Forwarding an IP Packet over a WAN Link

Forwarding an IP Packet over a WAN Link

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R1 has the same dilemma and the same solution that it had when it used Ethernet to connect to R2 back in Chapter 11. R1 needs to encapsulate the IP packet inside a data link layer frame.

Routers often support many options for data link layer protocols for point-to-point WAN links. The two most popular are high-level data link control (HDLC) and Point-to-Point Protocol (PPP). Regardless of which of these two is used, R1 does the same general thing: It encapsulates the packet into a frameeither an HDLC or PPP frame, depending on which of the two protocols the router is using. Figure 14-7 shows the encapsulated packet, using HDLC.