue. After all QoS will be different for every network.

So now that we have our queue defined we need to define how many packets from each queue the switch will transmit before working on the next queue. This to, is also done from within interface configuration mode.

When we are configuring the individual queues they start from queue 1 to queue 4 (left to right), so in this example we configured the switch to transmit:

  • 10 packets from queue 1
  • 20 packets from queue 2
  • 30 packets from queue 3
  • 50 packets from queue 4

Now we can configure queue 4 as the priority, meaning the switch will check the priority queue and transmit all packets in the priority queue after it checks any other queues. So the switch will check queue 3, transmit the packets then check queue 4 since its got priority transmit those packets, then check queue 2 transmit those packets and then go back to queue 4. We can do that with the following command:

You just have to be careful with what traffic is placed in the priority and how many packets are sent from that priority queue to avoid starving other queues or causing unnecessary latency for other traffic.

You can verify your configuration by issuing the following command sh mls qos interface queueuing

This command will show you the QoS configuration on an interface by interface basis, I only included interface fa0/1 for this example. You see’ll that the expedite queue is enabled (queue 4) and the bandwidth per queue. Right after that we will see the CoS mapping to each of the queues.

Written by Stephen J. Occhiogrosso

June 7, 2012 at 7:03 AM

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