Hi,
The commands you provide give this:
cat /proc/sys/net/ipv6/conf/enp3s0f1/accept_ra_mtu
1
cat /proc/sys/net/ipv6/conf/all/accept_ra_mtu
1
I have had run these commands on multiple computers running Debian 13.2:
sudo tcpdump -i enp3s0f1 -vv icmp6 and 'ip6[40] == 134'
tcpdump: listening on enp3s0f1, link-type EN10MB (Ethernet), snapshot
length 262144 bytes
16:11:30.899059 IP6 (flowlabel 0x188f8, hlim 255, next-header ICMPv6
(58) payload length: 160) fe80::7476:17ff:fe73:320d > ip6-allnodes:
[icmp6 sum ok] ICMP6, router advertisement, length 160
hop limit 64, Flags [managed, other stateful], pref medium, router
lifetime 165s, reachable time 0ms, retrans timer 0ms
source link-address option (1), length 8 (1): 76:76:17:73:32:0d
0x0000: 7676 1773 320d
mtu option (5), length 8 (1): 1468
0x0000: 0000 0000 05bc
prefix info option (3), length 32 (4): 2a05:f6c3:427e:1::/64,
Flags [onlink, auto], valid time 167s, pref. time 167s
0x0000: 40c0 0000 00a7 0000 00a7 0000 0000 2a05
0x0010: f6c3 427e 0001 0000 0000 0000 0000
prefix info option (3), length 32 (4): fde9:9ef3:3ff3:1::/64,
Flags [onlink, auto], valid time infinity, pref. time 604800s
0x0000: 40c0 ffff ffff 0009 3a80 0000 0000 fde9
0x0010: 9ef3 3ff3 0001 0000 0000 0000 0000
prefix info option (3), length 32 (4): fde9:9ef3:3ff3::/64, Flags
[onlink, auto], valid time infinity, pref. time 604800s
0x0000: 40c0 ffff ffff 0009 3a80 0000 0000 fde9
0x0010: 9ef3 3ff3 0000 0000 0000 0000 0000
rdnss option (25), length 24 (3): lifetime 165s, addr: router1.lan
0x0000: 0000 0000 00a5 fde9 9ef3 3ff3 0001 0000
0x0010: 0000 0000 0001
advertisement interval option (7), length 8 (1): 55000ms
0x0000: 0000 0000 d6d8
^C
1 packet captured
1 packet received by filter
0 packets dropped by kernel
So, Debian should accepts router advertisements, and it also recieves
them, but it stays on mtu 1500
Den 17.11.2025 kl. 15.07 skrev [email protected]:
On Mon, Nov 17, 2025 at 01:29:42PM +0100, Kenneth wrote:
Hi,
New on this list, so sorry if this topic already is active.
Im just trying to configure my network, which is with multople routers
running OpenWRT 24.10.3.
I use B.A.T.M.A.N. on this, so the MTU is set to 1468, and the routers do
send RAs, but my devices running Debian 13.x does not seem to change their
MTU to 1468.
Is this a known issue?
Hm. No idea. What does cat /proc/sys/net/ipv6/conf/<your
interface>/accept_ra_mtu
say? And cat /proc/sys/net/ipv6/conf/all/accept_ra_mtu?
(stabbing in the dark...)
Cheers
