One side effect of deciding on a default configuration is that most of the time, one size does not fit all.
This is especially true of the dual-Ethernet functionality used in the TS-4600, TS-4712, TS-4720, and TS-7250-V2 products.
For these products' default configuration, the SOM uses the CPU Ethernet (eth0) to connect to a Marvell managed switch chip. This switch chip is normally configured to use VLAN tagging to create two virtual Ethernet port interfaces in Linux (eth0.1 and eth0.2).
Unfortunately this VLAN tagging strategy is not compatible with all networks, especially those which use uncommon DHCP services or other more advanced networking strategies.
All is not lost, however. If your design does not require both ports to be separate, you can simply turn off the VLAN tagging functionality. Linux will return to using eth0 for its connection to the switch, and both switch ports will be available for more generic networking purposes so long as they are not both plugged into the same upstream switch. Note: Creating a network loop could cause the upstream switch to terminate (stop listening to) one or both ports.
To turn off VLAN mode, simply log into Debian on the SOM and edit the file /ts/config. The last lines will look like this:
### TS-4712 / TS-4720
## These boards include an onboard switch with 2 external ports. By default
## the switch will detect if it is on a known baseboard that supports the second
## ethernet switch port, and set up VLAN rules to define eth0.1 and eth0.2. The
## other option is to configure the switch to pass through the packets to eth0
## regarless of port.
## 2 Disable VLAN and pass through to eth0
## 1 Enable VLAN on all baseboards
## 0 Enable VLAN on supported baseboards (Default)
Remove the # from the last line, and change the "1" to a "2". Save the file and reboot.
That's all there is to it. Now the system has one Ethernet port called eth0, and the two external ports will react just as any other network switch on your network.