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.
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.
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 ?
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.
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).
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.