Very good idea. PPPoE is used on the WAN interface for connecting to my ISP. Running a cable from the Router WAN port to a free ethernet port on my computer, and then running a second cable from a second ethernet port on my computer to the NTU (using a fibre connection), and finally bridging the two ethernet ports on my computer should do the trick, right? And then simply use wireshark to listen to the passing traffic.
If the router runs lede it is even easier, simply "opkg update; opkg install tcpdump" and the you can use tcp dump directly on the router, yu might want to use a USB stick to store the capture.
Routing the traffic through another machine to snoop on it is possible but certainly not that easy as you need to forward these packets properly.
Should I dump the packets on the pppoe_wan interface? Or on the eth0.6 interface? My gut feeling says eth0.6, since the packets traveling through the PPPoE tunnel can be unencrypted/uncompressed even if the tunnel itself isn't. And since the tunnel runs through eth0.6, encryption/compression would only be visible there. Am I right in my assumptions?
edit: I have tried both interfaces and piping the output to grep, which is searching for specific content on a http site. Both interfaces are able to show the actual content of the packets. So no compression/encryption seems to be in use. It's definitely the encapsulation / decapsulation that is hitting the CPU hard and unfortunately not very well threaded.
I wonder how difficult it would be to get the accel-ppp software working on Lede. It's written from the ground up with performance in mind and is properly multithreaded. Roaring penguin PPPoE is showing its age 
Yep, eth0.6 would have been the one that would show compressed/encrypted PPPoE payloads as unintelligible payloads, actually comparing (concurrent) captures of eth0.6 and pppoe-wan was probably the best idea.
As an alternative to getting another pppoe client into lede, you might want to talk to your ISP anf try to convince them to use DHCP (with option 82, see https://slaptijack.com/networking/what-is-dhcp-option-82/) instead of PPPoE 
IPoE is definitely the easiest option, but I doubt my ISP would want to change their infrastructure just because I insist on using my own router instead of the ISP provided one. Officially this isn't even supported. I guess I could switch to an ISP that does use IPoE instead.. Or maybe buy something ARM based with more CPU power for PPPoE.
I switched from PPPoE to DHCP long ago and my ISP changed nothing...I can use either or, and sometimes do for testing.
I've tried DHCP, but it is not working. My ISP explicitly requires a PPPoE connection unfortunately
Or switch to an x86-based router board.
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Hello all,
i seem to be running into the same issue on a wrt1900AC router running LEDE. Jus wondering what this "fast-path" is and how can i use it.
Have there been any enhancements to the PPOE speeds in LEDE?
Update. Enabling "NAT software offloading" under firewall settings, resolved the speed issue.
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Hi all, how I configure fastpath ?
You don't, it's a modem feature - which is decided during the handshake between DSLAM and modem, but decided by the DSLAM.
How do I know if my modem has this feature? what do you mean by DSLAM?