Here's the setup:

Two companies (A & B) share office space and a LAN. A 2nd ISP is brought in and company A wants its own Internet connection (ISP A) and company B wants its own Internet connection (ISP B).

VLANs are deployed internally to separate the two companies' networks (company A: VLAN 1, company B: VLAN 2, shared VOIP: VLAN 3).

With separate VLANs it's simple enough to use separate DHCP servers (or separate scopes on the same server) to assign the default gateway to each company's gateway for their Internet connection. Static routes can be created on each gateway to point traffic destined for the other company's VLAN or the voice VLAN so that all nodes are reachable as expected.

However, I think this is a form of asymmetrical routing, right? (The path from node A1 to node B1 is not the same as the path back from node B1 to node A1).

Can I set up policy-based routing to correct this? In that case, can I assign the same default gateway to every device on all VLANs and create a routing policy on a L3 switch to look at the source address and forward traffic to the appropriate next hop? In that case, I want the routing logic to go like this:

1. If the destination address is known, forward the traffic (traffic destined for a different VLAN).
2. If the destination address is unknown, forward the traffic to ISP A's gateway if the source address is on VLAN A; or forward the traffic to ISP B's gateway if the source address is VLAN B.

Am I thinking about this problem in the correct way? Is there another way to solve this problem that I am overlooking?