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CCNA Guide to Cisco Networking Fundamentals Fourth Edition

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Title: CCNA Guide to Cisco Networking Fundamentals Fourth Edition


1
CCNA Guide to Cisco Networking Fundamentals
Fourth Edition
  • Chapter 11
  • PPP and Frame Relay

2
Objectives
  • Describe PPP encapsulation
  • Configure PPP encapsulation and its options
  • Describe and enable PPP multilink
  • Understand Frame Relay standards and equipment

3
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

4
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

5
PPP (continued)
  • PPP can be used over several different physical
    interfaces, including the following
  • Asynchronous serial
  • ISDN synchronous serial
  • High-Speed Serial Interface (HSSI)

6
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

7
PPP in the Protocol Stack (continued)
8
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

9
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10
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

11
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

12
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

13
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

14
Establishing PPP Communications (continued)
15
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

16
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

17
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18
Establishing PPP Communications (continued)
19
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)

20
Frame Relay Standards and Equipment (continued)
21
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

22
Frame Relay Standards and Equipment (continued)
23
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24
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

25
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)

26
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

27
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

28
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29
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

30
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

31
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

32
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

33
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

34
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

35
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

36
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37
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38
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39
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

40
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

41
Frame Format
42
Frame Relay Topologies
  • Frame Relay can use many different WAN
    topologies
  • Peer (point-to-point)
  • Star (hub and spoke)
  • Partial mesh
  • Full mesh physical

43
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44
Frame Relay Configuration
  • In this section, you will learn how to configure
    Frame Relay over serial interfaces
  • Using IP as the Network layer protocol

45
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

46
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47
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48
Basic Multipoint Configuration with Two Routers
(continued)
49
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

50
Multipoint Configuration Using a Subinterface
(continued)
51
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52
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

53
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54
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55
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

56
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

57
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

58
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

59
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60
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61
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62
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

63
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

64
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
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