Outbound QOS on 3750 for VOIP on routed link

[dan hale]

Hi All, we are implementing our new Shortel phone systems requires us to match IP DSCP EF, Protocol RTP Audio, IP DSCP AF31.

It requires us to reserve 20% of bandwidth in its highest priority queue and 20% of signaling in the medium queue.

Below is what I had mapped out but, It looks like the 3750 switches will not apply to the output side of the interface.

class-map match-any Audio
match ip dscp ef
match protocol rtp audio
class-map match-any Control
match ip dscp af31
!
policy-map voice
 class Audio
  priority percent 20
  set ip dscp ef
 class Control
  bandwidth 20
  set ip dscp af31
 class class-default
  set dscp default
  fair-queue

The error I get is "Warning: Assigning a policy map to the output side of an interface not supported"

How can I accomplish the above when I cannot apply to the output side of  my routed nterface?

Thanks,

Daniel

[Martin Hruby]

Hello Daniel

On these Catalyst switches you only have limited options for configuring QoS when compared to a router. The non-metro version of 3750 (and the 3750-X) doesn't support outbound CBWFQ / LLQ using MQC syntax. You are limited to 4 egress queues, one of which can be configured as a priority queue and then you can apply shaping to adjust the outgoing traffic rate. Remember, that using MQC you can police/mark your traffic (DSCP) in the inbound direction on an interface. Maybe you could split your service-policy into two sections and apply marking inbound and then queuing/scheduling outbound.

You might want to have a look at the configuration guide here http://www.cisco.com/c/en/us/support/docs/switches/catalyst-3750-series-switches/91862-cat3750-qos-config.html#qds and also at this post: https://supportforums.cisco.com/discussion/11238706/srr-queue-and-traffic-prioritization#3397944

Best regards,
Martin

[Joseph W. Doherty]

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Posting

Ditto to what Martin has noted.

In addition, on a 3750, you need to enable QoS globally to take advantage of its QoS features, and when you do, by default, ingress traffic that is already marked will have its markings erased.  Also the 3750's default QoS settings often increase egress drop rates without some buffer setting tuning.

Also unlike CBWFQ, there's no default class nor FQ equivalent.  You need to insure traffic goes to the egress queue (one of four) you desire.  Each hardware egress queue only supports FIFO although with one of three tail drop settings.

Martin mentioned shaping.  A 3750 can shape a queue or the port.  If your egress has some logical bandwidth limit, less that port speed, you normally want to shape the port, but also be aware the port shaper is a bit inexact, so you might need to shape slower than your really want to avoid exceeding downstream bandwidth caps.