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Cisco waas and QOS issues (WRED)


I manage a MPLS network with several routers and we have waas almost everywhere. We manage the QoS with CBWFQ and WRED and it's working pretty well.

We have noticed recently that during congestions we had some drops in the AF31 class despite the fact that the AF31 traffic is lower than the percentage of traffic allowed to that class and we are troubleshooting this issue.

I was wondering if the waas could not have a negative effect in the congestion management process:

WRED is supposed to queue packets regarding the type of class and discard some packets when the queue reaches a MIN threshold then all of them when the queue reaches the MAX threshold.

By dropping the packets, it assumes that the clients/servers on both sides will then decrease the TCP windows and limit the rates so that the congestion is stopped.

The Waas has a quite opposite effect by trying to push the maximum of traffic on the WAN link in order to use the bandwidth in a more efficient way than the TCP protocol allows it.

Would it be possible that when a link is in congestion, the WRED mechanism would be jeopardized by the waas efficiency and thus the waas would increase (or at least extend) the effects of the congestion rather than limitate its effects?

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