Some ISPs are doing QoS based on traffic type (online streaming), such as Comcast and Cogent. Why don't they reset IP DSCP at the edge so it is not as obvious that they are doing some kind of traffic shaping? Most CPEs are going to ignore DSCP anyway.

Any insight on how ISPs usually implement this type of QoS? I am thinking at exchange point, source IP (content networks) based mapping. What am I missing here (the linecard is going to be pretty expensive...on platform like ASR9K)?

  • 2
    FYI, providers don't need to mangle your dscp to implement backbone qos, the smartest way for them to mark traffic is using MPLS exp bits Nov 9, 2014 at 14:30
  • It's just unnecessary processing overhead. Altering a packet while forwarding is an expensive process.
    – Ricky
    Nov 9, 2014 at 23:29
  • The question was edited to something else... I know nobody (should) trust another network's CoS/DSCP unless it's something explicitly agreed upon. But why don't they reset the DSCP value at the edge to cover their tracks (so end-users in this case can't directly see some of their traffic is being deprioritized) Nov 10, 2014 at 2:08
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    If you are concerned about markings in your network, then reset them at your edge interfaces and apply your own. Like @Mike says, the markings you use do not have to be the DSCP information in the original packet.
    – cpt_fink
    Nov 10, 2014 at 7:12
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could post and accept your own answer.
    – Ron Maupin
    Jan 6, 2021 at 0:17

1 Answer 1


I have no practical experience in running CoS/DSCP in ISP networks, but a logical explanation would be that it doesn't make sense to just change fields in a header. Even if I as an intermediate ISP wouldn't be interested in these fields, the receiver or some other intermediate party could be. Resetting DSCP fields just because I'm not using them sounds like a waste of resources and unneeded meddling in my customers' packets.

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    Meddling with packets must be minimal for an ISP because anything changed potentially breaks the customer's requirements Nov 9, 2014 at 14:24

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