I've been searching for hours and just can't seem to get to the bottom of this one!
I am connected to an ISP which supports RFC 4638, but I'm trying to figure out exactly which Cisco I need to buy to support this. This RFC is related to using MTU 1500 over PPPoE by using "baby jumbo frames" I'd assume the ethernet MTU would effectively end up being 1508 in order to accomodate the PPPoE header.
Cisco does support RFC 4638 with "tag ppp-max-payload", but doing so requires an ethernet interface which can ingress/egress frames (slightly) larger than 1500 bytes. What I cannot figure out, is which routers can do this and on what ports. It seems fairly likely based on various internet chatter that the large ISRs support up to MTU 1600 on the FE ports, which would definitely do the trick. I have no doubt that this would work just fine on 1841/1921, but I'd rather not have to have such a large/power hungry piece of kit where it really isn't necessary. What of the 800 series?
I can confirm, and fully expect this to be impossible on the 10/100 switch ports (FastEthernet0-3), as the chip they're attached to (usually something like 88E6063 plain doesn't support it) but there is no such hardware limitation on the FE port attached directly to the CPU (WAN/FastEthernet4)
The question is basically: Has Cisco actually implemented "baby jumbo" (1500-1600 byte) frames on the 800 series FE WAN port, that would be needed to support the "tag ppp-max-payload" command! Thought this'd be an easy one!
After getting one to experiment with, I think I can now answer my own question.
Running 15.1(4)M2, Attempts to set MTU on FE4 resulted in "Interface FastEthernet4 does not support user settable mtu.". This lead me to suspect that all 5 of the rear ports are connected to the L2 switch, with a little software trickery to make FE4 appear like a real standalone ethernetnet interface, when it isn't. Likely to save $1 on parts.
Examination of the routers internals confirms this. Given that there's no "system mtu" configuration option, every port will always be stuck on MTU 1500.
(Interestingly there's an alternate path from FE4 back to the CPU, via a 'non-fitted' chip about the right size to be a 10/100 PHY. Maybe Cisco were going to give it a real WAN port but changed their mind at the last minute)
So does this mean RFC 4638 would work? If so, I have no idea how; but I suspect, probably not. Time for an ISR...
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