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If we try to implement router on a stick (ROAS) for try interVLAN communication, we must to use one Phisical interfase and split it into several "logicals" interfaces. Until Now all right.

But let´s see the following diagram:

enter image description here

In this diagram the "correct way" to implement ROAS would be to make one Trunking Link between Switch_1 and Switch_2 and eliminate the ethernet wire in interface F0/1 on the router.

But what happen I trying to use the two interfaces of the router ?. I can´t put the same IP addresses in the subinterfaces Fastethernet0/0 and Fastethernet0/1 because I get this message:

% 192.168.0.0 overlaps with FastEthernet0/0.2

So to implement RoaS in everywhere, Is it mandatory just to use one interface of the router ?. Because I can´t see we could use more than 1 interface.

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    The term router on a stick means using only one interface. If you use more than one, you're doing something else.
    – Ron Trunk
    May 17, 2021 at 18:53
  • 1
    It may be possible to use IRB. See this answer about that.
    – Ron Maupin ♦
    May 17, 2021 at 19:02
  • Did any answer help you? if so, you should accept the answer so that the question does not keep popping up forever, looking for an answer. Alternatively, you could post and accept your own answer.
    – Ron Maupin ♦
    Dec 23, 2021 at 19:27

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Router on a Stick refers to a router that's just using a single physical interface. That can be a reasonable setup when multiple VLANs are trunked to that interface (to multiple logical, virtual interfaces), using the router for inter-VLAN routing.

If you route between multiple physical interfaces that's no RoaS, just routing without physical-layer peculiarities.

I can´t put the same IP addresses in the subinterfaces Fastethernet0/0 and Fastethernet0/1

That's something a router would never allow you to do as it makes no sense.

So to implement RoaS in everywhere, Is it mandatory just to use one interface of the router ?

It's the other way around: if you route across virtual interfaces on a single physical port that's called RoaS. If you don't it's not.

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You should look into Integrated Routing and Bridging.

https://www.cisco.com/c/en/us/support/docs/lan-switching/integrated-routing-bridging-irb/17054-741-10.html

HTH

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