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Webcast-Catalyst9k
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CoS Tagging Question

Hi, good folks:

Just need to verify my thinking...

CoS values have no inherent significance. In and of themselves, a CoS value does not bestow any importance to a frame. The IOS does not see a frame with a CoS of 5 and somehow automatically give it more importance over a frame it sees with a CoS of 2. The diffferentiation between the handling of traffic occurs when the traffic is scheduled.

For example, a 3550 switch has 4 egress queues, and by default WRR is configured to service each queue equally; they all have the same weight. So, unless we change the weighting on the queues and direct the more important traffic to the "heavier" queue, all traffic will be handled the same. In other words, the actual CoS value in and of itself is meaningless. So, I could tag, say, VoIP traffic with a CoS of only 1, but as long as I direct that traffic to a heavily weighted queue, or a priority queue (queue 4 on the 3550), that is fine.

Is all this correct? If not, where exactly is my understanding wrong?

Thank you!

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Super Bronze

CoS Tagging Question

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Yes and no.  Actual preference for particular CoS (or L2 ToS) markings depends on device configuration.  You're correct, that in itself, CoS tags mean nothing and many default device configurations won't treat the traffic differently but when devices do treat differently, often there are expectations about the importance of different markings, so I would recommend not marking traffic in such a way that becomes a problem later on.

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