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[heavyrain]

Hello everyone

I am pretty new to understanding MTU but hopefully this is ok to ask.

I was curious about what happens if an MTU mismatch happens between the modem and the router (in regards to BT FTTC).

Scenario: You have a modem that operates at max of 1492 MTU (e.g. Billion 8800NL). The "router" in question is a Tp-link Deco product. It does not allow configuration of MTU. Apparently it defaults to 1500 and cannot be changed.

What happens in this regard? One would hope that the Tp-link in its non configurable state would automatically negotiate down to 1492? I don't have the device to test it.

Also something I wondered about PPPoE in relation to MTU size. I see alot of people in technical forums stating about baby jumbo frames and stuff which in turn means setting the mtu to 1508. I understand this because 8 bits get taken off due to 6 header + 2 ppp id to leave 1500.

However from my understanding, the PPPoE header resides within the actual payload amount of the l2 frame which according to wikipedia is 46-1500. Setting to 1508 then seems wrong because those 8 bytes basically are part of the payload anyway and already accounted for at a max of 1500 (i.e. so a max payload size should actually be 1500). I know I'm probably missing something there in the logic but I would also like to know why if you do account for pppoe outside the payload, shouldn't you also have to account for the 4 bytes of ptm transmission (s, ck, ptm-fcs) and actually set 1512 as the mtu?

Thanks

[Chrysalis]

There is a negotiation e.g. even if your router was set at 1492 to match the modem, client machines would still likely be at 1500 (windows as an example) and they negotiate downwards.

[j0hn]

The question is somewhat phrased wrong.

Your can't really have a modem set at 1492 and a router set at 1500.
Your modem in bridge mode will be set at 1500 by default and any changes you make to that should be ignored.

The router will default to 1500 the same as the modem.
As you are using PPP then 8 bytes are used by the router which brings the MTU to 1492.

Most things on the internet will run at 1500 MTU but because you are using PPP and are limited to 1492 then both ends of the link talk to each other and negotiate a 1492 MTU.

Some routers will use 1500 MTU by default but will automatically drop the MTU to 1492 when PPP is used.

Many of us use modems that support a 1508 MTU (often referred to as baby jumbo frames).
This means if the MTU is set to 1508 on the router you still get a 1500 MTU after PPP is used.