ROAS and VLANs

[PrimeYeti]

Going to try to keep this simple. In order for ROAS to function, the VLANs themselves don't need to be configured on the router right? Just the sub interfaces?

I only ask as I have setup subinterface gi0/0/0.10 for tag 10 and subinterface gi0/0/0.20 for tag 20 which works fine (e.g. interVLAN routing works, and if the trunk doesn't allow one of the tags I can't ping between VLANs as you would expect). I then configured VLAN 10 and VLAN 20 on the router and noticed that they were down which is also what I expected since I didn't have any trunk or access ports configured for them on the router.

It then got me thinking, do I actually need the VLANs themselves as well as the subinterfaces? I think I know the answer but affirmation would be great!

[M02@rt37 · Accepted solution]

Hello @PrimeYeti,

You're on the right track! In a Router on a Stick configuration, you only need to configure the subinterfaces on the router for each VLAN. The actual VLANs themselves do not need to be explicitly configured on the router.

The subinterfaces on the router are what allow it to perform inter-VLAN routing by acting as a virtual interface for each VLAN. Each subinterface is associated with a specific VLAN using the 802.1Q tagging, which carries VLAN information within the Ethernet frames as they traverse the trunk link between the router and the switch.

So you do not need to configure the VLANs themselves on the router, as long as you have the appropriate subinterfaces with the correct VLAN tags configured on the router's interface facing the trunk link.

Steps summary:

-Configure subinterfaces with VLAN tags on the router's interface facing the trunk link.

-Configure the VLANs on the switch and assign appropriate access ports or trunk ports for each VLAN as needed.

-The subinterfaces handle inter-VLAN routing, allowing traffic to flow between VLANs.

[PrimeYeti]

Thanks very much! Very helpful!

[M02@rt37]

You're welcome @PrimeYeti 

[Joseph W. Doherty]

"In order for ROAS to function, the VLANs themselves don't need to be configured on the router right?"

No, in fact, how would you define VLANs on a router?  We're talking router, not a L3 switch or router with a L2/L3 switch module too, correct?

On a router, using subinterfaces, you could have two interfaces, with a subinterface, using the same VLAN tag, but a router wouldn't assume they are the same L2 domain.  Conversely, you could have the forgoing using different VLAN tags, but if you try to assign the two subinterfaces to the same network, router would assume they are in the same L2 domain.

"Just the sub interfaces?"

Yes, although if you're thinking the subinterfaces are restricted to using VLAN tags, well, not really, although for a LAN based ROAS, that's very, very likely what you'll find.

Subinterfaces, I recall and/or believe (?), can also used with frame-relay (using DLCIs), ATM (using ILMIs), using channelized serial (using channel number[s]), MPLS (using MPLS tags).

I mention the forgoing to clarify subinterfaces, L3, are quite distinct from VLANs, L2, but, of course, L3 runs over L2, and most often you see VLANs being used with subinterfaces, which might lead one to believe they are bound together.  They are not.