Some installers did not like the Belden 7851NH cable that ETC specified for Cat 6 infrastructure prior to May 2009. The pairs separator could be difficult to work with. The alternate Belden 1872A was oval shaped and could be difficult to pull in conduit.
With these complaints (and since to us Cat 6 is Cat 6), on the suggestion of an installer we switched to Belden 7881A from for Cat6 installations. It is round and generally less expensive than the others. For a plenum version use 7882A.