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[GigabitEthernet]

Dear all,

I have put our TalkTalk-provided Huawei HG523a into bridge mode so we can use our router, the TP-LINK-WDR3600, to connect via PPPoE to the modem so we can access the internet.

What I would like to know is: what MTU should I be setting for the connection in the router? TalkTalk uses an MTU of 1432 for PPPoA connections. Is this the same when connecting via PPPoE like we are here?

[Berrick]

Hi AlecR,

Tis my understanding that Talk Talks MTU is set at 1432. Therefore you cant use an MTU larger than this value. If you do the packets will get fragmented resulting in potential performance and other issues such as webpages not loading fully or website login issues.

[kitz]

It all depends on the lowest set on their routing.  Presumably your traffic will still be traversing a lot if the same totes as when on dsl, so there's a strong chance the mtu will be the same.

You can always check yourself to work out your Brest mtu.   The instructions how to do thar are ragout 1/2 way down the page here under "working out your maxfimum MTU.
http://www.kitz.co.uk/adsl/MTU2.htm

[GigabitEthernet]

So is the MTU the same on PPPoE? It isn't the PPPoA MTU + 28?

[kitz]

Regardless of protocol, MTU is based on the lowest setting on ALL of the routers that your data passes through.    If TT routing on their network is set at 1432, then its when your data packets traverse on the TT network that the Maximum Segment Size will be at its lowest... and therefore a possible point of fragmentation. 

Your optimal MTU will be based on the lowest MTU set on the network routers of any network(s) which your data traverses from start point to end point.

[djamig]

Hello Alec;

I have the same HG523a router and am wondering if you have actually been able to set it up in bridge mode and have your attached router log in via PPPOE. I was under the impression that talktalk was PPPOA only.

The reason I ask is that I am using the HG523 connected to a ddwrt router and am having no luck setting it all up. it does not seem to log in via PPPOE. Is there anything special I need to do to set up bridge mode?

Thanks for your time.

David

[Chrysalis]

why does talktalk use such a low mtu for pppoa? that makes no sense.

They dont give reason but it does seem on FTTC it uses 1500.  Of course pppoe without rfc4638 maxes at 1492, so either talktalk fiber routers supportrfc4638 and are configured to use it or they got that wrong.

https://help2.talktalk.co.uk/what-mtu

As the OP is doing some bridge trick on adsl tho the answer it seems would be 1432 or left to auto.

[kitz]

Don't know...  But one of the things that gies through my head is that at one time TT 'inherited' large portions of the AOL network who were infamous for the low mtu value and merged them with their own.  Interestingly, Aol were actually one of the first ISPs to start installing their own MSANs into exchanges.   It was all done very quietly many years ago and because it was only adsl and no phone involved, few aol customers even knew they were LLU.   

[kitz]

Had to start another post to add a link
http://www.kitz.co.uk/adsl/telephone_exchange.htm

Near the bottom look at the photo of the fibre optic tray, showing the feed for the aol and TT msan both going over the same backhaul.  I was surprised to find out that aol was in most of the local exchanges around here, before tiscali, TT or BE, it was all done so quietly and before we even had SAMs checker, so no one ever possibly realised the extent of the aol network.     That photo is now quite old, but shows even back then how close the aol and TT networks were and how much they shared common equipment.

[Blackeagle]

 I was under the impression that talktalk was PPPOA only.

No.  It's been a while since I was on ADSL and even longer since I messed around with the TT network, but I do recall raising to them the fact that PPPoE worked as well as PPPoA.  As far as I recall, the response was something like "Yeah, we know it does, but it's not something we're looking to 'fix' anytime soon" !!

Some people have reported better connections using PPPoE, but this may simply be a 'placebo' effect.  See this thread for example, on their horrible looking forum !!

[Chrysalis]

wow what have they done to their forum, another site that has been mobilised.

[burakkucat]

I seem to remember, about four years ago, deciding to let the "Wizard" in a Netgear DG834Gv4 determine the configuration parameters for a TT LLU'd service. It was a surprise to see that PPPoE had been chosen. 

[kitz]

Over the years TT have taken over many other SP networks and at least for a while (years) left the old network more or less 'as was'.   It was tiscali (and the TT /ex Tiscali ) part of their network that would only work using pppoa, vcmux.

Btw the BTW network will also work when the router is set up as pppoe. As confirmed in about 2006 when plusnet moved some of their customers to tiscali retail network and suddenly their connection no longer worked.   

The reason was as a result of many routers auto configs...  What happens is during auto config, the router tries pppoe first and if it works continues to use it, if it doesn't it tries pppoa.   Whilst BT is actually pppoa, it also allows pppoe by emulating pppoe (PPPoEoA).  The solution for PN /tiscali LLU customers was to simply manually set their router to pppoa.