If I have multiple switches (all connected together) but only need FICON functionality on two of them, do I only need to license the two boxes?
Clearly I wouldn't be able to configure FICON ports on the unlicensed switches, but would there be any issues regarding trunking (ISL/EISL)?
You are correct, you would need the FICON license on both switches where the FICON devices are to be attached. Since FICON only supports 1 hop, (swtich--isl--switch) I'm not sure what you mean by trunking ISLs. It is perfectly okay to have a 6 switch fabric, and run FICON on only 2 of them. The FICON VSANs can only exist on the 2 FICON switches, and FICON devices can only be attached to them. You can not trunk a FICON VSAN from a FICON switch to a non-FICON switch, then onto a FICON switch. All switches in the path must be FICON enabled...and the path is limited to 2 switches.
If possible, you might want to have dedicated ISLs for the FCION traffic. FICON requires in-order-delivery, and a by product of this is that if a frame is dropped on an ISL carrying FICON traffic, the entire ISL is frozen for 1/2 second. SCSI traffic is not hat picky, and can usually survive a drop, but if the SCSI traffic is traversing the same ISL as FICON traffic, it will be impacted by the 1/2 second freeze. Just a design point to consider. There are many ISLs out there carrying both SCSI and FICON traffic.
Hope this helps,