In many projects, we are configuring VPNs for example between multiple cell gateways for a customer. When the carrier assigns IPs, unfortunately they often assign sequential IPs from the public static IP pool and then we end up getting something on two modems like
Above are real values on a current - no typos. Based on the above values, Modem B will not build a tunnel to Modem A as the target IP appears (based on its ipconfig) to be its broadcast address. We see similar issues with the DG of one being an IP of another, etc. etc.
From over a dozen similar projects with VZW and another modem manufacturer, it has been explained to us multiple times that VZW only assigns the IP to the modem (no mask and gateway) and the mask and gateway are assigned locally at the modem using some algorithm (often /30 mask and IP+1 = dg but you see above there are other versions) from the chipset. In order to overcome this default assignment creating issues, other vendor includes a “subnet selection mode” feature in which the user can “override” the default module mask and gateway assignment. Typically we “force 24 subnet” which then assigns a mask of /24 and a dg of .1. Alternatively we can use an “IP overrides” feature to manually set the mask and gateway (again most typically assign a /24 mask and .1 gateway. Either of these options lets us overcomes the issue and VPNs to be established.
Below is an excerpt from other vendors manual. Can Peplink add a similar override ability? It is critical to solve these kinds of issues as the only alternative is requesting different IPs from the carrier which can sometimes stall the project.
Subnet Selection Mode
Thisoption overrides the subnet mask that is assigned to the modem from thecarrier. In some cases, you may be assigned static IP addresses in the samenetwork at multiple locations, or a subnet mask that would designate that theIP address is a non-usable network or broadcast address and cause the internalrouter to reject it. In these cases, it is necessary to force a differentsubnet mask.
“Force24 Subnet” – this setting will alleviate the issue where you are assigned IPaddresses in the same network at multiple locations, or an IP address andsubnet mask combination that designates the IP address as a non-usable networkor broadcast address and is rejected by an internal router. If you have astatic IP address from you ISP this is the recommended setting.