Comment 5 for bug 1774203

Loopback interfaces are important for switches/routers. The Linux dummy interface provides the ability to create interfaces whose function is consistent with loopback interfaces widely used on switches/routers.

Dummy interfaces can be created with both Network Manager and systemd-networkd. Because of their simplicity, adding support to netplan should be easy and will not add debt to the netplan project. The nmstate project already supports dummy interfaces. There is no practical need to match or sync with any existing devices on the system. Netplan should not concern itself with anything but creating the interface and assigning the addresses.

There are very important uses for dummy/loopback interfaces.

- Link-State routing protocols like OSPF/IS-IS, or cluster identifier for iBGP
- Endpoints for systems with multiple tunnels
- Any situation where a layer-3 failover solution is required
- Load balancing with ECMP (equal-cost multipath routing)
- Configuring additional addresses on system which are being routed to the system via a physical interface from an upstream internet/cloud/hosting provider.

A dummy/loopback interface is NOT a bridge. A bridge is a layer-2 solution, a dummy/loopback is a layer-3 solution.

The proper location in the yaml hierarchy should be:

network:
  dummies: # or loopbacks:
    dummy0:
      addresses:
        - n.n.n.n/n

A bridge interface is not a great substitute for a dummy/loopback interface because it can complicate configurations and create confusion for users. In most bridge implementations, the bridge includes a physical interface for connectivity to an external gateway. During troubleshooting, it is very common for users to be misled into thinking they need to add their physical interface to the bridge and migrate the IP configuration to the bridge which solves nothing and usually results in additional problems.