WRR confusion

[Colin Higgins]

I am looking through the Cisco documentation on WRR configuration for 6500 series switches, and I have run across some really confusing stuff. Any help here would be great.

Question 1

If we look at this interface configuration

interface GigabitEthernet1/1

no ip address

wrr-queue bandwidth 50 75

wrr-queue queue-limit 100 50

wrr-queue cos-map 1 1 0 2

wrr-queue cos-map 1 2 3

wrr-queue cos-map 2 1 4

wrr-queue cos-map 2 2 6

priority-queue cos-map 1 1 5 7

switchport

It looks like the bandwidth assigned to queue 1 is 50 and queue 2 is 75

But I see on another 6500

wrr-queue bandwidth 20 100 200

What exactly does this mean? 3 queues? Is it bandwidth percentage? How can it be 200? I'm not sure what "weight" refers to here.

Question 2

Looking at the first example, I see that egress cos values 0 through 2 are assigned to queue 1 threshold 1 (

wrr-queue cos-map 1 1 0 2)

But the statement

priority-queue cos-map 1 1 5 7

seems to mean that egress cos values 5 through 7 are ALSO assigned to queue 1 threshold 1, which in this example is the "low priority" queue (low meaning better or worse?)

HELP! lol

[smehrnia · Accepted solution]

Hey,

Question #1:

with wrr-queue bandwidth; values are WRR weights; valid values are from 1 to 255. not percentages in a scale of 0 - 100.

depends on you Supervisor Engine, in some cases you could choose up to seven queue's.

Question #2:

first of all, wrr-queue cos-map 1 1 0 2 doesnt read CoS 0 through 2. it is Queue 1, Threshold 1, CoS 0 And CoS 2.

the priority-queue cos-map 1 1 5 7 is to set the "expedite queue" which is a strict priority queue that is serviced until it is empty, before using one of the WRR queues u set earlier. the priority-queue always uses the value of 1 for its use (aint the Queue 1). again with your input you chose CoS 1, CoS 5 and CoS 7 to be in the expedite priority queue.

plz Rate if helped.

Soroush.

[Colin Higgins]

OK, it is starting make sense, but ...

the values 1 - 255

How would those correspond to a standard 100mbs interface? I am assuming this doesn't really have anything to do with bandwidth, even though the "bandwidth" is in the statement. Do the weight values mean that the LOWER the value (1-255), the more attention the switch gives to the queue?

As far as the priority-queue cos-map 1 1 5 7 statement goes: this is a "different" queue 1? In other words, one that is reserved for the priority queue?

[Ivan Shirshin]

Hi Colin,

The switch transmits frames from one standard  queue at a time using one of these dequeuing algorithms, which use  weights to allocate relative bandwidth to each queue as  it is serviced in a round-robin fashion.

Lets say you have 2 wrr queues and the configuration is:

  wrr-queue bandwidth 50 75

Then queue 1 will use  40%=50/(50+75)*100%. of the available bandwidth after the  priority queue is serviced. Queue 2 will use 60%.

So the higher the weight that is assigned to a queue, the more transmit bandwidth is allocated to it.

Note that strict prority queue has absolute priority over all of the other  queues. Whenever there is a packet in the SP queue, the scheduler will  service that queue, which ensures the highest possibility of  transmitting the packet and the lowest possible latency in transmission  even in periods of congestion. The strict priority queue is ideal for  voice traffic because voice traffic requires the highest priority and  lowest latency on a network, and it also is a relatively low-bandwidth  traffic type, which means that voice traffic is not likely to consume  all available bandwidth on a port.

You can check "Configuring Qos on 6500" guide for more details and examples.

http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SXF/native/configuration/guide/qos.html

Kind Regards,

Ivan

**Please grade this post if you find it useful.

[Ivan Shirshin]

HI Colin,

As for strict priority configuration, you are right  - this is a  "different" queue 1. It is "strict priority queue 1", and number 1 is unrelated to other queues, has no relation to "standard queue 1". Hope my explanation is clear.

Kind Regards,

Ivan

**Please grade this post if you find it useful.

[Colin Higgins]

OK, then ...

wrr-queue bandwidth 20 100 200

would mean queue 1 gets a little over 6% of available bandwidth 20%=20/(20+100+200)

in a case where we have the 3 queues. Am I reading that correctly?

and

priority-queue cos-map 1 1 5 7

means dscp values 5 AND 7, not 5 through 7

yes?

[Ivan Shirshin · Accepted solution]

Hi Colin,

You are right, queue 1 will have 6.25% in such case.

But remember that WRR provides bandwidth sharing at the egress port. The WRR weights are used to partition the bandwidth between the queues if all queues are nonempty. So if queue 2 and 3 are empty, the bandwidth of queue 1 will be 100%. When queue has traffic in it, queue 1 bandwidth will be correspondingly reduced.

You are correct about priority-queue command as well. It is "AND" between the values and you have to list all values explicitly.

Kind Regards,

Ivan

**Please grade this post if you find it useful.

[Colin Higgins]

I got it now: thanks Ivan!

[Ivan Shirshin]

Welcome, Colin!

[smehrnia]

Hi,

Seriously, u didnt find my post helpful at all that you didnt rate it?  hehehe..! but seriously... not helpful??!!!

come on!

[Colin Higgins]

soroushim: it was helpful for sure

not sure if there is a way to rate multiple posts here (in fact, I didn't even get the option to assign stars--may be because I am on Safari with a Mac)