Connect a L3 sw to the internet via a dumb LTE/3G modem

[daviddelgadovega]

Using the following devices:

L3 3560 sw

3700 series AP with autonomous firmware

ISP LTE/3G modem

Using the AP with several SSID's to provide wireless connectivity. Each SSID has a VLAN on the SW to which the ISP modem connects. Each VLAN should have its own ISP modem to provide internet connectivity. The L3 switch is providing DHCP services for each VLAN. Up to this point everything seems to work fine.

The ISP modem only has a L2 interface. If a L3 interface and routing is configured to route traffic on the switch, then the ISP modem is not able to send packets to other subnets on the switch. I suppose that all ARP requests that fall outside the 3G modem L2 interface subnet are sent out to the default gateway on the 3G modem itself and out to the internet. I'm unable to ping the 3G modem from a different subnet, even though routing is fine as I can ping other devices on the same subnet as the 3G modem.

I thought about NAT/PAT to convert ip addresses before they reached the 3G modem , but the switch is not capable, however NAT/PAT is already done on the 3G modem. I'm also thinking on several subnets on the sw and to program the 3G router with a subnet that could communicate to all subnets on the switch. For example, two /25 subnets on the switch and the 3G router with a /24 subnet.  

I have also studied IP SLA, but I'm totally new to it.

Is there a way to accomplish this "My end goal is to try to have several 3G modems provide internet connectivity and for the switch to be able to provide redundancy to the internet with an HSRP protocol when a device fails" in my setup? 

Any suggestions will be greatly appreciated.

[Karsten Iwen · Accepted solution]

I would either change the 3G device to something that is able to handle static routes or change the switch to a device that can do NAT like a firewall.

But there is one more dirty workaround that is worth trying:

Put the link between the 3G modem and the switch in a /16 subnet like 10.10.0.0/16 and make sure that Proxy Arp is enabled. Then configure the individual VLANs as 10.10.1.0/24, 10.10.2.0/24 and so on. That could work.