User's Manual

Configuring GRE, NAT, RIPSO, and BFE Services
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The GRE tunnel can use any IP interface configured on the router as a physical
end point. To maximize the robustness of the tunnel, use a circuitless IP address as
a tunnel’s physical end point whenever possible. Because a circuitless IP address
is associated with the whole router, not one physical interface, the tunnel operates
as long as any slot that has a working IP interface stays up. (For instructions on
configuring a circuitless interface, see Configuring IP, ARP, RARP, RIP, and OSPF
Services.)
The following steps explain how GRE tunneling takes place. GRE tunnels support
both IP and IPX encapsulation. The example describes a GRE tunnel
encapsulating IP (refer to Figure 1-2
):
1.
The router interface on router 1 receives a packet from host 1, looks up the
packet’s destination address in its routing table, and determines that the next
hop to the destination address is the remote end of a GRE tunnel. The router
interface queues the packet at the tunnel interface for GRE encapsulation.
2.
Router 1 adds a GRE header to the packet and sends the packet to IP.
3.
IP looks up the route to the remote tunnel end point and sends the
GRE-encapsulated packet to the appropriate next-hop address.
4.
The remote tunnel interface on router 2 removes the outer IP header and the
GRE header.
5.
The remote router interface looks up the packet’s destination address in its
routing table and chooses the next hop to reach host 2.