[solved] Different MTU for IP v4 & v6 possible?
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I noticed that my first router will set different MTU for IP v4 & v6 on its LAN. It is a Fritzbox which usually knows what the local ISP wants and what is optimal.
My ISP uses PPPoE.PS C:\Users\Nutzer> Get-NetIPInterface | Format-Table -AutoSize ifIndex InterfaceAlias AddressFamily NlMtu(Bytes) InterfaceMetric Dhcp ConnectionState PolicyStore ------- -------------- ------------- ------------ --------------- ---- --------------- ----------- 14 Ethernet IPv6 1492 25 Enabled Connected ActiveStore 1 Loopback Pseudo-Interface 1 IPv6 4294967295 75 Disabled Connected ActiveStore 14 Ethernet IPv4 1500 25 Enabled Connected ActiveStore 1 Loopback Pseudo-Interface 1 IPv4 4294967295 75 Disabled Connected ActiveStoreSo I guess it makes sense that with IPv6, where there is no NAT and a GUA-Prefix gets delegated, a reduced MTU is used because of PPPoE.
When I tried to mimic this with pfSense, which is behind that router, and Custom DHCP Options, I had no luck.
It would be good if it would work automagically.
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J johnpoz moved this topic from DHCP and DNS on
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Hmm, not seen that. Is IPv6 using dhcpv6 over PPPoE there? Other way around perhaps?
I'd expect IPv4 to use MTU 1492 unless the have mini-jumbo frames (1508).
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@stephenw10 said in Different MTU for IP v4 & v6 possible?:
I'd expect IPv4 to use MTU 1492 unless the have mini-jumbo frames (1508).
Fritzbox is not very open, so I can only show you what is going on at a client on LAN-side, not WAN. I don't think they will use mini-jumbo frames. The ISP is using which most vDSL-providers use around here. And those will use the same settings for fiber, too, like PPPoE etc.
Hmm, not seen that. Is IPv6 using dhcpv6 over PPPoE there? Other way around perhaps?
Last time I used pfSense directly with that ISP, I had to activate Dhcpv6 and "Use IPv4 connectivity as parent interface" and the Fritzbox shows that "DHCPv6 Rapid Commit" is used.
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@Bob-Dig said in Different MTU for IP v4 & v6 possible?:
I noticed that my first router will set different MTU for IP v4 & v6 on its LAN.
That doesn't make sense. MTU refers to the size of the Ethernet payload, which should be the same for IPv4 and IPv6. Also, NAT has nothing to do with MTU size.
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@JKnott said in Different MTU for IP v4 & v6 possible?:
That doesn't make sense.
But packets are IPv 4 or 6, not both. I don't know this stuff, not working in tech, but I doubt the fritzbox would do that by mistake.
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I'm not saying it's not possible as the MTU is provided by 2 different methods in IPv4 & IPv6, so different values could be provided. However, the question is why? Back when I was using a tunnel for IPv6, the MTUs were different. Is there some sort of tunnel for one or the other? 1492 is often used for PPPoE. However, if it applies for one, it should apply for the other. Maybe whoever wrote Fritzbox goofed.
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@JKnott said in Different MTU for IP v4 & v6 possible?:
1492 is often used for PPPoE. However, if it applies for one, it should apply for the other.
I am sure they use 1492 for IPv4 on WAN too. But they don't use it for LAN IPv4. So it is a design consideration, maybe because they can. I saw and liked it. Now I want to replicate.
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@Bob-Dig said in Different MTU for IP v4 & v6 possible?:
Now I want to replicate.
Holy. PfSense is already doing it too?
I changed the MTU on pfSense WAN and I see basically the same on my pfSense LAN. So maybe it has nothing to do with anything.
PS C:\Users\Bobby> Get-NetIPInterface | Format-Table -AutoSize ifIndex InterfaceAlias AddressFamily NlMtu(Bytes) InterfaceMetric Dhcp ConnectionState PolicyStore ------- -------------- ------------- ------------ --------------- ---- --------------- ----------- 6 Ethernet 2 IPv6 1492 25 Enabled Connected ActiveStore 1 Loopback Pseudo-Interface 1 IPv6 4294967295 75 Disabled Connected ActiveStore 6 Ethernet 2 IPv4 1500 25 Enabled Connected ActiveStore 1 Loopback Pseudo-Interface 1 IPv4 4294967295 75 Disabled Connected ActiveStoreMaybe it's Windows.
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Looks like it is a Windows thing and has nothing to do with pfSense, Fritzbox etc.
Marked this thread as solved. -
Mmm, curious. I would have expected the same for both.
More likely I might have expected v4 at 1492 and v6 (perhaps incorrectly) at 1500 because a common setup if for v6 to use dhcpv6 over the pppoe once it's established.
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To conclude things, the effect that I saw last, that pfSense LAN was also showing different MTU sizes in Windows came from me changing the MTU on WAN to 1492. WAN was the parent interface of my LAN, which is a VLAN.
So changing the MTU of the parent also changes MTU on all the VLANs.
Realizing this made me change my WAN also to a VLAN and not using the parent at all.
Now only the WAN has 1492 (set by me), all the others have their usual default which is 1500. And Windows is showing 1500 for both IPv4&6.
This shows that the fritzbox is using 1492 for the LAN too, which Windows will change to 1500 for IPv4 for whatever reason. -
@Bob-Dig said in [solved] Different MTU for IP v4 & v6 possible?:
So changing the MTU of the parent also changes MTU on all the VLANs.
Yep. The only difference is the VLAN tag, which adds an extra 4 bytes to the frame, but does not affect the underlying MTU. They are not separate physical interfaces.
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@JKnott said in [solved] Different MTU for IP v4 & v6 possible?:
They are not separate physical interfaces.
Good to know.
Maybe it would be better if I had not changed from the parent to a VLAN. Because now, if I want to change the MTU, I will have to do it for every VLAN, which is a hassle.
But than, I only use IPv6 on three VLANs right now and for the rest, Windows would use 1500 for IPv4 anyways, so everything is good I guess.MTU kinda is a rabbit-hole...
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There is some info here:
IEEE 802.1QThis shows an Ethernet frame both without and with VLANs. You can see where the tag goes. The size of the payload is what is determined by the MTU.
If you run Wireshark you can examine the frames and see the VLAN tags and more.
