Gre Tunnel

[George Clark]

I have a situation where we have 2 buildings in production 2921 router they have requested to keep the same subnets on the lan at the old and new site temporary for a few weeks .while they move their equipment over in a few days.

Not sure how to accomplish this i thought of doing nat at the new location temporarily nating to a different network or with a temporary  gre tunnel not sure if you can have the same subnets on the lan on both ends of the tunnel.

Could also get a layer 2 from our ISP but costly .

Anyone with any ideas i would appreciate it ....

[Georg Pauwen]

Hello,

check if your devices support Local Area Mobility:

https://www.safaribooksonline.com/library/view/cisco-ios-cookbook/0596527225/ch24s02.html

[Richard Burts · Accepted solution]

There are several things about the original post that are not clear to me. But what is clear is that they are looking to have the same subnet in operation at two sites. That is a bit of a challenge. The BEST solution for this would be a layer 2 connection between sites which would allow hosts in the subnet at one site communicate directly (arp and respond and find destination mac address, and forward packets without needing a gateway) with hosts in the same subnet at the other site. But the original post mentions that this would be expensive which suggests that they do not want to do this. I would suggest that they look into L2TPv3 as a way to support the same subnet at both sites.

The original post asks about whether GRE could have the same subnets at both sites. This is a bit tricky to answer. Strictly speaking GRE does not care about the addressing of the subnets at each end. So GRE would not object if the same subnet was configured as the LAN at both ends. GRE would run just fine with no issues and no errors when the same subnet is the LAN on both sides. But the issue is how to route traffic over the GRE tunnel. If we assume that the LAN is subnet 10.1.1.0/24 at both sites then we can think about a host 10.1.1.1 at site 1 that wants to communicate with 10.1.1.2 which is at site 2. The router at site 1 is going to believe that 10.1.1.2 should be a locally connected device and why would it send that packet over the GRE tunnel? So GRE tunnel is not a solution here (not because of anything about GRE but because of the routing issue).

HTH

Rick

[George Clark]

Thnak you very much for your thoughts and time much appreciated

[Richard Burts]

You are welcome. Thank you for marking this question as solved. This will help other readers in the forum to identify discussions which have helpful content.

HTH

Rick