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    Question Problems with large UDP datagrams


    I have an application running on Windows which communicates using messages composed of UDP datagrams with a maximum size of about 31kb. What I've found is that some networking equipment doesn't seem to like passing these larger datagrams. For example, in one location we have two switches, let's call them A and B. The server and some clients are on switch A, with the remainder of the clients on switch B. The server and clients on switch A cannot exchange these datagrams when they approach the max size, however, the server on switch A and the clients on switch B have no problems with these messages.

    Any ideas on what might be going on here? What could switch B be doing that allows the communication though switch A? I have not yet had the opportunity to look at a packet capture between the machines, so I cannot say if the datagrams are being fragmented differently, but that is definitely on my todo list. A simple ping test also replicates the behavior outside of my application, when specifying a similar request size using the -l option.

    Thanks in advance!
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    As it turns out, the client found that some of their Cisco switches had some QoS settings enabled that were causing the last fragment of the large UDP datagrams to be dropped.
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    Interesting. Thanks for following up with the solution/explanation. Can you post what the QoS settings were?
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    Unfortunately the client shared very little information with us about the root cause. The most we were told was that there were some VoIP QoS settings enabled on some switches that should not have been.

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