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MTU: Difference between revisions
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Make it clearer that RFC4638 doesn't help for generic Ethernet links, just for PPPoE links.
m (Packets don't have to be 1500 octets - they can be smaller) |
m (Make it clearer that RFC4638 doesn't help for generic Ethernet links, just for PPPoE links.) |
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There are also ways to do over sized PPPoE using baby jumbo frames on [[Ethernet]]. The [[Ethernet]] specification still says 1,500 bytes maximum even for gigabit speeds, but it common for gigabit equipment to support jumbo frames - i.e. larger [[Ethernet]] packets typically up to around 9,000 bytes. This is more than you need to do PPPoE with 1,500 bytes - you only need 1,508. However there are other wrapping and tunnelling cases where just a bit more is useful. Baby jumbo support normally means a bit more than the usual 1,500.
As there is no real way to tell if baby jumbo frames are supported on an [[Ethernet]], RFC4638 defines an extra option for PPPoE to negotiate this at the [[Ethernet]] level. Of course two ends could simply agree to handle slightly larger [[Ethernet]] frames by configuration as well. Sadly this is not always the same level of operation or the same equipment that does the MRU negotiation at the PPP level, and if that knows PPPoE is involved it will not negotiate more than 1,492 MRU as per RFC2516. So typically some configuration is needed.
The upshot of all this? It is possible to get BT FTTC (Fibre to the Cabinet) circuits (which use PPPoE) working on full 1,500 byte PPP by using modified pppd on the customer end, a suitable network card that will handle 1,508 byte frames at 10/100Mb/s. We have done this! (Thanks to TonyHoyle and Simon, customers on irc, for tweaking pppd and testing this for us). The new FireBrick does, of course, support PPPoE with baby jumbo frames to handle 1,500 byte MTU and even bonds multiple lines. Using the right modem (and the DLINK 320B in bridge mode do this) you can negotiate and use 1508 bytes over ADSL as well.
=What is my MTU?=
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