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
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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
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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-topoint 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
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PPP (continued)
• PPP can be used over several different physical
interfaces, including the following:
– Asynchronous serial
– ISDN synchronous serial
– High-Speed Serial Interface (HSSI)
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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
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PPP in the Protocol Stack (continued)
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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
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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
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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
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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
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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
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Establishing PPP Communications
(continued)
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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
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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
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Establishing PPP Communications
(continued)
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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)
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Frame Relay Standards and
Equipment (continued)
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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
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Frame Relay Standards and
Equipment (continued)
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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
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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)
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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
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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
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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
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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
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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
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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
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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
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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
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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-topoint subinterfaces for each virtual connection
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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
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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
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Frame Format
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Frame Relay Topologies
• Frame Relay can use many different WAN
topologies:
–
–
–
–
Peer (point-to-point)
Star (hub and spoke)
Partial mesh
Full mesh physical
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Frame Relay Configuration
• In this section, you will learn how to configure
Frame Relay over serial interfaces
– Using IP as the Network layer protocol
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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
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Basic Multipoint Configuration with
Two Routers (continued)
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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
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Multipoint Configuration Using a
Subinterface (continued)
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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
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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
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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 framerelay ietf
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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
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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
show
show
show
interface
frame-relay pvc
frame-relay map
frame-relay lmi
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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
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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
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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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