Unable to create bridge using NetworkManager

I’m trying to work out why I cannot create a bridge for my Virtual Machines using NetworkManager.

I have now searched in several places for how to create a bridge network using NetworkManager to the point I cannot refer to a single tutorial. They are all more or less the same. Here are the steps that I do.

  1. Disconnect ethernet
    nmcli dev disconnect enp42s0
    
  2. Create bridge and slave
    nmcli con add type bridge ifname br0 con-name br0 autoconnect yes connection.zone home
    nmcli con modify enp42s0 master br0 connection.zone home
    
  3. Connect bridge and slave
    nmcli con up br0
    nmcli con up enp42s0-slave
    

Everything is successful except for the last command. It shows the following error:

Error: Connection activation failed: Unknown error
Hint: use 'journalctl -xe NM_CONNECTION=658de29d-4809-4113-9aa9-1f3f32ae40eb + NM_DEVICE=enp42s0' to get more details.

journalctl logs are not much more useful:

device (enp42s0): Activation: starting connection 'enp42s0-slave' (658de29d-4809-4113-9aa9-1f3f32ae40eb)
device (enp42s0): state change: disconnected -> prepare (reason 'none', sys-iface-state: 'managed')
device (enp42s0): state change: prepare -> config (reason 'none', sys-iface-state: 'managed')
device (enp42s0): state change: config -> ip-config (reason 'none', sys-iface-state: 'managed')
device (enp42s0): Activation: connection 'enp42s0-slave' could not be enslaved
device (enp42s0): state change: ip-config -> failed (reason 'unknown', sys-iface-state: 'managed')
device (enp42s0): released from master device br0
device (enp42s0): Activation: failed for connection 'enp42s0-slave'
device (enp42s0): state change: failed -> disconnected (reason 'none', sys-iface-state: 'managed')

For context I also asked this related question: NetworkManager showing different information than ip command

Asked By: MetalMathician

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I cannot believe this solved my problem!

sudo reboot

— Facepalm —

The reason why this did not happen earlier is that I kept deleting the bridge and slave, as it did not appeared to be working.

Answered By: MetalMathician
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