We already know that the DCX3400 reesponds to the traditional GI Cable IR protocol or to the XMP protocol. I don't know if a 30 second skip is implemented under XMP, so I gave you the Pronto Hex for the GI Cable protocol. Both sets of data are available at hifi-remote.com in the Cable section.  Basically the XMP stuff comes from the CBl 1982 setup code in Comcast remotes, and it applies to a variety of STBs.

I use MakeHex with the MakeHex GUI.  Jason wrote a pretty good explanation of how to use them, although not for XMP.  Use IRTool to check if your Pronto Hex is correct.