baby jumbo frame

[ramzez]

Hi, just curios I think BT Wholesale supports baby jumbo frames 1500 MTU over PPPoE connetion, do we know if this is reliable to use if my router supports it or it's best to be avoided?
Thanks.

[dseven]

Interesting question. I tried it, for giggles, and it seems to work....

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# ping -M do -s 1472 1.1.1.1
PING 1.1.1.1 (1.1.1.1) 1472(1500) bytes of data.
1480 bytes from 1.1.1.1: icmp_seq=1 ttl=55 time=16.8 ms
1480 bytes from 1.1.1.1: icmp_seq=2 ttl=55 time=17.0 ms
1480 bytes from 1.1.1.1: icmp_seq=3 ttl=55 time=17.0 ms

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root@opnsense:~ # tcpdump -nnvei pppoe0 host 1.1.1.1
tcpdump: listening on pppoe0, link-type NULL (BSD loopback), capture size 262144 bytes
09:21:02.772917 AF IPv4 (2), length 1504: (tos 0x0, ttl 63, id 0, offset 0, flags [DF], proto ICMP (1), length 1500)
    78.32.x.x > 1.1.1.1: ICMP echo request, id 43254, seq 1, length 1480
09:21:02.789324 AF IPv4 (2), length 1504: (tos 0x0, ttl 56, id 25553, offset 0, flags [none], proto ICMP (1), length 1500)
    1.1.1.1 > 78.32.x.x: ICMP echo reply, id 43254, seq 1, length 1480

It would be good to know if it's officially supported (by Aquiss)...

[ramzez]

yeah I read about it on some forums, first but then also on pfsense and later on AA ISP https://support.aa.net.uk/MTU

[Martin]

Good Morning

Yes this should work fine with us and our network is setup to support it.

The "baby jumbo" bit just means setting the CPE's physical WAN port MTU to 1508, giving you the extra overhead to allow setting the pppoe interface to 1500 so you don't get any fragmentation, and get true 1500 MTU to the internet.

I do this at home myself, works fine with our LNS's on BTW backhaul. We can also see several users doing this on CityFibre products too at the moment.

[dseven]

Great! I'll keep it in place then. Thanks, Martin... and ramzez for bringing up the topic!