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New Member

Question about where to put GRE Tunnel Source

Where would you suggest putting the GRE tunnel source on a router that is NATing? the IP NAT OUTSIDE interface as the source or the IP NAT INSIDE interface as the source? Just wondering because both work. Is there a proper way? Thanks for your input!

  • Other Network Infrastructure Subjects
4 REPLIES

Re: Question about where to put GRE Tunnel Source

You dont really need a nat in this case, because, you can configure the public ip you have on the outside serial interface, at either ends, and then tunnel all packets through the GRE tunnel.

tunnel source serial 0

tunnel destination

serial 0 should have public ip configured.

Also configure an ip address for both the tunnel interfaces at either ends, say 20.1.1.1/24 and 20.1.1.2/24 (this wont be used at all, this is for IP routing to be enabled on the tunnel interfaces).

Also add a default route at either ends,

ip route 0.0.0.0 0.0.0.0 tunnel 0.

This should help!

Now

New Member

Re: Question about where to put GRE Tunnel Source

why the default route? i just want to route 172.16.1.0/24 traffic to 172.16.2.0/24 traffic to each other through the gre tunnel. btw this router is also natting for internet access.

thanks for the help.

Re: Question about where to put GRE Tunnel Source

You can specify the specific route to 172.16.2.0 /24 traffic via tunnel 0.

(i just put the route in general).

So let the traffic nat the normal traffic going to internet via PAT (overloading the s0 interface ip address) while the tunnel doesnt need to nat, because packets are tunnelled (encapsulated inside) a packet with source and destination IP addresses, which are public routable ip addresses.

So the only change in my above configuration would be the specific routes to the 172.16.1.0/24 and 172.16.2.0/24 routes. You dont need default route pointing to tunnel interface in this case.

R1.

ip route 172.16.1.0 255.255.255.0 tunnel 0

R2

ip route 172.16.2.0 255.255.255.0 tunnel 0

HTH

New Member

Re: Question about where to put GRE Tunnel Source

thanks. a systems go!

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