On VyOS 1.3 rolling,
The ping command fails when 'interface' arguemnt is given:
```
vyos@vyos-vm-2:~$ ping 10.2.1.1 interface 10.2.6.21
/bin/ping: invalid option -- 'I'
Try 'ping --help' or 'ping --usage' for more information.
vyos@vyos-vm-2:~$ ping 10.2.1.1 interface dum0
/bin/ping: invalid option -- 'I'
Try 'ping --help' or 'ping --usage' for more information.
```
Apparently the -I switch is not included/supported in the version of ping shipped with the upstream distribution. The ability to specify source interface / address is very important for most network administrators so this is a noticeable problem.