Posted on 2011-04-23
Last Modified: 2012-05-19
Hello Experts,

This question is in regards to QOS on Cisco switches.
Is it a correct assumption to say that QOS only kicks in when there is congestion?
Question by:trojan81
    LVL 79

    Accepted Solution

    Yes and no.
    Yes, it only has an affect when there is contention for any given data path.
    No in the fact that once set up, it is "always on" and will always be doing the priority queueing, etc. It's just that it won't start dropping packets or anything unless and until there is contention for the space.
    IMHO, if your whole network is 100M to the desktop/phones, and you have good Gig backbone, there will almost never be any contention on the network to have to deal with, and we need only setup the bare minimum QoS policies to prevent data floods from a virus/malware or other application that has run amuk..
    Now, when you have WAN links that choke that 100M data path down to, say a 1.5M T1, then yes, you want to manage that bandwidth with proper QoS as much as possible.

    Author Closing Comment

    Very well said!

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