The Linux ping, confusingly, uses -I option for both source interface and source address. Right now we expose it as interface in our op mode CLI for ping.

However, people not familiar with that quirk of ping(1) may not realize that they can use an IP/IPv6 address with the "interface" option (I forget that it's traceroute -s but ping -I once in a while, and I bet I'm not alone).

Thus it's probably a good idea to add a separate source-interface option to make the CLI more intuitive, especially to those not familiar with the quirks of the Linux version of ping.