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To get the serial link in Figures 9-1 and 9-2 working, the router configuration is simple. Example 9-1 lists the configuration on R1 and R2.

Example 9-1 Minimal Configuration for IP on a Point-to-Point Link

Note that the IP addresses must be in the same subnet, because the two routers’ interfaces are not separated by some other IP router. In many cases, the no shutdown command is unneeded. However, when a Cisco router comes up, and the physical WAN link is not working, the router might place a shutdown command on the interface configuration. So the no shutdown interface subcommand would be needed to put the interface in service.

The clock rate command sets the clock rate on interfaces when a DCE cable has been used, as in Figure 9-2. In Example 9-1, the clock rate 56000 command sets the clock rate to 56,000 bps. If external CSU/DSUs were used, as in Figure 9-1, as is typical when you use an actual leased line from a provider, this command is unneeded. In fact, the router wouldn’t let you add the command to the configuration if a DTE cable was connected to the interface!

Data-Link Protocols for Point-to-Point Leased Lines

The more interesting part of point-to-point WANs relates to the choices of different data link layer protocols and how each protocol behaves differently. WAN data-link protocols used on point-to-point serial links provide the basic function of data delivery across that one link.

The two most popular WAN data-link protocols are High-Level Data Link Control (HDLC) and PPP. Each of these WAN protocols has the following functions in common:

■HDLC and PPP provide for the delivery of data across a single point-to-point serial link.

■HDLC and PPP deliver data on synchronous serial links; PPP also supports asynchronous serial links.

Each synchronous serial data-link protocol uses the concept of framing. Each data-link protocol defines the beginning and end of the frame, the information and format of a header and trailer, and the location of the packet between the header and trailer. In other words, synchronous WAN data-link protocols are frame-oriented, just like LAN data-link protocols.

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protocols. However, asynchronous protocols require less-expensive hardware, because there is no need to watch transitions and adjust the clock rate. For links between routers, synchronous links are typically desired and used. When you use a PC with a modem to dial up an Internet provider, you use an asynchronous link. All the protocols covered in this section support synchronous links, because that is what routers typically use between each other.

Another comparison criteria is error recovery. Do not confuse error recovery with error detection. Almost all data-link protocols, PPP and HDLC included, perform error detection. All the data-link protocols described here use a field in the trailer, usually called the frame check sequence (FCS), that verifies whether bit errors occurred during transmission of the frame. If so, the frame is discarded. Error recovery is the process that causes retransmission of the lost or errored frame(s). Error recovery can be performed by the data-link protocol or a higher-layer protocol, or it might not be performed at all. Regardless, all WAN data-link protocols perform error detection, which involves noticing the error and discarding the frame.

Finally, the people who created these protocols might or might not have defined a Protocol Type field. As described in more detail in Chapter 3, “Data Link Layer Fundamentals: Ethernet LANs,” from CCNA INTRO Exam Certification Guide, each data-link protocol that supports multiple network layer protocols needs a method of defining the type of packet encapsulated in the data-link frame. If such a field is part of the protocol specification, it is considered architected—in other words, specified in the protocol. If the protocol specification does not include a Protocol Type field, Cisco might add some other header information to create a Protocol Type field.

Figure 9-3 shows the framing details of HDLC and PPP, showing the proprietary HDLC Protocol field and the standardized PPP Protocol field.

Figure 9-3 PPP and HDLC Framing

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Table 9-3 lists the important comparison points between HDLC and PPP.

HDLC and PPP Configuration

HDLC and PPP configuration is straightforward. You just need to be sure to configure the same WAN data-link protocol on each end of the serial link. Otherwise, the routers will misinterpret the incoming frames, because each WAN data-link protocol uses a different frame format. Other than configuring some optional features, that’s all you need to do!

Tables 9-4 and 9-5 summarize the configuration commands and EXEC commands used for HDLC and PPP configuration. Most of the commands relate to the optional features.

Table 9-4 PPP and HDLC Configuration Commands

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Example 9-2 shows the configuration for PPP, followed by the changed configuration for a migration to HDLC. Assume that Router A and Router B have a serial link attached to their serial 1 ports.

Example 9-2 Configuration for PPP and HDLC

Changing serial encapsulations in configuration mode is tricky compared to some other configuration commands in a Cisco router. In Example 9-2, converting back to HDLC (the default) is accomplished with the encapsulation hdlc command, but it can also be accomplished with the command no encapsulation ppp, because that causes the router to revert back to the default encapsulation, which is HDLC. Additionally, any other interface subcommands that are pertinent only to PPP, but not HDLC, are also removed when the encapsulation hdlc command is used.

PPP-Specific Features

PPP was defined much later than the original HDLC specifications. As a result, the creators of PPP included many additional features that had not been seen in WAN data-link protocols up to that time. As a result, PPP has become the most popular and feature-rich WAN data link layer protocol.

PPP-unique features fall into two main categories:

■Those needed regardless of the Layer 3 protocol sent across the link

■Those specific to each Layer 3 protocol

PPP uses a protocol that focuses on the features that apply regardless of the Layer 3 protocol used and another protocol to support each Layer 3 protocol supported on the link. The PPP Link Control Protocol (LCP) provides the core features for PPP. For features related to a specific Layer 3 protocol, PPP uses a series of PPP control protocols, such as IP Control Protocol (IPCP). For example, IPCP provides for IP address assignment; this feature is used extensively with Internet dialup connections today.

PPP uses one LCP per link and one Control Protocol for each Layer 3 protocol defined on the link. If a router is configured for IPX, AppleTalk, and IP on a PPP serial link, the router