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QoS on multilayer switch only doing L2

I had someone show me a configuration for a 3550 which they set up for QoS'ing voice traffic. The QoS config looks all wrong to me, but I want to make sure I have my facts straight before I say anything. I apologize for not having config samples to post, but here's what I saw.

They have 6 3550 setup in a flat network, just 2 vlans, voice and data trunked between the switches and out to all the phones and PC's (non-cisco IP phones) with no L3 segmenting although they do have MLS turned on on all the switches. Next they don't do any kind of CoS marking, but do mark all voice traffic by setting the DSCP.

Am I correct when I look at that and believe that because they are only setting the DSCP and not the CoS that no packet will ever get prioritized or dropped because even though MLS is enabled it is still a flat network with no routing interfaces so the IP header with the DSCP is never inspected.

4 REPLIES

Re: QoS on multilayer switch only doing L2

Hi Stan,

You can " Trust DSCP " on the incoming interfaces and once done it works out fine with dscp - cos mapping and QOS works find in a complete L2 environment.

Pls go through this doc to have an idea on the same

http://www.cisco.com/en/US/products/hw/switches/ps646/products_configuration_guide_chapter09186a00801f0a5a.html

regards

vanesh k

New Member

Re: QoS on multilayer switch only doing L2

Thanks for the response Vanesh. My question remains "in the current environment, by only marking the DSCP and not CoS and using the 3550's as L2 switches (MLS enabled, but no routing interfaces configured). Are they, in effect, not doing QoS because they don't set CoS and no shaping or policing occurs based DSCP until they hit a L3 interface".

I contend that is the case, but I am not positive. To put it another way. "Does a 3550's switchport interface inspect the DSCP and queue/shape/police accordingly just by having MLS enabled globally on the switch?"

Re: QoS on multilayer switch only doing L2

Hi Victor ,

I request you to go through the link and then you would be clear and with DSCP-COS mappings , the dscp what you set is copied to COS field in L2 and You need to trust the DSCP or COS fieids in the incoming interface from other switches for QOS to take effect

regards

vanesh k

Re: QoS on multilayer switch only doing L2

queuing/buffering on 3550's are all based on DSCP. This is why all the QOS maps map to DSCP.

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