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To create a GRE tunnel, you must create virtual interfaces on the routers at each end, and the tunnel must begin and terminate at existing routable IP addresses. The tunnel is not a physical link—it is logical. As such, it must rely on routes already in place for its existence. The tunnel will behave like a physical link in that it will need an IP address on each side of the link. These will be the tunnel interface IPs. In addition, as the link is virtual, the tunnel will need to be told where to originate and terminate. The source and destination must be existing IP addresses on the routers at each end of the tunnel.

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Cover of Network Warrior, 2nd Edition


GRE tunnel configuration requirements