How To configure a DAL Router to operate in IP Passthrough mode with multiple Sources interfaces

A DAL router can be configured to operate in IP Passthrough mode, which means that the device passes the IP address assigned to it on a WAN or cellular modem interface, to a client connected to a LAN interface.

In this article will be shown an example of using more than one interfaces as source of IP Passthrough and how this will be handled by the router in case of failures.

The scenario considered is the follow (an IX20 is used in this example, but the configuration is similar on other models, just the interfaces name may differ):

- The ETH2 (LAN) interface has a client (laptop) connected with DHCP client enabled

- ETH1 (WAN) interface is the primary WAN interface (metric 1):

- Modem interface is the secondary WAN interface (metric 3):

The routing table confirm this WAN interfaces priority order:

To configure the ETH2 to acts in IP Passthrough mode with multiple interfaces, the settings need to be changed as following:

Note:

- The Source interfaces will be chosen in the order they are on the list, so, even if the Modem is not the primary WAN interface for the DAL router, it will be the "primary" source of the IP Passthrough for the LAN client

- The Ancilliary address will be needed/used in case BOTH Source interfaces will not have a valid IP configuration.

Applying this configuration, the status of the interfaces is the following: (Status > Interfaces):

ETH1 (WAN) and Modem interfaces are both showing to provide the IP Passthrough to the ETH2(LAN) Interface:

But, due to the Modem being the first in the Source Interfaces list, the ETH2 is shown as getting the IP Passthrough from that:

In this condition, a Laptop connected to the ETH2 port with a DHCP client enabled, will get the Modem Interface IP address:

In case the Modem interface fails for some reason and does not have a valid IP configuration to pass, then the ETH2 interface will start getting the IP Passthrough from the ETH1 interface:

So, the Laptop connected to the ETH2 interface, will be now getting the ETH1 IP address:

If also the ETH1 (WAN) interface fails, then the Ancilliary address will be assigned to Laptop and no IP Passthrough is shown in the ETH2 (LAN):