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Author Topic: PPP MTU 1512 for UK G.993.2/VDSL2/FTTC  (Read 559 times)

Weaver

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PPP MTU 1512 for UK G.993.2/VDSL2/FTTC
« on: July 15, 2018, 10:59:30 AM »

Regarding the use of FTTC with a PPPoE modem in the UK, I heard some mention of 802.1Q VLAN tags being required by BT. If you have a separate modem and router, what happens to the tags ? Are they typically thrown away downstream when they reach the modem or do they get passed on to the router?

On the DSL link, is PPP in use? I am not sure what the protocol stack looks like on the DSL link as I have never even seen FTTC.

Regarding MTU does this mean that you need both modem and router to be able to cope with a PPP MTU of 1500 + 8 + 4 = 1512 on the modem-to-router link side (PPPoEoE)? ie With four more bytes for the VLAN tag? Not just 1508? So even jumboer still.
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burakkucat

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Re: PPP MTU 1512 for UK G.993.2/VDSL2/FTTC
« Reply #1 on: July 15, 2018, 06:54:31 PM »

With the standard UK FTTC product from Openreach, the PTM, G.993.2, link uses a VLAN tagged as 101 upstream of the modem.

--- G.993.2 link --- Modem --- Ethernet frames ---

<----- Upstream ---- Modem ---- Downstream ----->
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Weaver

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Re: PPP MTU 1512 for UK G.993.2/VDSL2/FTTC
« Reply #2 on: July 15, 2018, 07:51:05 PM »

And VLAN-tagged only upstream of the modem ?
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DaveC

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Re: PPP MTU 1512 for UK G.993.2/VDSL2/FTTC
« Reply #3 on: July 15, 2018, 08:19:43 PM »

And VLAN-tagged only upstream of the modem ?

Yes, although the HG612 has an option to vlan-tag the packets downstream.  But normally you would configure it to strip them.

I had a play with that feature recently (when I had one VDSL line, and one ADSL line, both using HG612s).  The option worked fine with the VDSL, but didn't work with ADSL.

But it did allow "daisy-chaining" the HG612s in following configuration:

1) LAN1 on HG612#1 connected to my router
2) LAN2 on HG612#1 connected to LAN1 on HG612#2
3) VDSL vlan-tagged
4) ADSL not vlan-tagged
5) Both HG612s configured with a LAN IP in the same subnet
6) Both HG612s configured with "port binding" disabled for LAN1 and LAN2

The port on my router (Firebrick FB2700) was configured with an IP in the same range as the HG612s, and both PPP connections on also on the same port (the VDSL with a VLAN tag).

It all worked fine.  It's a shame the ethernet ports on the HG612 are only 100Mbits/s, otherwise I would probably use the same configuration with my current setup of 2 VDSL lines (both syncing at approx 80/20).
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