qos trust question

[from88]

Hello,

If the router or switch gets two packets, one marked with better dscp/cos and the other one with no marks - just best effort and the port trusts QOS. So my question is: do the switch/router will automatically treat the marked packets better or some QOS mechanism (service input-policy) should be implemented ?

Thanks in advance !

[Navanshu Sehgal]

QOS majorly involves classification marking/trusting and queuing. As you have already marked the traffic the switch/router needs to know which queue should be used to pass traffic in different queues which can be configured to police shape give bandwidth etc...

On a switch if QOS is enabled queues are used based on the mls-qos maps where as on router we need to create service-policy to define egress queuing. Each class-map in the service-policy defines a queue which can then be configured as needed.

[from88]

Thanks for answer.

so another question. Ok we have service input policy with classes. DSCP Ef mapped to class EF, BE traffic mapped to class BE. Now, if the congestion occurs do the packet with EF will be serviced first before BE packets ? or it's a must to implement mechanism like priority queing ?

By the way the qos CLASS equals QUEUE ?

Thanks again !

[Mark Malone]

voip traffic will take preference in qos at all times when theirs congestion usually as it real-time traffic and cannot be re-sent or voice would be out of sync in the call , it gets the highest priority available expedited forwarding EF 46 DSCP or COS 5

BE is burst excess is that what you mean policing of traffic ?  can you post your qos config it will help

check this chart

http://www.netcontractor.pl/blog/?p=371

[from88]

thanks all for input..

My question in general was: it is enough to enable trust on ingress port to make the device prioritize DSCP EF packets ? Or i need to enable some type of specified qos like WFQ, priority quening etc.? ?

[Joseph W. Doherty · Accepted solution]

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Posting

It depends on the actual device.  But generally speaking, for DSCP EF packets, which are usually used for something like VoIP packets, additional configuration is often required to properly prioritize and treat them.

[Navanshu Sehgal]

On egress interface you will have to define a class which matches ef and then give the priority command to enable priority queuing. without this the router is not going to prioritize the traffic.

Priority queue makes sure that it is serviced if a packet arrives in that queue regardless of the packets in other queue.

A class with a queuing action (eg: priority bandwidth or shape) creates a queue.

Implementing QOS varies based on platform but the overall concept remains the same. Some catalyst switches like 6500 have priority queue by default if qos is enabled.

-Navanshu

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

Cisco QoS router/switch behavior, varies per device kind, generally more so, between switch series; it's definitely not one-size-fits-all.

That said, many Cisco "true" routers, by default, don't provide any different/special treatment for ToS marked packets, for ingress or egress, unless so configured.  However, even without QoS specific settings, if original WFQ was enabled on router interfaces, it would provide different egress treatment based on IP Prec markings.

Again, Cisco QoS varies much between Cisco switches.  Many will also, by default, provide no special/different treatment of packets with ToS or frames with CoS, for ingress or egress.  Many, though, by default (although may require globally [manually] enabling QoS), will direct certain ToS or CoS marked packets/frames to specific hardware egress queues; often such queues, again by default, will get equal treatment.  Many Cisco switches, if QoS is globally enabled, will not trust ingress CoS or ToS and reset such markings to zero.

For Cisco routers and switches, if you're dealing with CoS/ToS marked traffic, you'll want to review the device's QoS defaults, to insure the device doesn't do anything you don't want it to do.  If you want special/different QoS treatment, you'll then really need to study the QoS features of the device.  Be prepared, not all Cisco devices can provide the same QoS features, let alone provide the same QoS feature in exactly the same manner.

[from88]

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