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direct PPPoE gets 77/19 but PPPoE through any router gets 30/16?

nhackett
Member

‎18-02-2013 12:18 AM - edited ‎18-02-2013 12:37 AM

I have Infinity 2 and my line speed is fine. If I connect using a windows PC directly to BT's VDSL modem (logging in with PPPoE directly) I get 77/19 compared to a theoretical 80/20.  Tested both on BTspeedtester and speedtest.net. Have also treid direct "fetch" (All figures Mbps unless otherwise stated)

 

However if I connect via ANY router (with the router logging in via PPPoE) the speed drops to 28-33/15-17.  For the upload this seems reasonable to drop a little for the extra overhead, but the download is losing more than half the speed.

 

I have tried various routers with similar results - including hardware based and Linux and BSD based. Have tried different NICS. All of these routers are capable of much higher speeds - subsituting internal server for fttc sees speeds in excess of 400.

 

I am beginning to suspect MTU - have been using normal 1492 on all routers, as per RFC1516. I gather that BT's implementation of fttc may be compatible with MTU up to 1500 as per RFC 4638 - so direct connection to PC may use this.Is this correct and could it be relevant?

 

For example - is it possible that upsteram from VDSL modem uses 1500 - which is passed through with direct PC connection, but via router (with MTU of 1492) fragmentation occurs - reducing the speed?

 

Frankly I am at a loss to know how to even determine the real cause - let alone fix it. It may well be nothing to do with MTU - but I have no other even vague ideas?

 

Any suggestions as to either how to fix the problem - or at least how to further troubleshoot?

 

Thanks to anyone in advance for suggestions

 

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2 REPLIES 2

MHC
Guru

on ‎18-02-2013 10:48 AM

 

 

The BT FTTC Hub (2wire 2701) will default to an MTU of 1492 so any PC needs to be that or lower.

 

 

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nhackett
Member
In response to MHC

‎18-02-2013 05:06 PM - edited ‎18-02-2013 05:36 PM

The only MTU setting I don't know for sure is direct PC PPPoE connection but I guess this defaults to 1492.  In any case - as this is direct PC direct to VDSL modem using PPPoE (No HUB) this is the only one that gives maximum speed.

 

All the other settings are definitely 1492 - as per my message above.

 

 

ADDITIONAL

I suddenly thought - why not check mtu under PPPoE.  So I connected direct from PC to VDSL modem using PPPoE.  Checked speed - 76/18 - pretty good.  Then checked mtu (netsh interface ipv4 show subinterfaces) - very surprised to see that PPPoE was 1480. Normally win7 would give 1500 for LAN interface.   Not sure what this means - but I will try setting different routers to lower mtu to see if any effect.

 

 

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