Unanswered Question
Oct 4th, 2007

Is it possible to set up a VPN with the same subnet at both ends? For example, router 1 FA0/0 interface IP address, serial interface Router 2 FA0/0 interface, serial interface I need to connect the 10.1.1.x subnet through the 192.168.100.x link to connect a remote office to the main office.

Any help will be greatly appreciated.


I have this problem too.
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jimcalano Thu, 10/04/2007 - 11:30


Thank you VERY much. Sorry I didn't have much time to look this up myself. I'm under the gun here.

I'll let you know how it goes.

Jon Marshall Thu, 10/04/2007 - 11:46


No problem. I've used IPSEC config docs quite a few times myself so i'm familiar with where to look.

Hope it goes okay.


sundar.palaniappan Thu, 10/04/2007 - 11:37


I just edited the post in which I had stated that you can't do IPSEC when overlapping addresses are in use. Though it's true in a straightforward scenario because of reasons I cited. However, Jon has posted a link that offers a workaround (NAT w/IPSEC) to this problem.

Good Luck!!

Jon that's a good link you have provided.



jimcalano Thu, 10/04/2007 - 14:36

The crypto command doesn't seem to be a recognized command in global, or any, mode. I'm using a 2621 router with software version 12.3(17a).

sundar.palaniappan Thu, 10/04/2007 - 14:58

You probably do not have a crypto image, it would have k9 embedded in the filename of the IOS, running in the router to do IPSEC. Use software advisor on CCO to select the IOS that supports IPSEC functionality. Here's one that supports IPSEC c2600-jk9s-mz.12.2-46a.



jimcalano Thu, 10/04/2007 - 18:43

Thank you Sundar. I'll continue with this tomorrow when I get back to the office.

timothykerr Thu, 10/04/2007 - 19:40

Hi Jim,

If I recall, and someone correct me if I am wrong. But you will need to have a 2600XM router to support the k9 IOS's. This is due to the memory restrictions on the 2600 routers. They only support a max of 16/64 flash/dram. You will need a 2600XM router in order to upgrade the memory that will be required to download the newer IOS images that support the crypto command set.



jimcalano Fri, 10/05/2007 - 06:41

Don't have those. Does anybody else have any suggestions?

This whole exercise is an attempt to manage routers across a serial link from a single point of contact. Both sides of the link are on a 10.1.1.x network because of a wireless mesh configuration.


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