We have been using the Sierra Wireless GX440 modem on Verizon 4G service with a Mikrotik with great results. Yesterday however we upgraded the GX440 modem which broke our solution. I hoping a networking guru can shed some light.

The modem is setup as a modem, no routing functions, or security enabled. It simply passes the outside IP to the Mikrotik to via DHCP. This has worked perfectly. After the firmware upgrade, we lost internet connection and a check of the routing table shows the default route is unreachable.

The modem is assiging the following:
IP 10.170.27.235/32 with a gateway of 10.17.227.1

The route is unreachable becuase the gateway is not included in the /32 network, I get that. What I don't get is if you plug the modem direclty into a computer. You are assigned the same IP 10.170.27.235/32 with a gateway of 10.17.227.1. However, it works you can get to the internet, even with the 255.255.255.255 subnet.

Sierra Wireless states that this was an intentional change in the firmware, that it works as evidence by pluggin it into the PC and don't understand why it wont work on our Mikrotiks.

Can anyone explain why they would have built in this "feature" and why these settings work on Windows but not the Mikrotik?
Thanks.

This link explains it, basically it is an ARP trick they use to make it work. It's probably something that Windows automatically enables, that the Mikrotik doesn't.

On that interface, try changing the ARP to proxy-arp and see if it makes a difference.

http://wlug.org.nz/Half%20bridge%20with%20PPPoA