Openreach Fibre PPPoE MTU of 1500?

[njh]

Is having a PPPoE MTU of 1500 (and Ethernet MTU of 1508 to the ONT) well supported these days on Openreach FTTP? Any gotchas?

I think it tried it years ago on DSL but ran into problems...

Thanks!

[nowster]

Ours seems to have negotiated 1492:

Code Select Expand

13: ppp0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1492 qdisc pfifo_fast state UNKNOWN mode DEFAULT group default qlen 3

[njh]

Yeah, 1492 is the default. I guess there isn't an automatic way of knowing if Ethernet between the ONT and your router supports Baby Jumbos / RFC4638.

I believe using a PPPoE MTU of 1500 improves performance by matching the standard 1500-byte MTU of most networks. Standard PPPoE typically forces a smaller 1492-byte MTU, causing fragmentation of larger packets, which are common on LANs.

No fragmentation means less processing for the routers - so I think it is worth giving it a go.

[nowster]

If the kit at both ends supports Path MTU Discovery, there's no fragmentation.