SixXS::Sunset 2017-06-06

setting mtu
[pl] Carmen Sandiego on Wednesday, 19 October 2011 22:09:44
is it possible to set different MTU values for ipv4 and ipv6 on one interface? I did it on windows but I'm wondering how to do that in unix.
setting mtu
[ch] Jeroen Massar SixXS Staff on Wednesday, 19 October 2011 22:21:35
Which UNIX? Also, as MTU is an interface level thing, it is interface specific and should be the same for all the NICs connected to that Ethernet. Thus what kind of odd situation do you have where you need to have different IPv4 and IPv6 MTUs? Note that if there is an upstream MTU change, Path MTU discovery will take care of it. As such, Ethernet interfaces should always have 1500, unless you have Jumbo-frames enabled on that L2 segment.
setting mtu
[pl] Carmen Sandiego on Wednesday, 19 October 2011 22:35:58
FreeBSD 8.2 I wanted to set mtu=1500 for ipv4 traffic and 1428 for ipv6 traffic (I use ayiya). should I both leave with 1500?
setting mtu
[ch] Jeroen Massar SixXS Staff on Wednesday, 19 October 2011 23:18:52
As the MTU is applied to the interface, it has to be the MTU of that link. If it is an Ethernet Interface it thus is 1500, always (except as mentioned above for jumbo frames) One then configures the TUNNEL interface to have an MTU of 1428. AICCU will already do this for you btw. (though I am not sure if that is really the case on FreeBSD now) When a packet from your Ethernet then hits the 1428 ayiya interface that host will reply big with a ICMPv6 Packet Too Big (max = 1428) and voila, other hosts have learned that they should make packets a max size of 1428. Long live Path MTU Discovery (one of the major reasons to not blindly filter out ICMP)
setting mtu
[pl] Carmen Sandiego on Wednesday, 19 October 2011 23:21:52
then I have to change firewall rules for icmpv6 thanks, I wasn't sure it works that way
setting mtu
[pl] Carmen Sandiego on Wednesday, 19 October 2011 23:54:17
one more question: am I able to check of actually mtu transfered packed size? tcpdump? wireshark?
setting mtu
[ch] Jeroen Massar SixXS Staff on Thursday, 20 October 2011 08:13:32
Both can work; do note that it is a maximum, thus packets can be smaller.

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