Dave,
Sorry, but I have to disagree and correct some of the things you've stated in your post.
Let's look at a situation where someone can get two circuits from the same ISP. These two circuits are on different subnets. This means there are two different gateways. While Windows allows the configuration of multiple default gateways, it is NOT recommended. Windows cannot handle multiple default gateways in a very predictable manner. The end result would be erratic routing behavior.
Now let's look at the situation where both circuits are on the same subnet. While I'm not exactly sure how Windows will behave when there are two NICs on the same subnet with a common default gateway, I suspect there is a pecking order or preferred path selection where Windows will constantly defer to one outbound NIC.
There are methods to aggregate physical links into a single logical link but these are in the domain of a common layer 2 locally switched network and not over WAN type circuits. A method for utilizing multiple network paths where the next hop gateways on each of the network paths are of the same metric cost is called ECMP (equal cost multipath). ECMP allows a router to simultaneously send packets down multiple paths available to the router when the next hop gateway of each of the paths have the same metric cost where no one path has an advantage over another. ECMP is a function of more advanced dynamic routing protocols such as OSPF. Windows doesn't do OSPF routing nor will an ISP ever allow any of their customers to connect into their routers to do ECMP via OSPF.
Another issue is multiple independent circuits just don't work with SPI or DPI firewalls. If a packet is sent down one circuit and returns on another circuit, the firewall will drop the return packet as it didn't return to the originating port....it breaks the statefulness of the session.
Sorry, but I have to disagree and correct some of the things you've stated in your post.
Let's look at a situation where someone can get two circuits from the same ISP. These two circuits are on different subnets. This means there are two different gateways. While Windows allows the configuration of multiple default gateways, it is NOT recommended. Windows cannot handle multiple default gateways in a very predictable manner. The end result would be erratic routing behavior.
Now let's look at the situation where both circuits are on the same subnet. While I'm not exactly sure how Windows will behave when there are two NICs on the same subnet with a common default gateway, I suspect there is a pecking order or preferred path selection where Windows will constantly defer to one outbound NIC.
There are methods to aggregate physical links into a single logical link but these are in the domain of a common layer 2 locally switched network and not over WAN type circuits. A method for utilizing multiple network paths where the next hop gateways on each of the network paths are of the same metric cost is called ECMP (equal cost multipath). ECMP allows a router to simultaneously send packets down multiple paths available to the router when the next hop gateway of each of the paths have the same metric cost where no one path has an advantage over another. ECMP is a function of more advanced dynamic routing protocols such as OSPF. Windows doesn't do OSPF routing nor will an ISP ever allow any of their customers to connect into their routers to do ECMP via OSPF.
Another issue is multiple independent circuits just don't work with SPI or DPI firewalls. If a packet is sent down one circuit and returns on another circuit, the firewall will drop the return packet as it didn't return to the originating port....it breaks the statefulness of the session.
