Using same subnet on 2 diffrent interfaces on same router

Answered Question

I have  dilema. I am getting ready to move a metro ethernet off a switch and onto a router. Unfortunatly it of forcing me to use same subnet on 2 diffrent interfaces on the same router. The ISPs gateway/peer is using an address that is the same subnet as the class c they provide. Also, have an issue with archtecture change to accomodate this.



I have never had a need to do this before. Any suggestion on how to get this can be down.



I get an error message when I try to do this.


See attached drawing for Visual.

Attachment: 
Correct Answer by Giuseppe Larosa about 7 years 3 months ago

Hello Eferro,

only on serial interfaces or POS interfaces this is possible.


on lan interfaces you get an "address overlapping error" and ip address of second interface is rejected.


as noted by Jon you should ask for a separate s/30 or the current subnet has to be subnetted in two parts  (if this is possible)


Hope to help

Giuseppe

Correct Answer by Jon Marshall about 7 years 3 months ago

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I have  dilema. I am getting ready to move a metro ethernet off a switch and onto a router. Unfortunatly it of forcing me to use same subnet on 2 diffrent interfaces on the same router. The ISPs gateway/peer is using an address that is the same subnet as the class c they provide. Also, have an issue with archtecture change to accomodate this.



I have never had a need to do this before. Any suggestion on how to get this can be down.



I get an error message when I try to do this.


See attached drawing for Visual.


I'm afraid you can't do this on a router with ethernet interfaces. You could subnet down the class C but trouble is you could have a /30 on the outside to the ISP but then you would need multiple subnets on the inside interface and hence multiple secondary addresses.


It might be worth asking the ISP if you could have a /30 just for the handoff.


Jon

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Correct Answer
Jon Marshall Wed, 02/17/2010 - 12:36
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[email protected]


I have  dilema. I am getting ready to move a metro ethernet off a switch and onto a router. Unfortunatly it of forcing me to use same subnet on 2 diffrent interfaces on the same router. The ISPs gateway/peer is using an address that is the same subnet as the class c they provide. Also, have an issue with archtecture change to accomodate this.



I have never had a need to do this before. Any suggestion on how to get this can be down.



I get an error message when I try to do this.


See attached drawing for Visual.


I'm afraid you can't do this on a router with ethernet interfaces. You could subnet down the class C but trouble is you could have a /30 on the outside to the ISP but then you would need multiple subnets on the inside interface and hence multiple secondary addresses.


It might be worth asking the ISP if you could have a /30 just for the handoff.


Jon

Correct Answer
Giuseppe Larosa Wed, 02/17/2010 - 13:02
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Hello Eferro,

only on serial interfaces or POS interfaces this is possible.


on lan interfaces you get an "address overlapping error" and ip address of second interface is rejected.


as noted by Jon you should ask for a separate s/30 or the current subnet has to be subnetted in two parts  (if this is possible)


Hope to help

Giuseppe

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