in addition to what Ernes mentioned, i will add that whenever i setup ports as ISL i typically leave them in VSAN 1 and simply trunk allow VSANs that need to go over that link. Why ? Because let's say you were using VSAN 20 and your ISL port was in that VSAN ..if you were to delete that VSAN, that port will move to VSAN 4094..which is an isolated VSAN so any VSAN that were going over that link with become isolated. By keeping the port in VSAN 1, you are sure that will never happen because you can't delete VSAN 1. My 2 cents [IMAGE]
To enable trunking on an interface  do we need to give command  switchport trunk allowed VSAN 3-6  , or we need to make the interface part of the  vsan 3,4,5,6.  Thanks 
Cisco MDS