Is it okay to not assign an IP address to the physical interface of router when used for "Router on a Stick"?

Question

When setting up "Router on a Stick" interVLAN routing, the steps I have seen only covered creating sub-interfaces for each VLAN and assigning an IP address to each of these sub-interfaces. However, there is no mention about assigning an IP address to the physical interface of the router, i.e. the parent interface.

I have several questions regarding this:

1. Would there be any problems if no IP address is assigned to that physical interface of the router?
2. Are there any possible use-cases for me to assign it an IP address?
3. Can the physical interface also be assigned a VLAN and treated just like a sub-interface?

---

EDIT: From the two answers received and this question, my understanding is that:

The router's physical interface can optionally be configured as default gateway for the native VLAN on the switch that it is connected to (by assigning the physical interface an IP address and if the native VLAN exists and is allowed on the trunk port of the switch).

• The physical interface on the router cannot be used for any other VLANs apart from native VLAN
• The native VLAN (if configured), can choose to use either the physical interface or another sub-interface

If there are any errors please feel free to correct me thanks.

Answer 1

No, there would be no problem at all

unless on the given 802.1q trunk coming from the switch, one of the VLANs would have to be untagged (for whatever reason, usually not recommended), and you would want that given VLAN/subnet included in routing-on-a-stick.

In that case you'd have to add an IP config to the "parent" interface, appropriate for the given VLAN/subnet.

To be precise:

1. no problem

2. use case see above: one of the VLANs you want to do inter-VLAN-routing for happens to be the untagged VLAN of the given 802.1q trunk.

3. no. VLAN-Tags can only be set for subinterfaces.

Answer 2

Generally, you only need to assign an IP address to an interface that is used to talk to the gateway.

If the parent interface is not used with an untagged/native VLAN in a trunk then it doesn't need an IP address of its own. Some (few) devices may require some configured address to activate the physical interface though.