Each Fibre Channel interface has an associated trunk-allowed VSAN list. In TE-port mode, frames are transmitted and received in one or more VSANs specified in this list. By default, the complete VSAN range (1 through 4093) is included in the trunk-allowed list.
The common set of VSANs that are configured and active in the switch are included in the trunk-allowed VSAN list for an interface, and they are called allowed-active VSANs. The trunking protocol uses the list of allowed-active VSANs at the two ends of an ISL to determine the list of operational VSANs in which traffic is allowed.
In the following figure, switch 1 has VSANs 1 through 5, switch 2 has VSANs 1 through 3, and switch 3 has VSANs 1, 2, 4, and 5 with a default configuration of trunk-allowed VSANs. All VSANs configured in all three switches are allowed-active. However, only the common set of allowed-active VSANs at the ends of the ISL become operational as shown in below.
Figure 3. Default Allowed-Active VSAN Configuration
You can configure a selected set of VSANs (from the allowed-active list) to control access to the VSANs specified in a trunking ISL.
Using the figure above as an example, you can configure the list of allowed VSANs on a per-interface basis (see the following figure). For example, if VSANs 2 and 4 are removed from the allowed VSAN list of ISLs connecting to switch 1, the operational allowed list of VSANs for each ISL would be as follows:
The ISL between switch 1 and switch 2 includes VSAN 1 and VSAN 3.
The ISL between switch 2 and switch 3 includes VSAN 1 and VSAN 2.
The ISL between switch 3 and switch 1 includes VSAN 1, 2, and 5.
VSAN 2 can only be routed from switch 1 through switch 3 to switch 2.
Figure 4. Operational and Allowed VSAN Configuration