RAs trigger neighbor discovery which allows users to query the
LL address of the container/VM via `ip neighbor get dev`. This is
useful as it gives users an easy way to connect to the container
without needing LLMNR or mDNS to resolve the hostname of the container
to an IP address. In practice, this allows connecting with only
networkd enabled and without resolved running in the host/container.
This changes the default .network files we ship for nspawn containers to set
EmitLLDP=customer-bridge in order to allow propagation of the LLDP packets
across bridges. This is useful so that "networkctl status" shows all peers
connected to a virtual container network, collecting this data via LLDP. This
is safe since the default configuration for these interfaces does not bridge
these links to external interfaces, but relies on IP routing for this.
Daan De Meyer