Same VLAN id on WAN and LAN

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cgandr
DD-WRT Novice


Joined: 27 Sep 2015
Posts: 3

PostPosted: Sun Jun 12, 2022 20:09    Post subject: Same VLAN id on WAN and LAN Reply with quote
My ISP supplies a locked, preconfigured router but they allow subscribers to use their own routers at their own risk, for which they gracefully provide PPPoE credentials and advise to use VLAN id 20 on the WAN side.

I intend to replace their router with this device:

Model: Asus RTN-18U
CPU: Broadcom BCM4708
Firmware: DD-WRT v3.0-r48432 std (03/01/22)

Replacing the [ISP router] with my [DD-WRT router] was easy. Just had to configure WAN connection type as PPPoE and enter the credentials. Then, on the VLAN tab (Setup - Switch config) changed the VLAN number on the WAN port to 20. It works like a charm.

So far so good. Now here comes the fun...

I decided to also contract voice services (a telephone line) with my ISP. The service is provided by connecting an analog telephone device to a specific 'phone line' with RJ11 connector on the [ISP router]. So, if I replace the [ISP router], I loose the telephone service. The ISP specifically advises that using a different router is incompatible with their telephone service. I'm willing to challenge that.

My targeted solution: Still use my DD-WRT device as the first-line device and configure it as PPPoE server with same credentials as ISP, and hook the [ISP router] WAN port to one my [DD-WRT router] LAN ports. So my DD-WRT router will become PPPoE client on the WAN side and PPPoE server on the LAN side. I chose LAN Port #4. That also means that I have to go again to [Setup - Switch config] and create a new VLAN with tag 20 and assign it to LAN port #4 to allow the ISP router to connect to.

And here is where everything breaks. Actions above end up creating a switch between WAN and LAN #4 ports. After scratching my head for a while, I noticed that all [WAN and LAN] VLAN interfaces are created over the same physical interface eth0. In other words: My router has a single physical interface for WAN and LAN and I suspect for this reason it is not possible to use the same VLAN tag on two WAN-LAN ports without physically switching them. As stated above, my router has a SoC system with CPU Broadcom BCM4708. After reading this (and as opposed to a BCM4704 in other routers), most Broadcom devices provide just a single interface.

At this point, here are the questions:

1.- Is it possible, with all the community wisdom, to somehow allow same VLAN ids on both WAN and LAN, even on a single physical interface?

2.- I may likely end up replacing my Asus RTN-18U with a more capable device with separate interfaces for WAN and LAN. Any sensible suggestion among DD-WRT supported devices (maybe based on Atheros...)?

3.- As a short-term workaround I may reverse routers: Leave the ISP router as the first-line device connected to the ONT and hook up my [DD-WRT router] WAN side to the one of [ISP router] LAN ports as DMZ. I hate this solution because it introduces double NAT, and I fear also a good deal of latency. It also means being defeated in the purpose of replacing the [ISP router] as the first-line device. Still need to try that. OK, this is more a comment than a question.

4.- Any other solution or workaround that I have not think of...?

Thanks for your help.
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the-joker
DD-WRT Developer/Maintainer


Joined: 31 Jul 2021
Posts: 2146
Location: All over YOUR webs

PostPosted: Sun Jun 12, 2022 20:28    Post subject: Reply with quote
3: dont do it, ISP routers have built in hidden management protocols with access to pretty much everything, its how ISPs are able to push configurations and change settings/run diagnostics, and see whats connected to mention a couple, without any permission from you or even needing current router credentials.

Then by default their firmware like any stock firmware is based on outdated EOL kernels, libraries and their mothers, mostly unpatched to known security flaws.

Further DMZ bypasses all firewalls, its a free for all zone.

Or do it... what do I care Wink

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jonsalam
DD-WRT Novice


Joined: 12 Jun 2022
Posts: 1

PostPosted: Sun Jun 12, 2022 21:52    Post subject: wan to lan port Reply with quote
sorry im not good in english. its possible ddwrt do that setup on router 1?
same isp with different ppoe account. and vlan id must be put on lan 2
if can how to do that? im running ddwrt on ac68u.
thanks in advance



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Last edited by jonsalam on Tue Jun 14, 2022 15:41; edited 2 times in total
Zyxx
DD-WRT Guru


Joined: 28 Dec 2018
Posts: 732

PostPosted: Mon Jun 13, 2022 7:20    Post subject: Reply with quote
@cgandr:
I have a similar setup.

My solution? Using this setup:
modem --> WAN of DD WRT Router LAN --> VoIP to POTS adapter.
Something like this:

https://www.grandstream.com/products/gateways-and-atas/analog-telephone-adaptors

I had a hard time getting all needed SIP credentials, but finally one technician and a wireshark trace gave insights. It is working. Years and years without trouble, excellent voice quality.
cgandr
DD-WRT Novice


Joined: 27 Sep 2015
Posts: 3

PostPosted: Tue Jun 14, 2022 20:00    Post subject: Reply with quote
@Zyxx

Your solution would be my favourite, for that it means completely ditching the ISP router. However...

Zyxx wrote:

I had a hard time getting all needed SIP credentials...

I didn't even consider that because I can't imagine extracting my SIP credentials will be anywhere near easy Surprised . But now you mentioned it, good point Idea , nothing to loose just giving it a try. Let's see what Wireshark can yield here.

In general I'd appreciate some educated comment in particular on questions 1.- and 2.-

Thanks!
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