[SOLVED] MTU not what is set in ifcfg file

bradvan · 04-04-2019, 06:53 AM

Our kickstart server sets the MTU of active interfaces to 9000. This works fine on all of our existing locations. Recently we expanded to a new remote site. They don't want to support jumbo frames, so we have to keep the default MTU setting. In a post script, I edit the ifcfg file to change the MTU setting to 1500. I can see that the setting is changed from the actual file and the post log. NetworkManager has been disabled. However, after the system reboots, the NIC MTU is set to 9000. If I get on the console and do an ifdown eno1;ifup eno1, then the MTU changes to 1500. However, if I then do a systemctl restart network or simply reboot, MTU is back to 9000. Does anyone have a clue what keeps setting the MTU to 9000?

System has CentOS 7.4 installed.

bradvan · 04-04-2019, 07:37 AM

Found the answer. Even though we disabled NetworkManger, it was up and running at kickstart time and MTU was set to 9000 then. Not sure why after it has been disabled it still affects things? I restarted NetworkManager and ran:

Code:

# nmcli connection show eno1 | grep mtu
802-3-ethernet.mtu:       9000
802-3-ethernet.mtu:       1500

Not sure why there are two entries? I then ran:

Code:

# nmcli connection modify eno1 802-3-ethernet.mtu 1500
# nmcli connection show eno1 | grep mtu
802-3-ethernet.mtu:       1500
802-3-ethernet.mtu:       1500
# systemctl stop NetworkManager
# systemctl restart network
# ifconfig eno1
eno1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500

Not sure why NetworkManager should be affecting things when it has been disabled? I will have to add that to the post script.

tyler2016 · 04-04-2019, 07:45 AM

I have never used kickstart, but is there some reason you can't configure it to not install the NetworkManager* packages?