In the past when setting up a 2820 with a 120, I have always assigned a static IP to the 120 on the same subnet as the 2820, and connected it to a LAN port rather than the WAN2 port directly. Then patched the WAN2 port to another LAN port. This has always worked well, and allows the 120's management interface to remain accessible on the LAN.

I tried this with a 2830 earlier today, and it ddid not seem to work. With the 120 connected directly to to WAN2 it connects and behaves correctly, but connected via a network switch (either the internal 4 port one or an external one), and the 2830 does not establish the PPPoE connection.

Any idea why? (alas I did not have the chance to setup a mirror port and capture a wireshark log of what was going on)