2509 router does not accept wan ip with subnet

Unanswered Question
Nov 27th, 2007

hi i hv 2509, 2501. while configuring wan ip on serial interface its ot allowing me to configure ip with other subnet mask , its allowing only ip with default subnet mask, for eg- if i giv on s0 of 1st router then router does not allow me to configure , but if i give then it configure smoothly , can any body tel me why?

I have this problem too.
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Overall Rating: 5 (1 ratings)
Richard Burts Tue, 11/27/2007 - 10:56

I am not confident that ip classless will help this issue. ip classless has to do with how we interpret the content of the routing table and not to do with how interfaces are configured. It is more likely that the ip subnet-zero command might be the solution for the symptom described.

I would ask for some clarification from Gulshan. When you attempt to configure the router with does it give error message saying invalid mask? If so it is a symptom of attempting to configure an address that falls into subnet zero (which your example does). If you configure the router with ip subnet-zero and then attempt to configure the interface it should work ok.

Restriction on the use of subnet zero was common many releases of IOS ago. But those restrictions are not common in more recent versions of code. It sounds like your router is running code which enforces this restriction by default.



Collin Clark Tue, 11/27/2007 - 11:05

Rick is right (once again), I just tried it:

MKE-MPLS-Test(config)#no ip subnet-zero

MKE-MPLS-Test(config)#int fa0

MKE-MPLS-Test(config-if)#ip add

Bad mask /30 for address


MKE-MPLS-Test(config)#ip subnet-zero

MKE-MPLS-Test(config)#int fa0

MKE-MPLS-Test(config-if)#ip add


Points well earned.

Richard Burts Tue, 11/27/2007 - 11:35


Thanks for the compliment (and for the rating).

It is an interesting error message - why does it say bad mask when the mask is perfectly ok?

What it is really saying is that the mask puts the address in an incorrect place.



Collin Clark Tue, 11/27/2007 - 11:40

I was reading too fast... I thought ip subnet-zero until I saw .10 which I thought was the last octet.


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