Inter-VLAN Communication Tips
Although most older routers (routers running IOS 12.2 and earlier) support both ISL and dot1q, some switch models support only dot1q, such as the 2960, 2960-x, 3650, and 9200 series. Check with the version of IOS you are using to determine whether ISL or dot1q is supported.
ISL will probably not be an option, as it has been deprecated for quite some time.
If you need to use ISL as your trunking protocol, use the command encapsulation isl x, where x is the number of the VLAN to be assigned to that subinterface.
Recommended best practice is to use the same number as the VLAN number for the subinterface number. It is easier to troubleshoot VLAN 10 on subinterface fa0/0.10 than on fa0/0.2.