different ports, same subnet and gateway

Question

Hello Forum,

First, i've the following situation in one of our customers: their ISP is about to change their internet connection, they had 4 wan connection, each in their own /29. Today they will have two connections, two modems and 5 IP addresses.

The problem? Their ISP told them that:

[ul]

IP1 is on modem1/port1.

IP2 is on modem1/port2.

IP3 is on modem1/port3.

IP4 is on modem2/port1.

IP5 is on modem3/port2.

All IPs are in the same /24.

The default gateway is the same.[/ul]

So, this is a bit tricky... I can't configure two ports on the same subnet. Even if they allow me to use any port for any of the IPs and since the gateway is the same, outgoing packets will go out on one link OR the other.

Did anyone here have this situation? How did you solve it?

TIA.

Agent_1994 · Answer

FWIW, here how the story ends:

There's a configuration switch that allows subnet overlap:

config system settings
  set allow-subnet-overlap [enable/disable]

This will solve one problem, and i've tested it on a lab: FortiGate will consider and honor the incoming/outgoing port, ie: if you have a virtual ip on port1, all traffic regarding that VIP will be on port1, regardless if port2, port3 and port4 are in the same subnet.

But... there's always a "but", this particular ISP didnt allow the same MAC address on different IP addresses (clearly violating the standard), so even when we figured the addressing and routing, we weren't able to implement it :(

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