Is each office having their own router coming off the main router as secure as each office having their own internet line

Question

Company A owns a building and leases out 2 offices to other companies.

Telecom 1 provides internet to Router 1 with ip address x.x.x.121

Company A connects their router, Router A, to it and gives it IP address x.x.x.122
Company B connects their router, Router B, to it and gives it IP address x.x.x.123
Company C connects their router, Router C, to it and gives it IP address x.x.x.124

Is that effectively the same as each company having a seperate line?

Assume that A has control of the main router and each has access to their own router but no one else's.

EDIT: In my case the main router is a Cisco 2811
Company A is using a Ubiquiti EdgeMax
Company B unsure
Company C a BT home hub

Answer 1

Since they use different routers (telecom + companies), it would be more secure to have them behind another router. An attacker would need to attack the telecom router and then the company routers, instead of just the company routers.

Additionally, if the telecom router has client isolation capabilities enabled, then the companies would have to attack the telecom router before attacking the other company routers.

One possible drawback with having them behind the telecom router is if they're able to attack each other, then they may have more bandwidth available with which to launch attacks.