Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

New Member

GRE Tunnel and Natting

Dear All

I have a simple set up with a hup GRE/NHRP and a spoke. the spoke has 2 WAN interfaces, f0/0 has a public IP and the GRE tunnel works fine.

The spoke f0/1 gets a private IP from a non cisco VDSL router/modem. On that side the GRE tunnel will not come up. I assume it is because of the NAT/PAT on the VDSL device. I can not figure out what is exacly going on, and what can be done, as I have no control/access over that device..

Thanks ofr your inputs

Best Regards



Re: GRE Tunnel and Natting


Maybe it's just me, but I cannot reconstruct your topology in my head

Do you have a drawing with your topology, or can you make just a basic one and share it with us?



New Member

Re: GRE Tunnel and Natting

just one hub and one spoke linked by GRE/NHRP. on he spoke I can test the tunnel on either an interface with a public IP or an interface with a private IP behind a VDSl box. The second option does not work.

Hall of Fame Super Gold

Re: GRE Tunnel and Natting

Generally speaking, GRe cannot work behing third-party NAT.

As a workaround, you can congirue GRE over IPsec, that generally work fine with third-party NAT.

New Member

Re: GRE Tunnel and Natting

It 's also my feeling, I guess it has something to do with NHRP resolution but

I would like to understand the details.