One Network, Three Routers

[alephpp]

Hi, I'm a bit new to Cisco, I'd like to know if this is possible

This was presented by my teacher and he explain that in theory this is possible, but I know he likes to play tricks so I'm a bit skeptical on that. Is there any way this is possible? Thanks!

[DanielP211]

Hello,

What is the question here? If the same segment can be assigned to two different interfaces of a switch? It can if you are using VRF's.

BR

[alephpp]

Hi there,

The question that is presented is if it's possible to have the same network segment across three routers? As far as I know, they'll just overlaps.

Thanks for replying!

[DanielP211 · Accepted solution]

It's possible. As long as the middle router has the interfaces in two seperate VRF's there is no issue here, and I assume the IP's from the subnet are different.

BR

[M02@rt37 · Accepted solution]

Hello @alephpp,

Router R1 cannot have two interfaces in the same subnet --- not possible!

You could use VRF as @DanielP211 proposed.

[alephpp]

Thanks for helping, y'all!

[M02@rt37]

You're very welcome.

[Joseph W. Doherty]

Well, it depends what your "tricky" teacher has in mind.

What M02@rt37 suggests might be done but if you want to exchange traffic between

R0 and R1,

it gets "complex".

What teacher might have in mind, on

R2 is IRB.

  (What's that?  Student exercise - look it up.)

[Giuseppe Larosa]

Hello @alephpp ,

I think that @Joseph W. Doherty is right here the teacher is presenting a possibile use case for

IRB

or at least on the possibility to configure the router in the middle to bridge between two interfaces.

With

IRB

Integrated Routing and bridging a L3 interface can also be associated to the bridge domain and would make the router in the middle able to speak IP in the common subnet.

to be noted in more modern

IOS XE IRB is configured using BDI 

instead of BVI as L3 interfaces.

This is more a change in syntax of commands but from conceptual point of view it is still

IRB.

Solutions using VRFs would probably be an overkilling tool and also would require addition of NAT to work as the subnet would be overlapping.

Hope to help

Giuseppe

[Joseph W. Doherty]

"Solutions using VRFs would probably be an overkilling tool and also would require addition of NAT to work as the subnet would be overlapping."

Yea, part of the complexity of a VRF solution, two way NAT.  Although, you can use the whole

/24 address

space on both sides.

Again, though, doable with VRFs.

I've also wondering whether something like

L2TPv3

might be used with, or without, VRFs to join the two segments at L2.  Much would depend whether want R2 to also have an address in that

/24.