these periods decreases the likelihood that increased debug command processing overhead will affect system use. Lab Activity Lab Exercise: Troubleshooting a Serial Interface In this lab, the students will configure a serial interface on the London and Paris routers. Lab Activity e-Lab Activity: Troubleshooting a Serial Interface In this lab, the student will configure a serial interface on the London and Paris routers.

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Content 3.2 PPP Authentication 3.2.1 PPP layered architecture PPP uses a layered architecture. A layered architecture is a logical model, design, or blueprint that aids in communication between interconnecting layers. The Open System Interconnection (OSI) model is the layered architecture used in networking. PPP provides a method for encapsulating multi-protocol datagrams over a point-to-point link, and uses the data link layer for testing the connection. Therefore PPP is made up of two sub-protocols:

• Link Control Protocol – Used for establishing the point-to-point link.
• Network Control Protocol – Used for configuring the various network layer protocols.

PPP can be configured on the following types of physical interfaces:

• Asynchronous serial
• Synchronous serial
• High-Speed Serial Interface (HSSI)
• Integrated Services Digital Network (ISDN)

PPP uses Link Control Protocol (LCP) to negotiate and setup control options on the WAN data link. PPP uses the Network Control Protocol (NCP) component to encapsulate and negotiate options for multiple network layer protocols. The LCP sits on top of the physical layer and is used to establish, configure, and test the data-link connection. PPP also uses LCP to automatically agree upon encapsulation format options such as:

• Authentication – Authentication options require that the calling side of the link enter information to help ensure the caller has the network administrator's permission to make the call. Peer routers exchange authentication messages. Two authentication choices are Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP).
• Compression – Compression options increase the effective throughput on PPP connections by reducing the amount of data in the frame that must travel across the link. The protocol decompresses the frame at its destination. Two compression protocols available in Cisco routers are Stacker and Predictor.
• Error detection – Error detection mechanisms with PPP enable a process to identify fault conditions. The Quality and Magic Number options help ensure a reliable, loop-free data link.
• Multilink – Cisco IOS Release 11.1 and later supports multilink PPP. This alternative provides load balancing over the router interfaces that PPP uses.
• PPP Callback – To further enhance security, Cisco IOS Release 11.1 offers callback over PPP. With this LCP option, a Cisco router can act as a callback client or as a callback server. The client makes the initial call, requests that it be called back, and terminates its initial call. The callback router answers the initial call and makes the return call to the client based on its configuration statements.

LCP will also do the following:

• Handle varying limits on packet size
• Detect common misconfiguration errors
• Terminate the link
• Determine when a link is functioning properly or when it is failing

PPP permits multiple network layer protocols to operate on the same communications link. For every network layer protocol used, a separate Network Control Protocol (NCP) is provided. For example, Internet Protocol (IP) uses the IP Control Protocol (IPCP), and Internetwork Packet Exchange (IPX) uses the Novell IPX Control Protocol (IPXCP). NCPs include functional fields containing standardized codes to indicate the network layer protocol type that PPP encapsulates. The fields of a PPP frame are as follows:

• Flag – Indicates the beginning or end of a frame and consists of the binary sequence 01111110.
• Address – Consists of the standard broadcast address, which is the binary sequence 11111111. PPP does not assign individual station addresses.
• Control – 1 byte that consists of the binary sequence 00000011, which calls for transmission of user data in an unsequenced frame. A connection-less link service similar to that of Logical Link Control (LLC) Type 1 is provided.
• Protocol – 2 bytes that identify the protocol encapsulated in the data field of the frame.
• Data – 0 or more bytes that contain the datagram for the protocol specified in the protocol field. The end of the data field is found by locating the closing flag sequence and allowing 2 bytes for the frame check sequence (FCS) field. The default maximum length of the data field is 1,500 bytes.
• FCS – Normally 16 bits or 2 bytes that refers to the extra characters added to a frame for error control purposes.

Interactive Media Activity Drag and Drop: PPP Layered Architecture When the student has completed this activity, the student will understand the basic PPP layered architecture. Web Links Point-to-Point Protocol (PPP) http://www.cisco.com/univercd/cc/ td/doc/cisintwk/ito_doc/ppp.htm

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Content 3.2 PPP Authentication 3.2.2 Establishing a PPP session PPP session establishment progresses through three phases. These phases are link establishment, authentication, and the network layer protocol phase. LCP frames are used to accomplish the work of each of the LCP phases. The following three classes of LCP frames are used in a PPP session:

• Link-establishment frames are used to establish and configure a link.
• Link-termination frames are used to terminate a link.
• Link-maintenance frames are used to manage and debug a link.

The three PPP session establishment phases are:

• Link-establishment phase – In this phase each PPP device sends LCP frames to configure and test the data link. LCP frames contain a configuration option field that allows devices to negotiate the use of options such as the maximum transmission unit (MTU), compression of certain PPP fields, and the link-authentication protocol. If a configuration option is not included in an LCP packet, the default value for that configuration option is assumed. Before any network layer packets can be exchanged, LCP must first open the connection and negotiate the configuration parameters. This phase is complete when a configuration acknowledgment frame has been sent and received.
• Authentication phase (optional) – After the link has been established and the authentication protocol decided on, the peer may be authenticated. Authentication, if used, takes place before the network layer protocol phase is entered. As part of this phase, LCP also allows for an optional link-quality determination test. The link is tested to determine whether the link quality is good enough to bring up network layer protocols.
• Network layer protocol phase – In this phase the PPP devices send NCP packets to choose and configure one or more network layer protocols, such as IP. Once each of the chosen network layer protocols has been configured, packets from each network layer protocol can be sent over the link. If LCP closes the link, it informs the network layer protocols so that they can take appropriate action. The show interfaces command reveals the LCP and NCP states under PPP configuration.

The PPP link remains configured for communications until LCP or NCP frames close the link or until an inactivity timer expires or a user intervenes. Interactive Media Activity Drag and Drop: Establishing a PPP Session When the student has completed this activity, the student will know the three steps in establishing a PPP Session. Lab Activity e-Lab Activity: show interfaces In this activity, the student will demonstrate how