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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 at 18:53
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    It may be possible to use IRB. See this answer about that.
    – Ron Maupin
    May 17 at 19:02
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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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