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New Member

3750 QoS

Hello,

20M WAN Link is connected to Catalyst 3750(100M I/F).We have voice traffic and important application.

(Q1) When we use 'srr-queue bandwidth limit 20', does the 3750 work like shaping? Is the input policing necessary?

(Q2) We want to use 3750 QoS like LLQ and CBWFQ. We make all 4 Egress queues as shape queues and 'Priority queue out' is enabled. (Voice is set to COS7/dscp56,application is set to COS4/dscp32. We changed mls queue maps.)

But QoS does not work fine.

When we make only voice traffic ( 30M),it looks like good (all test pings are OK).

When we make voice and application and other traffic (3M/3M/3M),it does not work fine.Please advice me.

Best Regards,

Aya Yoshihara

6 REPLIES
Purple

Re: 3750 QoS

Hi,

Q1. The srr-queue bandwidth limit performs output rate-limiting (policing) to restric traffic to 20% of the interface bandwidth, in your case. It is not shaped.

Q2. When you make the first queue an expedite queue, it will not be shaped. It will simply be serviced to exhaustion. I would also change the rest of the queues to operate in shared more, not shaped. Pls give that a go and see how you do.

Hope that helps - pls rate the post if it does.

Paresh

New Member

Re: 3750 QoS

Thank you for your response.We have questions.

Q1. When we use the srr-queue bandwidth limit, will the voice traffic drop at random ?

Q2. We maked the rest of the queues to operate in shared. When we make big traffic (30M/COS0), COS7 traffic (voice) is influenced. When the rest of the queues shaped, it is not influenced. Is it strange ?

Aya Yoshihara

New Member

Re: 3750 QoS

If you enable priority queue, that queue is served first irrespective of the other queues. You need to map COS value to the respective queue (I think queue 1 is expedite queue when you enable priority queing).

Please note that, QOS does not make any sense in this case as there is a mismatch between link bandwidth and interface speed. all traffic over 30Mbps (sbscribed bandwidth) is blindly dropped in carrier netwrok irrespective of voice or data.

HTH,

Subba

New Member

Re: 3750 QoS

Hi,

In your case, the WAN link bandwidth does not match your switch interface speed. To avoid packet drop in carrier network, you need to either police at ingress or shape at egress. Cat3750 cannot do shaping, so use some router (like 1800 series) at WAN edge to leverage vast variety of QOS tools like CB shaping with LLQ.

Though Cat3750 supports policing at ingress, it has negitive impact over regular application performance.

"srr" is not really a shaping mechanism, rather it avoids starvation among output queus by giving weights to each queue (If you enable priority queue-out only 3 quques participate in SRR). Do not use switch at WAN edge if there is a mismatch between interface speed and subscribed bandwidth from carrier (Cat3750 Metro is an exception).

HTH,

Subba

Purple

Re: 3750 QoS

Hi,

Is this a GigE interface or a FastEthernet interface ? If it's a FastEth interface, I don't think you should be surprised since 30M exceeds the 20M that you are rate-limiting the port to.

The limiting applied by the 'srr-queue bandwidth limit' will be indiscriminate - it will drop voice or data traffic depending on which particular packets are not conformant.

Paresh

New Member

Re: 3750 QoS

It is FastEth interface on our 3750, but it is GigE interface on customer's 3750 with 'speed 100'.

Is there any difference between FastEth interface and GigE interface with spee 100 ?

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