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
(No Transcript)
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
(No Transcript)
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
(No Transcript)
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
(No Transcript)
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
(No Transcript)
37
(No Transcript)
38
(No Transcript)
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
(No Transcript)
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
(No Transcript)
47
(No Transcript)
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
(No Transcript)
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
(No Transcript)
54
(No Transcript)
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
(No Transcript)
60
(No Transcript)
61
(No Transcript)
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