Qos packet drops

[phamthecong]

Hi all,  We have an output hierarchical qos policy on our WAN interface.  Our Bc = 500 000bit/interval (25ms) Be = 500 000 . When apply the policy we see a lot of packet dropped. We remove it and reapply it number of times, then ask ISP if they see any packets dropped at their end  (incoming traffic). They said there was not any packet dropped at their end. Is that weird?  Could anyone explain why we see packets dropped at our end but not at ISP end?  Thanks

[Laurent Aubert]

Hi,

You have output drops because the router is trying to send traffic at a higher rate than your CIR. In this case the queues will start to be filled but once they are full, traffic will start getting dropped.

This could happen with UDP traffic as UDP doesn't have any congestion control mechanism like TCP.

On the SP side, it's hard to say as it really depends how they enforced their policy regarding the Bw they sold to you.

HTH

Laurent.

[phamthecong]

Hi Laurent,

Thank you for your quick response. I understand why packets drop at my end. the thing I can not explain is that ISP claims they dont see any packet dropped. I have asked them to check rate limit on switch and QoS on layer 3 router but always get "no packet dropped"

Regards,

Cong

[Laurent Aubert]

It depends how their network is provisioned from Bw perspective. It's not because they sold you 20Mbps circuit that they will drop anything they receive which is above. Different SP's may have have different implementation some being more strict than others.

Also the guy you are talking to may also not be fully aware of all the SP network implementation.

HTH

Laurent.