Can someone explain what portcfgfillword/Dword is and does?
In order to explain what portcfgfillword does, we have to know that a fillword is.
A fillword is a primitive (a pre-defined 4-character frame) inserted between each Data, or control, frames that helps to maintaing the line synchronization between boths ends of the link.
Also, this very same primitive is used during the link initialization protocol in order to set the link online.
The command portcfgfillword is used to set the fillword used.
When it comes to Dwords, I am afraid that I don't know that concept. Do you mean Dport? If so, a Dport is a feature introduced by Brocade in FOS 7, by which when you configure both ends of an ISL as a Dport, and we can perform link-level diagnostic tests in order to detect a link issue. The good part of it is that by doing it this way, the rest of the fabric is not affected by the tests.