Thank you for asking this question in the support community. Unfortunately, the NetVanta 1638P unit does not support the ability to modify the "Don't Fragment" bit.
Other ADTRAN products that support Policy-based Routing (PBR) have the ability to modify the "Don't Fragment" bit. To understand which units support PBR, please reference the AOS Feature Matrix. Also, the Configuring Policy Based Routing in AOS document explains how to configure this with the set ip df command.
If you have any additional questions, please do not hesitate to reply to this post. I will be happy to help in any way I can.
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