While interconnecting two Brocade VDX VCS fabrics using normal ethernet trunks i stumbled on the following interface status during a migration:
TenGigabitEthernet 2/0/17 is up, line protocol down (ISL DOWN)
The interface configuration was configured as follows on both ends:
interface ten 2/0/17
no fabric isl enable
no fabric trunk enable
switchport mode trunk
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.