If you do that you'll need VLAN capable switches or else you'll need a 2nd network run between the router/modem..
What'd you do is assign one port on the market switch to say, vlan 100.. then another port on the tractor switch to assign it to vlan 100.. then you plug your cable modem into the market port and the router into the tractor port you just configured then configure the 2 buildings uplink to add vlan 100 as tagged.
still with me? now's a good time to tap out
With you in concept. Special Switches that support Vlan feature; an assignment of a dedicated portal in each switch for the modem-router communication. Whats not clear is what SW is used to make these assignments?
If the cost of the special switches exceeds the cost of the router then its not worth the hoop jumping.