301/200 william street

301/200 william street.10.1.2.2 source = l.l.l destination = 2.222 GRE payload-header text header packetFigure 7-9 Encapsulating a Source IP Packet in a GRE Formatted PacketTip: If the tunnel created by the two routers is also configured with the IPsec encryption portion of the tunnel, the sending router will first encrypt the source packet before encapsulating it as shown in Figure 7-8.GRE details the use of two headers to create a tunnel. the GRE defines its own header for managing the tunnel itself. GRE also defines the use of a 20-byte IP header, called the "delivery header," which uses the header from the insecure networkIP address from the unsecured network 0. In this example, the delivery IP header displays the l.l.l.l Internet IP address of R1 as the source and the 2.222 Internet IP address of R2 as the destination.When this packet passes over the Internet, a router in the Internet uses this external GRE submit IP header to route the packet. The fact that this packet happens to contain another entire IP packet is irrelevant to the IP forwarding process in these routers; they simply forward this IP packet based on the 222.2 destination IP address. Figure 7-10 illustrates this concept. note that this packet may be routed by many routers on the Internet before it reaches R2.When the GRE packet in Figure 7-10 eventually reaches R2 over the Internet on the right, R2 simply removes the header from the incoming data link as is the normal practice. For GRE-encapsulated packets, the receiving router (R2) also needs to remove the "header text" and the GRE packet, as shown in Figure 7-11, leaving the source data exposed.4.2 Basic Contents 150------ destination= 10.1.2.2 (ignored) 'destination= 2.222 (used)