Maximum MTU for PPPOE

[#1]

I have previously read that the maximum MTU size should be 1492 with the other 8 for the header
Sky and TalkTalk recommend an MTU of 1500 for their routers
This leads me to the obvious question
Should a router on Plusnet Fibre be set to 1492 or 1500 (note that with the 2704n it appears to be locked and hidden)
If the router is set to 1500 should the client machines also be set to 1500 or to 1492

[#2]

It doesn't really matter with IPv4 because of the way packets fragment, but it's an issue with IPv6. I set everything my end to 1500 though.
You can check the 2704N by sending an do-not-fragment ping to see the MTU size, e.g. ping -f -l 1500 192.168.0.1 (keep lowering the packet size until you get a ping response).

[#3]

I thought the 1492 limit was to allow for the Ethernet link between the router and the separate modem. A combined modem/router should be capable of 1500. But I can't find a nice clear statement in any official documentation to confirm that.
Wouldn't pinging the router's LAN IP be pointless for this? - you need to ping something that's going over the WAN connection, i.e. something on the Internet.

[#4]

The WAN and LAN/br0 are normally set the same though. But, I see what you're saying about pinging an external address.

[#5]

use 1492 unless you have applied the known workarounds, if you unaware of these workarounds then you probably didnt set them.
as andy says ipv6 is affected much worse than ipv4, but ipv4 can be problematic as well with incorrect mtu.

[#6]

Do find the 1508 better which  chrcoluk hints at  

[#7]

I've been looking at MTU for a little while today and came up with a few results that don't add up.
1450 is as far as I can get while testing.
[tt]ping -c 3  -M do -s 1450 www.bbc.co.uk
PING www.bbc.net.uk (212.58.244.69) 1450(1478) bytes of data.
1458 bytes from bbc-vip114.telhc.bbc.co.uk (212.58.244.69): icmp_req=1 ttl=54 time=12.5 ms
1458 bytes from bbc-vip114.telhc.bbc.co.uk (212.58.244.69): icmp_req=2 ttl=54 time=11.6 ms
1458 bytes from bbc-vip114.telhc.bbc.co.uk (212.58.244.69): icmp_req=3 ttl=54 time=12.1 ms
--- www.bbc.net.uk ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2002ms
rtt min/avg/max/mdev = 11.611/12.089/12.543/0.380 ms
[/tt]
[tt]« SpeedGuide.net TCP Analyzer Results »
Tested on: 2015.06.23 09:38
IP address: 87.114.xx.xx
Client OS/browser: Mac OS (Safari 8.0.5)

TCP options string: 0204059e010303050101080a073d35e10000000004020000
MSS: 1438
MTU: 1478
TCP Window: 131168 (NOT multiple of MSS)
RWIN Scaling: 5 bits (2^5=32)
Unscaled RWIN : 4099
Recommended RWINs: 63272, 126544, 253088, 506176, 1012352
BDP limit (200ms): 5247kbps (656KBytes/s)
BDP limit (500ms): 2099kbps (262KBytes/s)
MTU Discovery: ON
TTL: 49
Timestamps: ON
SACKs: ON
IP ToS: 10000000 (128)
    Precedence: 100 (flash override)
    Delay: 0 (normal delay)
    Throughput: 0 (normal throughput)
    Reliability: 0 (normal reliability)
    Cost: 0 (normal cost)
    Check bit: 0 (correct)
DSCP (DiffServ): CS4 100000 (32) - class 4 (RFC 2474). Similar forwarding behavior to the ToS Precedence field.[/tt]
So... 1450 with overheads comes to 1478 which is what I get from SpeedGuide.net, however, when I test here http://www.letmecheck.it/mtu-test.php It gives me a result as follows...
"From the tests we did, we can assume that 1464 bytes is the largest unfragmented packet
size. The MTU size would be 1492, made up from 1464 payload and 28 ICMP/IP Headers
and payload information."
Any pointers please ?

[#8]

Quote from: 11110_110

however, when I test here http://www.letmecheck.it/mtu-test.php It gives me a result as follows... "From the tests we did, we can assume that 1464 bytes is the largest unfragmented packet size. The MTU size would be 1492, made up from 1464 payload and 28 ICMP/IP Headers and payload information." Any pointers please ?

Is that with the 2704n router?
It may not work with that router because the site needs the router to reply to pings from WAN.
With my router's DOS protection enabled http://www.letmecheck.it/mtu-test.php gives this:

Quote

From the tests we did, we can assume that 1453 bytes is the largest unfragmented packet size. The MTU size would be 1481, made up from 1453 payload and 28 ICMP/IP Headers and payload information.

With DOS protection disabled http://www.letmecheck.it/mtu-test.phpt gives this:

Quote

From the tests we did, we can assume that 1472 bytes is the largest unfragmented packet size. The MTU size would be 1500, made up from 1472 payload and 28 ICMP/IP Headers and payload information.

Both are wrong the routers actual MTU is 1492.
That sites test is not to be trusted IMO.

[#9]

Yes my 2704n has Wan ping enabled.
Ping Results:
PING 87.114.x.x (87.114.x.x) 56(84) bytes of data.
64 bytes from 87.114.x.x: icmp_seq=1 ttl=51 time=117 ms
64 bytes from 87.114.x.x: icmp_seq=2 ttl=51 time=116 ms
64 bytes from 87.114.x.x: icmp_seq=3 ttl=51 time=116 ms
64 bytes from 87.114.x.x: icmp_seq=4 ttl=51 time=116 ms
64 bytes from 87.114.x.x: icmp_seq=5 ttl=51 time=116 ms
--- 87.114.x.x ping statistics ---
5 packets transmitted, 5 received, 0% packet loss, time 4006ms
rtt min/avg/max/mdev = 116.709/116.886/117.343/0.319 ms
http://www.speedguide.net/networktools.php TCP/IP Analyzer
is telling me…..
MTU = 1478
MTU is not fully optimized for broadband. Consider increasing your MTU to 1500 for better throughput. If you are using a router, it could be limiting your MTU regardless of Registry settings.
MSS = 1438
Maximum useful data in each packet = 1426, which is less than MSS because of Timestamps, or other TCP/IP options used.
Macbook is giving me MTU 1478 with overheads,  1450+28
ping -D -s 1450 www.bbc.co.uk
PING www.bbc.net.uk (212.58.244.27): 1450 data bytes
1458 bytes from 212.58.244.27: icmp_seq=0 ttl=53 time=12.093 ms
1458 bytes from 212.58.244.27: icmp_seq=1 ttl=53 time=14.926 ms
^C
--- www.bbc.net.uk ping statistics ---
2 packets transmitted, 2 packets received, 0.0% packet loss
round-trip min/avg/max/stddev = 12.093/13.509/14.926/1.417 ms
Which site would you not trust npr ?

[#10]

@11110_110 That could mean the MTU on your Mac is set to 1478, but your router 1492

[#11]

1492 is the largest safest setting, if you need to go lower then something isnt working right.
1500 is achievable when using jumbo frames.

[#12]

Quote from: 11110_110

Yes my 2704n has Wan ping enabled.

To save me a bit/lot of digging - how did you do it

[#13]

Quote from: 11110_110

Yes my 2704n has Wan ping enabled. Which site would you not trust npr ?

Didn't know that was possible with that router.
http://www.letmecheck.it/mtu-test.php does not give reliable results with my Zyxel router.
http://www.speedguide.net/analyzer.php I've been using this site since dialup days and it's always been correct for me.
If you're using a mac then I'll go with g1000 comments.
Windows machines from visa on will auto determine the MTU and would match the router setting.
@Oldjim
For what it's worth:
Plusnet TG582n router will by default use a MTU of 1492 when configured for pppoe (fibre).
When configured for ADSL it defaults to a MTU of 1500.
Don't know about the 2704n.

[#14]

I've sent you a pm Oldim, good luck.
Mac is set to 1500  
I'll test a bit more on another machine just incase it is that, more curiosity than anything else... thanks for your thoughts everybody.

[#15]

If you ping the router's LAN IP, is the MTU 1478? It could be the router's LAN MTU.