CoS values

Posted on 2011-10-29
Last Modified: 2012-05-12
Is there any difference between setting priority on traffic and CoS values? Are they the same...
Question by:totaram
    LVL 9

    Accepted Solution

    As stated by HP

    Given that there will always be points in the network where multiple traffic streams merge or where network links will change speed and capacity, it is important to move traffic on the basis of relative importance. Without CoS prioritization, less important traffic can consume network bandwidth and slow down or halt the delivery of more important traffic. For example, without CoS, most traffic received by a switch is forwarded with the same priority it had upon entering the switch. In many cases, such traffic is ‘‘normal’’ priority and competes for bandwidth with all other normal-priority traffic, regardless of its relative importance to your organization’s mission. CoS helps to keep the most important network traffic moving at an acceptable speed, regardless of current bandwidth usage. This means you can manage available bandwidth so that the switch transmits the most important traffic first.

    Or take a look at the official definition of IEEE P802.1p
    The QoS technique developed by the working group, also known as class of service (CoS), is a 3-bit field called the Priority Code Point (PCP) within an Ethernet frame header when using tagged frames on an 802.1 network. It specifies a priority value of between 0 and 7 inclusive that can be used by QoS disciplines to differentiate traffic.

    Therefore to answer your question traffic priorities are usually set on a software level while this is a hardware technology.

    Author Comment

    So, if we set the priority on traffic on wholesale model (w/o CoS) values, it can get overwritten
    as some point?? In order for an end to end gauranteed traffic BW usage, user should set the CoS values.

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