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MTU: Difference between revisions
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clean up, typos fixed: over sized → oversized, thigns → things, However → However, (2)
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Unfortunately the specifications are not that helpful here. RFC1661 defines PPP and states If smaller packets are requested, an implementation MUST still be able to receive the full 1,500 octet information field. RFC2516 defines PPPoE The Maximum-Receive-Unit (MRU) option MUST NOT be negotiated to a larger size than 1,492. But they are not incompatible statements - negotiating 1,492 does not mean you don't have to accept 1,500 byte packets (as per RFC1661), but you can't send on if PPPoE bridging, for example, so logically you would have to fragment the IP packet. Most routers do not do that! This is one of the reasons we get problems with MTU being smaller than 1,500 bytes.
There are also ways to do
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.
rtt min/avg/max/mdev = 54.108/54.108/54.108/0.000 ms
The
If 1500 bytes cannot pass, then a ping result will look like:
time of writing (Dec 2014) TTW does not support an MTU of 1508, so users of PPPoE cannot achieve an MTU of more than 1492 bytes.
[Above is the official position. However, there are some experimental results which indicate one might be able to do better. Using a TG582n as the router running PPPoE
and configuring it to support baby jumbo frames:
:eth ifconfig intf=eth_WAN mtu=1508
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