Ensuring DSL/WAN and LAN are Isolated

I am looking to get a Netgear DM200 working and noticed that on the main ToH wiki page it states:

Because the device has only one wired ethernet port ( eth0 ), you will need to use tagged VLANs to prevent traffic from your LAN from leaking out the VDSL connection.

I'd never considered needing to explicitly isolate the DSL interface from the regular Ethernet ones. Could anyone help me better understand this? Is this purely a side-effect of it having only one Ethernet port (why would that be)? I'm especially interested as I have another OpenWRT/DSL device deployed and I'd like to know that I'm not leaking anything unintentionally!


If you plan to use this modem as a bridge then you need to consider that.
However I presume that you want to use it as a router and terminate the PPPoE on it, rather than on another router, so this warning doesn't apply to you.
More to read: https://en.wikipedia.org/wiki/Multiprotocol_Encapsulation_over_ATM