OK, my team has done some extensive research and analysis and this is the current position. I apologise for some techy bits but it is needed.
Everyone knows that data is transferred on modern networks using packets which have certanl characteristics. The packets have lengths and, within them, contain the data and some control bits. The exact make up is not need for this discussion. One of the control bits within each packet is a "do not fragment" flag which can be set to off or on. We think the VSS has fixed sized packets of 1500 MTU and the eF flag is set to disallow (i.e. do not fragment).
The two wide area lan confifurations in the UK use PPPoA and PPPoE as the format standard. Most UK hoseholds use PPPoA which is a hangover from old style telephony equiment which uses ATM. Nothing wrong with that.
If you connect the VSS to an ATM style connection using PPPoA, such as ADSL systems in the UK, the MTU can be set up to 1500 without the need to fragment. Even though the VSS packets do not allow fragmentation this was never a problem as none of the packets needed any and would be sent on their way by the broadband router (I assume for this discussion that all correct ports are open and the VSS is set up correctly).
However, the BT Infinity product uses PPPoE which meets the international standards and has a maximum MTU of 1492 which means any packets which are presented to it which are larger that this (called Jumbo packets) need to be fragmented. There is no way that you can send an un-fragmented packet of MTU 1500 through a properly configured PPPoE network such as the FTTC BT Infinity one.
So, any router confronted with this problem usually fragments the packet. This is a little inefficient but it does allow the data to pass (whether in a VPN tunnel or not). As some of the data sent is not superfluous, the speed will diminish but most will not see this effect.
With the VSS, the MTU 1500 packets need to be fragmented. But the control bit within each packets dissallows fragmentation. Any modern router cannot deal with this issue and just discards the packet presented in this way. They can do nothing else.
Some older routers do allow you to ignore the eF control bit and fragment the packet anyway. BT's older hubs do this to allow for older equipement using ATM connections and MTU's of 1500. The newer routers, including BT's latest version 3, 2Wire routers stick to the standard completely and do not support legacy ATM products (such as the VSS). Modern euipment, such as PC's etc use a facility called MTU Path Discover which tests the path of the packets to their destination and set their MTU (i.e. packet length) to the minimum that they find. This is why anything running Windows XP or 7 works fine. Other than the VSS we have had no other issues swapping from PPPoA to PPPoE.
There are four possible solutions to this, some of which people have already found.
1) Use a router (usually and older one) which ignores the eF flag and fragments packets anyway down to the PPPoE standard of 1492 or less. This does waste packets but it will work.
2) Find a router as in 1) above but put it as an interface between the VSS and your network (set the eF flag to ignore within the router software). This will fragment the packets down to a lower number and the BT Infinity router sees it as normal traffic.
3) Get Draytek (and others) to allow for a eF flag enable/disable oiption in their software which then will fragment the packets ignoring the eF flag set within the packets themselves.
4) Get Vodafone to implement MTU Path Discover within the software of the VSS and set the MTU to the minimum value found, i.e. an update to the network management software to meet the more modern standards.
We are looking for an interface router which will allow for legacy ATM products (such as telephone exchanges of older design). We think a Buffalo Airstation has the necessary requirements but, as yet, can't find one to try.
I'll keep you all informed.
I can't believe in this day and age we are dependent on luck?
