Tunneling with Cisco Routers

Definitions

207.126.96.98/30 interface address on router A.
207.106.31.242/30 interface address on router B.
10.40.40.0/30 generic private point to point addresses.

Router A

interface Tunnel0
 ip address 10.40.40.2 255.255.255.252
 tunnel source 207.126.96.98
 tunnel destination 207.106.31.242
 
ip route 207.106.31.242 255.255.255.255 207.126.96.97

Router B

interface Tunnel0
 ip address 10.40.40.1 255.255.255.252
 tunnel source 207.106.31.242
 tunnel destination 207.126.96.98
 
ip route 207.126.96.98 255.255.255.255 207.106.31.241

Notes

Make sure to read http://www.cisco.com/warp/public/105/56.html

Among other things, tunnels can be used to obscure the path data is taking while circuits are being provisioned. Another use could be to "warp" address space through outside networks where it might not be appropriate to get them to announce the "warped" block.

Some networks monitor GRE because of concern over the potential abuse of GRE tunnels at exchange points. You need to make sure that the path of the tunnel is via transit in both directions otherwise you will be in effect getting a free ride in the nontransit direction. Static routes are used to ensure the path the tunnel takes.

The example above creates a GRE tunnel (the default). To create an IP over IP tunnel use the command:

tunnel mode ipip

The private point to point interface addresses do not show up in the traceroute so their use won't break PMTU discovery. Normal point to point addresses would also work fine.

The above commands are simply an example. There are no tunnels in Hurricane Electric's core infrastructure.

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