Subject: IPv6 MTU of a tunnel: 2 different switches, which wins?

From: Peter Bieringer <[email-masked]>
Date: Mon, 02 Sep 2002 09:10:19 +0200

Hi,

sure here anyone can help me:

Let's say I have an internet connection via PPPoE through ppp0/eth1

MTU eth1: 1500
MTU ppp0: 1492

Over this connection, an IPv6 (named tun6to4) tunnel will be
established. Now I have 2 different switches to adjust the MTU of the
tunnel.

a) using utility ip
b) using sysctl

After unmodified MTU setup:

a)
# ip link show dev tun6to4 | grep -w mtu
42: tun6to4@NONE: <NOARP,UP> mtu 1480 qdisc noqueue

b)
# sysctl -a| grep tun6to4 | grep mtu
net.ipv6.conf.tun6to4.mtu = 1480

But proper MTU will be

MTU tun6to4: 1472   MTU(ppp0) - 20 for IPv4 header

where to proper set now?

1) via ip
2) via sysctl in net.ipv6.conf.tun6to4.mtu
3) both switches

BTW: host should act either as router or standalone.

Thanks for any clarifies,

        Peter

---
Dr. Peter Bieringer
mailto: pb at bieringer dot de
http://www.bieringer.de/pb/
Key 0x958F422D : B501 24F4 9418 23E2 C0F3  F833 7B57 AA7B 958F 422D

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