To allow a new VLAN 100 between VSX pair inter-switch-link 256 for port 1/45 and 2/45, you need to use the command vlan trunk allowed 100 in LAG256. This will add VLAN 100 to the list of allowed VLANs on the trunk port LAG256, which is part of the inter-switch-link between VSX peers. The other options are incorrect because they either do not use the correct command or do not specify the correct port or VLAN. Reference: https://www.arubanetworks.com/techdocs/AOS-CX/10.04/HTML/5200-6728/bk01-ch07.html https://www.arubanetworks.com/techdocs/AOS-CX/10.04/HTML/5200-6728/bk01-ch02.html