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GRE tunnel configuration help

Hi there,

I have a need to use a GRE tunnel to connect two subnets across my WAN, but I've never created a tunnel before and I'm not quite sure the correct process.

In SiteA I have, and at SiteB I have

I have a host in SiteB ( that needs to route to a series of networks that are reachable via the SiteA network.

So I know I need to create a tunnel interface at each site, and set destination and source IP's. Does it matter what I use for the tunnel interface IP? Also, the subnets at each site are on Vlan interfaces, so do I use those as the destination and source IP's for the tunnel?

Thanks in advance,


Hall of Fame Super Silver

Re: GRE tunnel configuration help


One of the basic requirements for successful GRE tunnel is that the tunnel destination address must be reachable from the source before the tunnel comes up. Based on this it might be advantageous if you configure the GRE tunnel to have its source and destination addresses as the outward facing interfaces on each router.

You can choose a subnet for the GRE tunnel. As long as the subnet is unique (does not overlap with addresses used somewhere else in the network) it should be ok. Without knowing more about your topology and how the existing IP addresses are organized I do not believe that we can give you good advice about what subnet to choose.

Once you get the tunnel up and passing traffic then you can set up routing to send traffic through the tunnel. You might use static routing or you might run a dynamic routing protocol over the tunnel. In setting up routing (either static or dynammic) be careful that you do not create a situation where the destination address of the tunnel appears to be reached through the tunnel. If the tunnel destination is reached through the tunnel this causes an error of recursion and the tunnel will be brought down from the error.



Hall of Fame Super Gold

Re: GRE tunnel configuration help

I don't know what the network design is going to be like so you'll need to go through these configuration examples/guides and pick which is suitable.

IPSec/GRE with NAT on IOS Router Configuration Example

GRE Tunnel with VRF Configuration Example

IPSec Virtual Tunnel Interface

Hope this helps.