The gist of it can be found here (searching for ‘dhcp option 121‘ or ‘classless static route’ should provide some good add’l detail).I'm trying to figure this out, could you give more information/details/steps to DHCP option 121? I'm trying to find more information about it.
The specific configuration syntax varies by the router running your DHCP server, but the end result is that DHCP clients get pushed an *additional* route (to a VLAN, a set of VLANs, or a specific host - which should be via your L3 switch) along with the default gateway (which in this case would be pfsense, VyOS, etc., for accessing your WAN - or even VLANs you *don’t* designate for routing via the L3 switch).
edit: ninja’d by @jjacobs - good explainer on the difference. tl;dr DHCP option 121 gives you a way to set that gateway and a route without manually configuring individual routes on each client