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[aesmith]

Are you using an IP MTU below 1492 bytes? Is that for IPv4, IPv6 or both? What are the circumstances ? Any problems encountered in the Internet as a result

Not me but a customer, on their WAN they have MTU 1476, MSS 1360.   They had some odd malfunctions until we forced that MSS.  On their Internet VPN they have MTU 1434 with the same MSS over-ride.  It's been a while so can't remember, maybe we set MSS deliberately low on the WAN so it's the same as the VPN.  Wouldn't swear to that.  I just remember we had to set that MSS over-ride when the started using DMVPN over the private WAN around 5 years ago.

[burakkucat]

I use an MTU of 1478, <snip>

I need a memory refresh, please.

I know that an ATM cell is 53 bytes, made up of a 5 byte header and a 48 byte payload. Dividing the "magic number", 1500, by 48 and rounding down to an integer gives 31. Multiplying 31 by 48 gives 1488.

So what is the reason for (or the significance of) the 10 byte difference?   

[Weaver]

[aesmith]

My example was all IPv4.  As for why it wasn't done automatically I don't really know, all I know is that we've seen occasional funnies with tunnels or VPNs, for example, one customer everything worked except RDP.  I'm going blame Microsoft IP stack and applications, given that I've seen Lync think it's a good idea to send video in 9K jumbo frames - with do not fragment set!

[Weaver]

[Weaver]

[burakkucat]

[Weaver]

[aesmith]

Just out of interest I checked our LTE interface at home, never having done so before.  It reports 1500 bytes MTU, and if I not losing my marbles that's confirmed by being able to ping with data length of 1472 without fragmentation.  Are you factoring in L2TP for you 3G, or does it just have a low MTU for some reason?

[Weaver]

The 3G USB NIC just has a really annoyingly low IP MTU anyway, 1440 iirc.