While interconnecting two Brocade VDX VCS fabrics using normal ethernet trunks i stumbled on the following interface status during a migration:

The interface configuration was configured as follows on both ends:

Note that ISL is disabled and fabric trunking is disabled as well. No problem to connect two separate fabrics this way you would think….. :)

After some attempts getting this interface up and running, it appears there is a neighbor discovery protocol on the VDX that still checks for the VCS ID configured on the fabrics ( you cant turn it off).  When both fabrics that you would like to interconnect are using the same VCS ID you will not be able to get a normal trunk up even if you disable ISL on these links. Solution is to renumber one of the fabrics to another VCS id. Sad thing is that renumbering a VCS id means you will loose the current configuration of the fabric member.

I hope this post helps, as it took me some time to figure this stupid thing out.