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Cisco Employee

Serial interfaces in the same subnet?

Hello,

Recently I was surprised that a student of mine had configured two serial interfaces on a 2500 router by mistake into the same subnet, e.g.:

interface serial 0/0

ip address 10.0.0.1 255.255.255.0

no shutdown

interface serial 0/1

ip address 10.0.0.2 255.255.255.0

no shutdown

The IOS did not complain about this configuration and it happily accepted it. I have also tested it on 2600 router, with the same behavior.

Now I am confused. Why is it possible to configure two serial interfaces with the same IP subnet? What is this good for? Where can it be used and why?

Thank you for any suggestion!

Best regards,

Peter

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Gold

Re: Serial interfaces in the same subnet?

Peter

It is kind of surprising when you discover this behavior. The IOS treats serial interfaces differently from LAN interfaces. If you attempt to configure two LAN interfaces into the same subnet the IOS gives an error message and will not allow it. But the IOS does allow it for serial interfaces. This behavior has been consistent for a very long time - so I do not think it is a mistake in the IOS.

I have not been clear why IOS does this and believe that the reasons have to do with using serial links to back each other up by putting them into the same subnet.

HTH

Rick

3 REPLIES
Hall of Fame Super Gold

Re: Serial interfaces in the same subnet?

Peter

It is kind of surprising when you discover this behavior. The IOS treats serial interfaces differently from LAN interfaces. If you attempt to configure two LAN interfaces into the same subnet the IOS gives an error message and will not allow it. But the IOS does allow it for serial interfaces. This behavior has been consistent for a very long time - so I do not think it is a mistake in the IOS.

I have not been clear why IOS does this and believe that the reasons have to do with using serial links to back each other up by putting them into the same subnet.

HTH

Rick

Re: Serial interfaces in the same subnet?

Hi Rick,

Can this be due to the nature of media like ethernet is a broadcast media and serial is not bydefault so can this be a reason that ethernet discovers if same subnet is assigned on 2 interfaces on same router?

Regards,

Ankur

Community Member

Re: Serial interfaces in the same subnet?

hi,

I also came to know when i started my routing CCNP paper however we can stop this behevior by using no ip classless and why this command stops that behevior you all know very well.

one more interesting this which happend with me was i did configured same ip address on two different interface of the same router. i took R1 so and s1 having 10.1.1.1/24 and R2 so and s1 10.1.1.2/24 and it did work. you can try this

i did run routing protocol's also and it was doing load balancing too.

thanks

HTconfuse

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