How to determine of multi-layer switch is needed?

[jan.mierczi]

Dear Friends,

<BLUF>: How to determine and prove that we need change current 'router-on-the-stick' design on L3 switches solution and move inter-VLAN routing on L3 switches?

I am a network specialist and my company rather install small sites with router-on-the-stick solution. However, sites grow up and very often becomes network performance issues on site. Part of cases we could fix by replacing router-on-the-stick on L3 switch. Sometimes not.

What is criteria that we need transform site to L3 switches solution?

In real life you need prove so that your manager accepted L3 switch purchase. 

[Joseph W. Doherty]

When you say your company has been implementing router-on-a-stick, do you mean using a L2 switch and a software based (e.g. ISR) router?

If so, the simplest criteria you might use if there's any local inter VLAN routing, most small software routers do not offer multiple FE and/or gig LAN routing capacity.  For that, you generally need a L3 switch.

[jan.mierczi]

When you say your company has been implementing router-on-a-stick, do you mean using a L2 switch and a software based (e.g. ISR) router?

Yes, I do. ISR, switch L2, sub-interfaces.

If so, the simplest criteria you might use if there's any local inter VLAN routing, most small software routers do not offer multiple FE and/or gig LAN routing capacity.  For that, you generally need a L3 switch.

How can I see it? With router-on-a stick you do not need multiple L3 interfaces.

[Joseph W. Doherty]

How can I see it? With router-on-a stick you do not need multiple L3 interfaces.

Physically, you're correct, but logically you have multiple sub-interfaces, each a L3 interface.

With a L3 switch, you often have L3 interfaces which are VLAN interfaces, those too are logical.

If you have other L2 switches, just as the router-on-a-stick can have one physical interface to them, and the L2 switch uses a trunk so can a L3 switch, i.e. on its side, instead of subinterfaces it too would use a trunk to the L2 switch.