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					 CCNA Guide to Cisco
Networking Fundamentals
       Fourth Edition


        Chapter 11
    PPP and Frame Relay
                                 Objectives

•   Describe PPP encapsulation
•   Configure PPP encapsulation and its options
•   Describe and enable PPP multilink
•   Understand Frame Relay standards and equipment




CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   2
                   Objectives (continued)

• Describe the role of virtual circuits and performance
  parameters in Frame Relay
• Understand the Frame Relay topologies
• Understand the difference between multipoint and
  point-to-point configurations
• Configure and monitor Frame Relay




CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   3
                                         PPP

• PPP
    – Internet standard protocol defined in RFCs 2153 and
      1661
    – Provide point-to-point, router-to-router, host-to-router,
      and host-to-host connections
    – Considered a peer technology based on its point-to-
      point physical configuration
    – Commonly used over dial-up or leased lines to provide
      connections into IP networks
• Serial Line Internet Protocol (SLIP) was the
  predecessor to PPP
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   4
                          PPP (continued)

• PPP can be used over several different physical
  interfaces, including the following:
    – Asynchronous serial
    – ISDN synchronous serial
    – High-Speed Serial Interface (HSSI)




CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   5
               PPP in the Protocol Stack
• You can use PPP over both asynchronous and
  synchronous connections
    – At the Physical layer of the OSI reference model
• Link Control Protocol (LCP)
    – Used at the Data Link layer to establish, configure,
      and test the connection
• Network Control Protocols (NCPs)
    – Allow the simultaneous use of multiple Network layer
      protocols and are required for each protocol that uses
      PPP

CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   6
 PPP in the Protocol Stack (continued)




CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   7
                             Frame Format

• PPP is based on the High-Level Data Link
  Control (HDLC) protocol
• The difference between PPP frames and HDLC
  frames is that PPP frames contain protocol and
  Link Control Protocol (LCP) fields
• LCP
     – Described in RFCs 1548, 1570, 1661, 2153, and
       2484
     – Describes PPP organization and methodology,
       including basic LCP extensions

CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   8
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   9
               Frame Format (continued)

• LCP field of the PPP packet can contain many
  different pieces of information, including the
  following:
     –   Asynchronous character map
     –   Maximum receive unit size
     –   Compression
     –   Authentication
     –   Magic number
     –   Link Quality Monitoring (LQM)
     –   Multilink

CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   10
               Frame Format (continued)

• LCP link configuration process
     – Modifies and enhances the default characteristics of
       a PPP connection
• Includes the following actions:
     –   Link establishment
     –   Authentication (optional)
     –   Link-quality determination (optional)
     –   Network layer protocol configuration negotiation
     –   Link termination


CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   11
     Establishing PPP Communications

• Involves the following actions:
     – Link establishment
     – Optional authentication
     – Network layer protocol configuration negotiation
• The link establishment phase involves the
  configuration and testing of the data link
• The authentication process can use two
  authentication types with PPP connections: PAP
  and CHAP


CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   12
     Establishing PPP Communications
                 (continued)
• PPP is an encapsulation type for serial interface
  communications
     – To configure a PPP connection, you must access the
       interface configuration mode for the specific
       interface you want to configure
• After LCP has finished negotiating the configuration
  parameters
     – Network layer protocols can be configured
       individually by the appropriate NCP



CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   13
     Establishing PPP Communications
                 (continued)




CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   14
     Establishing PPP Communications
                 (continued)
• Configuring PPP Authentication
     – Using authentication with PPP connections is
       optional
     – You must specifically configure PPP authentication
       on each PPP host in order for the host to use it
     – You can choose to enable CHAP, PAP, or both on
       your PPP connection, in either order




CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   15
     Establishing PPP Communications
                 (continued)
• Once you set the authentication type
     – You must still configure a username and password
       for the authentication
• You must exit interface configuration mode and
  enter global configuration mode
     – Type username followed by the host name of the
       remote router
     – Then type password followed by the password for
       that connection
• Confirming PPP Communications
     – With the show interface command
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   16
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   17
     Establishing PPP Communications
                 (continued)




CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   18
            Frame Relay Standards and
                   Equipment
• Frame Relay
     – A packet switching and encapsulation technology
       that functions at the Physical and Data Link layers of
       the OSI reference model
     – A communications technique for sending data over
       high-speed digital connections
• ITU-T and ANSI define Frame Relay
     – As a connection between the data terminal
       equipment (DTE) and the data communications
       equipment (DCE)

CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   19
            Frame Relay Standards and
              Equipment (continued)




CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   20
            Frame Relay Standards and
              Equipment (continued)
• The physical equipment that is used on a network
  may vary from one organization to another
• Some routers have built-in cards that allow them to
  make WAN connections




CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   21
            Frame Relay Standards and
              Equipment (continued)




CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   22
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   23
            Frame Relay Standards and
              Equipment (continued)
• Frame Relay access device (FRAD)
     – Network device that connects to the Frame Relay
       switch
     – Also known as Frame Relay
       assembler/disassembler
• Frame Relay network device (FRND)
     – The Frame Relay switch




CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   24
                             Virtual Circuits

• You can use Frame Relay with nearly any serial
  interface
     – Operates by multiplexing
• Frame Relay separates each data stream into
  logical (software-maintained) connections
     – Called virtual circuits
     – Which carry the data transferred on the connection
• Two types of virtual circuits
     – Switched virtual circuits (SVC)
     – Permanent virtual circuits (PVC)

CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   25
                                        DLCI

• Frame relay connections identify virtual circuits by
  Data Link Connection Identifier (DLCI) numbers
• A DLCI number associates an IP address with a
  specific virtual circuit
• DLCI numbers have only local significance
• DLCI numbers are usually assigned by the Frame
  Relay provider
     – Most likely not the same on either side of the Frame
       Relay switch


CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   26
                         Frame Relay Map

• Frame Relay map
     – A table in RAM that defines the remote interface to
       which a specific DLCI number is mapped
• The definition will contain a DLCI number and an
  interface identifier
     – Which is typically a remote IP address
• The Frame Relay map can be built automatically or
  statically depending on the Frame Relay topology



CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   27
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   28
          Frame Relay Map (continued)

• Subinterfaces
     – Virtual interfaces associated with a physical interface
     – Created by referencing the physical interface
       followed by a period and a decimal number
• For the purposes of routing, however,
  subinterfaces are treated as physical interfaces
• With subinterfaces, the cost of implementing
  multiple Frame Relay virtual circuits is reduced
     – Because only one port is required on the router


CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   29
                                         LMI

• LMI basically extended the functionality of Frame
  Relay by:
     – Making the DLCIs globally significant rather than
       locally significant
     – Creating a signaling mechanism between the router
       and the Frame Relay switch, which could report on
       the status of the link
     – Supporting multicasting
• Providing DLCI numbers that are globally
  significant makes automatic configuration of the
  Frame Relay map possible
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   30
                           LMI (continued)

• LMI uses keepalive packets to verify the Frame
  Relay link and to ensure the flow of data
• Each virtual circuit, represented by its DLCI
  number, can have one of three connection states:
     – Active
     – Inactive
     – Deleted
• The Frame Relay switch reports this status
  information to the Frame Relay map on the local
  router
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   31
                               Inverse ARP

• In multipoint configurations
     – Routers use the protocol Inverse ARP to send a
       query using the DLCI number to find a remote IP
       address
• As other routers respond to the Inverse ARP
  queries, the local router can build its Frame Relay
  map automatically
• To maintain the Frame Relay map, routers
  exchange Inverse ARP messages every 60
  seconds by default

CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   32
                     Encapsulation Types

• LMI has several different protocol encapsulation
  types that it can use for management
  communications
• Cisco routers support these types of LMI
  encapsulation:
     – cisco
     – ansi
     – q933a
• Cisco routers (using IOS Release 11.2 or later) can
  “autosense” the LMI type used by the Frame Relay
  switch
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   33
       Encapsulation Types (continued)
• The basic LMI type has three information elements:
  report type, keepalive, and PVC status
• Information concerning the status of the virtual
  circuit:
     –   New
     –   Active
     –   Receiver not ready
     –   Minimum bandwidth
     –   Global addressing
     –   Multicasting
     –   Provider-Initiated Status Update
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   34
       Encapsulation Types (continued)
• Split horizon
     – Routing technique that reduces the chance of
       routing loops on a network
     – Prevents routing update information received on one
       physical interface from being rebroadcast to other
       devices through that same physical interface
     – People also refer to this rule as nonbroadcast
       multiaccess (NBMA)
     – Can cause problems for Frame Relay routing
       updates
• The best solution is to configure separate point-to-
  point subinterfaces for each virtual connection
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   35
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CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   38
                Performance Parameters

• Service contract specifies parameters by which the
  connection is expected to function:
     –   Access rate
     –   Committed Information Rate (CIR)
     –   Committed Burst Size (CBS)
     –   Excess Burst Size (EBS)
     –   Oversubscription




CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   39
                                Congestion

• Frame Relay switches attempt to control
  congestion on the network
• When the Frame Relay switch recognizes
  congestion
     – Sends a forward explicit congestion notification
       (FECN) message to the destination router
• In addition, the switch sends a backward explicit
  congestion notification (BECN) message to the
  transmitting, or source, router


CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   40
                             Frame Format




CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   41
                 Frame Relay Topologies

• Frame Relay can use many different WAN
  topologies:
     –   Peer (point-to-point)
     –   Star (hub and spoke)
     –   Partial mesh
     –   Full mesh physical




CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   42
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   43
              Frame Relay Configuration

• In this section, you will learn how to configure
  Frame Relay over serial interfaces
     – Using IP as the Network layer protocol




CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   44
     Basic Multipoint Configuration with
                Two Routers
• LMI will notify the router about the available DLCI
  numbers
     – Inverse ARP will build the Frame Relay map
       dynamically




CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   45
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   46
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   47
     Basic Multipoint Configuration with
         Two Routers (continued)




CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   48
        Multipoint Configuration Using a
                  Subinterface
• The Frame Relay map will have to be built statically
  on RouterA
     – To configure a multipoint subinterface, you map it to
       multiple remote routers using the same subnet
       mask, but different DLCI numbers




CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   49
        Multipoint Configuration Using a
           Subinterface (continued)




CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   50
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   51
     Point-to-Point Configuration Using
               Subinterfaces
• Point-to-point Frame Relay configurations do not
  support Inverse ARP
• You will have to configure each subnet separately
     – Use the frame-relay interface-dlci command to
       associate the DLCI numbers with a specific
       subinterface




CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   52
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CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   54
            Frame Relay Static Mapping

• Sometimes you have to define the DLCI numbers
  manually
     – This is called making a static address to DLCI
       Frame Relay map
• You statically configure your DLCI entries in the
  following situations:
     – The remote router does not support Inverse ARP
     – You need to assign specific subinterfaces to specific
       DLCI connections
     – You want to reduce broadcast traffic
     – You are configuring OSPF over Frame Relay
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   55
                       Non-Cisco Routers

• Non-Cisco routers use a different Frame Relay
  encapsulation than Cisco routers
• If you are configuring Cisco routers to connect to
  other Cisco routers
     – They will automatically use the Cisco Frame Relay
       encapsulation
• If you are connecting a Cisco router to a non-Cisco
  router, you must specify ietf Frame Relay
  encapsulation using the following command:
        RouterA(config-if)#encapsulation frame-
        relay ietf
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   56
                 Keepalive Configuration

• By default, keepalive packets are sent out every 10
  seconds to the Frame Relay switch
• You can change the keepalive period by typing
  keepalive followed by the time in seconds
        RouterA(config-if)#keepalive 15




CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   57
                 Monitoring Frame Relay

• You can check your Frame Relay configuration by
  using show commands
     – These commands allow you to verify that the
       commands you previously entered produced the
       desired effect on your router
• The most common show commands for monitoring
  Frame Relay operation are:
          show       interface
          show       frame-relay pvc
          show       frame-relay map
          show       frame-relay lmi
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   58
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CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   61
                                  Summary

• Many WAN connectivity options are available for
  modern networks, including digital lines, Frame
  Relay, and analog modems
• The Point-to-Point Protocol (PPP) is the most
  widely used WAN protocol today
• PPP provides link establishment, quality
  determination, Network layer protocol
  encapsulation, and link termination services
• Frame relay is a flexible WAN technology that can
  be used to connect two geographically separate
  LANs
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   62
                    Summary (continued)

• Frame relay is both a service and type of
  encapsulation
• Service parameters for Frame Relay include the
  access rate, Committed Information Rate (CIR),
  Committed Burst Size (CBS), and Excess Burst
  Size (EBS)
• Frame relay connections employ virtual circuits that
  can be either permanent or switched
• Virtual circuit connections across Frame Relay
  connections are defined by Data Link Connection
  Identifier (DLCI) numbers
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   63
                    Summary (continued)

• Most Frame Relay providers support LMI, which
  allows Frame Relay maps to be dynamically
  created via Inverse ARP
• Static mappings of DLCI numbers to remote IP
  addresses can be configured when routers do not
  support Inverse ARP
• Inverse ARP is not enabled on point-to-point links
  because only one path is available
• Frame relay circuits can be established over serial
  interfaces or subinterfaces on Cisco routers

CCNA Guide to Cisco Networking Fundamentals, Fourth Edition   64

				
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