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New Member

Two serial interfaces, same IP Address?

My understanding was that you could not configure two interfaces on a router in the same subnet, no matter what type of interface.

This is true with etherent, but I have two serial interface on a router configured exactly the same and they are both showing up/up when I connect the somulated WAN link to another router.

Why can you do this on a serial interface and not etherent?

Am I missing something?

Hall of Fame Super Blue

Re: Two serial interfaces, same IP Address?


No you are not really missing something. Ethernet is a LAN interface (generally) and as such IOS will not allow 2 interfaces to be configured out of the same subnet but serial interfaces are treated as point to point WAN interfaces.

I know that's not a complete explanation - i'm afraid it's just one of those quirks in IOS.


Hall of Fame Super Silver

Re: Two serial interfaces, same IP Address?

Hello Richard,

I agree with Jon's explanation but I think it is a wanted exception to the check rule to allow WAN redundancy:

in a branch office if the router has two serial interfaces and only one is used you can configure the second one exactly as the first.

If the first interface is damaged or not working you can have someone move the serial cable to the second interface and restore WAN connectivity without any change in configuration.

I think we used this for some remote sites in our network.

For SDH APS the router has two interfaces configured the same, here there is a network based automatic protection mechanism but the idea is the same.

Hope to help


New Member

Re: Two serial interfaces, same IP Address?


I was trying to come up with a way to remotely upgrade a link from single T1 frame relay to x2 T1 Multilink.

I was thinking I couldn't do it, but I can.


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