IP addreesing on router interface

Unanswered Question
Mar 17th, 2007

Dear All,

I want to know how can I address two router interfaces in one subnet.


I have this problem too.
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hoogen_82 Sat, 03/17/2007 - 02:07

hmm.. not quite understodd your question but i guess what you want is a /30 subnet for your point to point link.


emadehsan Sat, 03/17/2007 - 02:18


acctully I want give into one of my route interfaces and two other router interface,


CSCO10892433 Sat, 03/17/2007 - 02:53


I guess you are asking the way to assign ip addresses to the interfaces of your routers.

Depending on the way you access the routers, here is some user guides that might help you.

CLI command:




If you have any furrther question, just let us know.



emadehsan Sat, 03/17/2007 - 05:21

Thank you for your reply

but acctually my problem is I want give two IP address in same subnet to my router interfaces.

it gives me a overlap error..

CSCO10892433 Sat, 03/17/2007 - 11:05

Sorry for the mis-understanding.

You can not assign overlapping addresses to interfaces in the same router. (or, more precisely, in the same vrf) The reason for that is the forwarding decision that a router must make when it received a packet destined to that subnet. For example, suppose fa0/0 has ip address and fa0/1 has ip address, then when the router received a packet destined to, it is difficult for this router to make a forward dicision: should I forward it out the fa0/0, fa0/1 or both? If it forward to one of these two interfaces, there is a 50% possibility that this packet get to the wrong place. If it forwards to both interfaces (a multicast behavier), there is 100% possibility that one of the link's bandwidth get wasted. A smart design is to disallow this configuration beforehand so this dilemma will never happen.

Anyway, if your intention is that hosts connected these two interfaces are in the same subnet, there is a workaround: a feature called IRB that most routers support. Instead of assigning overlapping addresses to two interfaces, you first associate these two interfaces to the same bridge-group, then you create a BVI interface and assigign an ip address to this BVI interface. Here is a sample config of IRB

bridge irb

bridge 1 protocol ieee

bridge 1 route ip

int fa0/0

no ip address

bridge-group 1

int fa0/1

no ip address

bridge-group 1

int BVI1

ip address

This config will work in case the hosts connected to fa0/0 and fa0/1 are in the same subnet

I'm trying to find some pages regarding IRB for your reference. I will post them later if I find a good one.



Richard Burts Sat, 03/17/2007 - 18:20


As has been pointed out, the answer to your specific question is that you are not permitted by IOS to configure 2 Ethernet interfaces on the same router in the same IP subnet. And as SSLIN has pointed out one workaround is to configure IRB and his example config should point you in the right direction.

Perhaps if you would give us some more detail about your environment and some more specifics about what you are trying to achieve we might be able to find better answers to suggest to you.




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