In fedora40 the IPv4 DAD (duplicate address detection) - or ACD (address conflict detection) was enabled by default.

This feature provides a mechanism to detect address conflicts by sending ARP probes for each IP address it wants to use; if another host replies, the address is already in use and can’t be configured on the interface.

When we attempt to create a VM having a primary UDN attachment, the guest's interfaces do not have IPs configured, since NetworkManager in the guest refuses configuring the IP address due to an ACD conflict whenever plugging a *primary* UDN interface:

"""
IP address 192.168.88.4 cannot be configured because it is already in use in the network by host 0E:6D:11:51:94:8D
"""

Any VM w/ IPv4 DAD configured. It's the default for fedora >= 40.

100 %

1. provision a layer2 UDN w/ role primary
2. provision a VM in the namespace w/ the primary UDN configured - assure the binding type used is `l2bridge`
3. voilà

NetworkManager in the guest refuses configuring the IP address due to an ACD conflict whenever plugging a *primary* UDN interface:

"""
IP address 192.168.88.4 cannot be configured because it is already in use in the network by host 0E:6D:11:51:94:8D
"""

IP address configured in the guest successfully.

Setting the ARP ignore sysctl to 1 addresses this issue:
0 - (default): reply for any local target IP address, configured on any interface 
1 - reply only if the target IP address is local address configured on the incoming interface

Since we have the IP address in the "dummy" NIC in the launcher pod, ARP replies are sent by default.