This isn't really a problem as such, more of a lack of understanding on my part. I have tried reading up on this with my understanding not increasing much.

I have a Synology RT2600ac router connect to my Vigor 130 in PPPOE pass-through mode. What I don't quite understand is how I can access my modem's management web service while it's connected to the router without having to do any configuration.

From looking at my router, the routing table shows the vigor subnet (192.168.2.X) and looking at my modem, I see my router was assigned an IP address from my Vigor as the DHCP server is enabled.

So, it appears the cable running between modem and router is passing on PPPOE frames from my ISP along with a LAN IP traffic at the same time.

I belive part of my problem is that I don't properly understand how PPPOE passthough works. It's the recommended setting and it works fine, what I don't quite get is why so many want to use the modem in bridge mode.

It appears PPPOE pass-through allows DHCP which allows me to monitor my modem from my router. Bridge mode allows this only after configuring static routes manually.

Have I got this right?

Some info if it helps.

Router has my public IP so no double nat'ing here (not even sure the Vigor can do this anyway).
It's the UK model, maybe bridge mode is a requirement for other regions or those with IPTV issues?
I am using the last firmware (3.8.2BT)

Again, not an issue or a complaint (it's great I have less work to do), I just want to understand how my modem is working. Any help would be appreciated.

Maybe this is the reason PPPOE passthortugh is the default for UK models.

Thanks.

I don't know anything about your Synology router but one reason for using the 130 in Bridge mode with a Draytek router is so that you can access the 130 GUI directly, which is not possible in the PPPoE configuration.

https://www.draytek.com/en/faq/faq-connectivity/connectivity.wan/how-to-access-vigor130-in-bridge-mode/

This is how I use mine and it works fine. Note that the dsl connection needs a VLAN tag so this probably doesn't work with adsl lines, only vdsl.

It is reported on here that certain Asus routers IIRC can access the 130's LAN directly, but Drayteks can't.

Thanks for replying.

Checked the link and this is the same result I get with doing nothing in PPPOE pass-through mode. Just plug it in and it all works. If it's the case a Dray-Tek router requires bridge mode in order to access the modem then that will explain why it's commonly used (although I would have thought a Dray-Tek modem would default to the setting easiest for another Dray-Tek product).

Maybe my router is doing something clever but it seems normal to me, router has an IP from the modem (via DHCP) on the WAN port, this same port is also used for PPPOE pass-through.

However, as I don't really know how it works, this could be nonsense.

Would it be correct to say that the modem is transparent is PPPOE pass-through mode? When I type my public IP address into a browser on the LAN, I get the router management page (which I would expect as the router has the public IP, not the modem).

I have searched for the difference between bridge and PPPOE pass-through and, to be honest, it wasn't clear to me. I have seen different answers to the same question.

The manual appears to be out of date for the firmware version (this has been noted in a few posts) so I can only assume that PPPOE does refer to pass-through as there is no longer an option to disable this.

The Vigor 130 is in pass through mode unless you alter it to run in bridge mode.

I connect my 130 to my Asus AC3200 and can call the Vigor's stats page via 192.168.2.1 whilst it is still connected to the router.

With other routers you need to disconnect the modem from the router and plug direct to a laptop/device using a PPOE connection to see the stats.

Thanks.

Seems to be the case for others in PPPOE pass-through so I'll just accept that it works