CCNA PPP and Frame Relay

CCNA Guide to Cisco Networking Fundamentals 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

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

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

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

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

PPP in the Protocol Stack (continued) CCNA Guide to Cisco Networking Fundamentals

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

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

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

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

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

Establishing PPP Communications (continued) CCNA Guide to Cisco Networking Fundamentals

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

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

Establishing PPP Communications (continued) CCNA Guide to Cisco Networking Fundamentals

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)

Frame Relay Standards and Equipment (continued) CCNA Guide to Cisco Networking Fundamentals

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

Frame Relay Standards and Equipment (continued) CCNA Guide to Cisco Networking Fundamentals

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

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)

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

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

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

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

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

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

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

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

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

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

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

Frame Format CCNA Guide to Cisco Networking Fundamentals

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

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

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

Basic Multipoint Configuration with Two Routers (continued) CCNA Guide to Cisco Networking Fundamentals

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

Multipoint Configuration Using a Subinterface (continued) CCNA Guide to Cisco Networking Fundamentals

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

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

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

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

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

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

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

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 PPP and Frame Relay

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