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Asked: 2023-10-22 00:05:19 +0800 CST

NetworkManager is not changing MTU setting

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I'm trying to change my MTU size on my ethernet interface.

I'm just using Ubuntu 22 Desktop, i've set my MTU value to 10000 through the Gnome network settings: enter image description here

I've then restarted NetworkManager sudo restart NetworkManager.

My MTU does not seem to change when I check with ifconfig:

ifconfig | grep mtu
enp39s0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500

This is my whole NetworkManager profile from /etc/NetworkManager/system-connections/Wired connection 1.nmconnection:

[connection]
id=Wired connection 1
uuid=02d40a9e-d675-390b-a1d1-f28ff3b23880
type=ethernet
autoconnect-priority=-999
interface-name=enp39s0
metered=2
timestamp=1697903662

[ethernet]
mtu=10000

[ipv4]
address1=192.168.178.2/24,192.168.178.1
method=manual

[ipv6]
addr-gen-mode=stable-privacy
method=auto

[proxy]

I have not manually modified this file, only used the GUI.

The NetPlan configuration is unchanged from default:

# Let NetworkManager manage all devices on this system
network:
  version: 2
  renderer: NetworkManager

nmcli appears to be set correctly nmcli connection show 'Wired connection 1' | grep mt:
802-3-ethernet.mtu: 10000

I'm stumped, does the interface inside ifconfig not represent what NetworkManger changes? Has it actually been changed? I can't tell.

The output of ip -d link list dev enp39s0 is:

2: enp39s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP mode DEFAULT group default qlen 1000
    link/ether <MAC was here> brd ff:ff:ff:ff:ff:ff promiscuity 0 minmtu 68 maxmtu 9194 addrgenmode none numtxqueues 1 numrxqueues 1 gso_max_size 64000 gso_max_segs 64 parentbus pci parentdev 0000:27:00.0
networking

1 Answers

  1. Best Answer

    Maximum MTU supported by the card is 9194 bytes, as shown by ip -d link list dev enp39s0.

    Setting it to 10k fails, and thus the default of 1500B is used.

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