CCNA Certification Guide

There are a number of different data link layer protocols that can be implemented on a point-to-point WAN. 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. Both of these protocols provide for the delivery of data across a single point-to-point serial link and deliver data on synchronous serial links. In addition, PPP also supports asynchronous serial links.

Each synchronous serial data-link protocol is frame-oriented, with each data-link protocol defining 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. Data-link protocols also send idle frames. This is because synchronous WAN links require that the CSU/DSUs on each end of the link operate at the exact same speed. To accomplish this, the CSU/DSUs on each side of the WAN link agree to use a certain clock rate, or speed, to send and receive bits. After they agree to a particular speed, both CSU/DSUs try to operate at that speed. One CSU/DSU is responsible for monitoring the clock rates between itself and the other CSU/DSU by noticing changes in the electrical signal received on the physical line. When a change occurs, the CSU/DSU monitoring the clock rates responds by adjusting its clock speed. If no traffic was sent across the link, there would be no electrical signal and clock synchronization would be lost. Therefore synchronous data-link protocols send idle frames when there is no end-user data to be sent over the link. The idle frames are called Receiver Ready. This need to monitor and adjust the clock rates for synchronous protocols requires more expensive hardware than asynchronous protocols. However, synchronous protocols allow more throughput over a serial link than asynchronous protocols. For links between routers, synchronous links are typically desired and used.

Almost all data-link protocols, including PPP and HDLC, perform error detection. These protocols use a field in the trailer called the frame check sequence (FCS) for this purpose. The FCS is used to verify whether bit errors occurred during transmission of the frame. If bit errors occurred, the frame is discarded. However, error recovery, which is the process that causes retransmission of the lost or errored frame, is not guaranteed. Error recovery can be performed by the data-link protocol or a higher-layer protocol, or it might not be performed at all.

PPP was defined much later than the original HDLC specifications. As a result, PPP includes many new features that are not implemented in HDLC. For this reason, PPP has become the most popular WAN data link layer protocol.

PPP uses a protocol that offers features regardless of the Layer 3 protocol used, and a protocol to support each Layer 3 protocol supported on the link. The PPP Link Control Protocol (LCP) provides the core features for PPP that operate regardless of the Layer 3 protocol used, while a series of PPP control protocols, such as IP Control Protocol (IPCP), provide features related to a specific Layer 3 protocol. Thus, 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 configured for PPP encapsulation automatically tries to bring up the appropriate control protocols for each Layer 3 protocol. Cisco routers also use a PPP CP for supporting CDP traffic, called CDPCP.

LCP provides a variety of optional features for PPP. These are:

• Error detection, which is provided by Link Quality Monitoring (LQM). The router can be configured to take down the link after a configured error rate has been exceeded. By taking down a link that has many errors, you can cause packets to use an alternative path that might not have as many errors but this is only useful when you have redundant routes in the network.

• Looped link detection, which is provided by magic numbers. Using different magic numbers, routers send messages to each other. If a router receives its own magic number, it knows that the frame it sent has been looped back. If configured to do so, the router can take down the interface through which the frame was sent, and effectively close the loop. This will improve convergence.

• Multilink support, which is provided by Multilink PPP and allows PPP to load-balance fragments of packets across multiple links.

• Authentication, which can be provided by Password Authentication Protocol (PAP) or Challenge Handshake Authentication Protocol (CHAP) and allows for the exchange of names and passwords so that each device can verify the identity of the device on the other end of the link. CHAP is the preferred method because it uses a Message Digest 5 (MD5) one-way hash to encode the password while PAP sends passwords in clear-text.