Chapter 3 Frame Relay-1

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Frame Relay

Accessing the WAN – Chapter 3



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Objectives

Describe the fundamental concepts of Frame Relay technologyin terms of Enterprise WAN services including Frame Relayoperation, Frame Relay implementation requirements, FrameRelay maps, and LMI operation.

Configure a basic Frame Relay PVC including configuring andtroubleshooting Frame Relay on a router serial interface and

configuring a static Frame Relay map.

Describe advanced concepts of Frame Relay technology interms of Enterprise WAN services including Frame Relay sub-interfaces, Frame Relay bandwidth and flow control.

Configure an advanced Frame Relay PVC including solving

reachability issues, configuring Frame Relay sub-interfaces,verifying and troubleshooting Frame Relay configuration.



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Frame Relay

As organizations grow and depend or reliable datatransport leased line solutions become prohibitivelyexpensive. The pace of technology change andexpansion geography into other areas requires moreflexibility.

Frame relay reduces network costs by using lessequipment, less complexity, and easier implementation.

Frame Relay provides greater bandwidth, reliability andresiliency than private or leased lines.



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Basic Concepts

Invented by Eric Scace at Sprint International.Sprint implemented using StrataCom switches, Cisco acquiredin 1996 and entered the carrier market.

Implemented for voice and data as an encapsulationtechnique between LANs over a WAN.

Each user gets a private line to a Frame Relay node

Frame Relay network handles the transmission over multiplepaths, transparent to all end users.

Frame Relay has become one of the most extensively

used WAN protocols primarily because it is inexpensivecompared to dedicated lines.

Benefit of Frame Relay over Leased line or ISDN:

Customers only pay for local loop and bandwidth purchased.



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Basic Concepts …

Error Control

Frame Relay services depend on the upper layer protocols tohandle error recovery.

The receiving device drops any frames that contain errorswithout notifying the sender.



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Frame Relay Operation

The connection between a DTE device and a DCEdevice consists of both a physical layer component anda data link layer component:

Physical component defines the mechanical, electrical,functional and procedural specifications.

Data link layer component defines the protocol that establishesthe connections between the DTE and DCE device

When carriers use Frame Relay to interconnect LANs

A router on each LAN is the DTE, a serial connection connectsto the carrier’s Frame Relay switch (DCE). Network switchesmove frames across the network and deliver to DTEs.

Computing equipment can also send data across a FrameRelay network by using a Frame Relay Access Device (FRAD)



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Concepts of Frame Relay Technology



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Virtual Circuits

Virtual circuits provide a bidirectional communication path between two DTE devices and are uniquelyidentified by a DLCI (data link connection identifier)

Circuits are virtual because there is no direct electricalconnection from end to end.

Connection is logical and data moves from end to end without adirect electrical circuit.

With VC, Frame Relay shares the bandwidth among multipleusers, and any single site can communicate with any othersingle site without using multiple dedicated paths

Two categories of VC

Switched Virtual Circuit (SVC)

Permanent Virtual Circuit (PVC)



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Virtual Circuits

SVCs are temporary connections used for onlysporadic transfer between DTE devices across theFrame Relay network.

Four operational states:call setup, data transfer, idle, and call termination

PVCs are permanently established connections that areused for frequent and consistent data transfers

between DTE devices.Does not require the call setup and termination states

Always operate in one of two states: data transfer or idle.



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Virtual Circuits



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Virtual Circuits



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DLCI

VCs are identified by DLCIs

Values are assigned by the Frame Relay service provider

DLCIs have local significance. (values are not unique to the

WAN)DLCI identifies a VC to the equipment at an endpoint

Two devices connected by a VC may use a different DLCI valueto refer to the same connection.

DLCIs are assigned by the service provider and range from 16

to 1007.

VC can be multiplexed to save equipment.



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DLCI



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Frame Relay Encapsulation

Frame Relay takes packets from a network layerprotocol (IP, IPX) encapsulates them as the dataportion of a FR frame then passes the frame to thephysical layer for delivery.

Encapsulation Process

Frame Relay accepts packet from IP

Wraps it with Address field that contains the DLCI and achecksum

Flag fields are added to indicate the beginning and end of theframe.

Frame Relay Address Header contains:

DLCI, C/R (not defined), Extended Address, Congestion

control



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Frame Relay Encapsulation



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Frame Relay Topologies



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Frame Relay Address Mapping

Before a Cisco router can transmit data over FrameRelay, it needs to know which local DCLI maps to theLayer 3 address of the remote destination.

Inverse ARP obtains Layer 3 addresses of otherstations from Layer 2 addresses.

Dynamic Mapping relies on Inverse ARP to resolve anext-hop network protocol address to a local DLCI

value. The frame relay router sends out Inverse ARP requests

on its PVC to discover the protocol address of theremote device connected to the Frame Relay network.



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Example of show frame-relay map command:

R1# show frame-relay mapSerial0/0/0 (up): ip 172.16.4.3 dlci 201 (0xC9, 0x3090), dynamic,

broadcast, status defined, active

Interface is up Destination IPaddress

Logical connection to reach interface.Displayed as decimal (201), hex value(0xC9) and value in address field(0x3090)



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Address Mapping

R1(config)# interface serial 0/0/0R1(config)# ip address 10.1.1.18 255.255.255.0R1(config)# encapsulation frame-relayR1(config)# frame-relay map ip 10.1.1.22. 22

R1(config)# exit

Question:Which two outcomes occur from this configuration?

Answer:1. The router will use DLCI 22 to forward data to 10.1.1.12. Frames arriving on Serial 0/0/0 will have a data-link

address of 22.



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Local Management Interface (LMI)

Frame Relay design provides packet-switched datatransfer with minimum end-to-end delays. The originaldesign omits anything that might contribute to delay.

When vendors implemented Frame Relay as aseparate technology there was a need to DTEs todynamically acquire information about the network’sstatus. An extension to the Frame Relay protocol calledLMI is used to provide this capability.

Basically, the LMI is a keepalive mechanism thatprovides status information about the connectionbetween the router (DTE) and the Frame Relay Switch(DCE)



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Configure Frame Relay

Step 1: Set the IP address on the interface

Step 2: Configure the encapsulation

encapsulation frame-relay

Step 3: Set the bandwidth (optional)

EIGRP and OSPF use the bandwidth value to calculate anddetermine the link’s metric.

Step 4: Set the LMI type (optional)

Cisco routers autosense the LMI type. Support 3 types:

Cisco, Ansi, and Q933-A, default is Cisco.



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Static Frame Relay Maps

Cisco routers support all network layer protocols over frameRelay. The address –to-DLCI mapping can beaccomplished by either dynamic or static address mapping.

Dynamic mapping is performed by the Inverse ARP feature

by default.LMI status messages also help in mapping data link layer tonetwork layer addresses.

Static mapping is manually configured on a router and isdependant on network needs.

Frame-relay map protocol-address dlci [broadcast]

Note: Frame Relay, ATM, and X.25 are nonbroadcast multiaccess (NBMA) networks. Because NBMA does not support broadcast traffic,using the broadcast keyword for the map command is a simplified way offorwarding routing updates. Should be used if RIP is used.



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Configure Static Frame Relay Map

Include “broadcast” in thecommand



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Configure Frame Relay Map

R1

R2

DLCI 103DLCI 301

S 0/0/0.1

S 0/0/0.2

R3

DLCI 201

S 0/0/0.3

DLCI 102

Question:What configuration option should be configured onthe R2 and R3 serial interfaces for correct

connection?

R2(config)# frame-relay map ip 10.1.1.1 201 broadcastR2(config)# frame-relay map ip 10.1.1.3 201 broadcastR3(config)# frame-relay map ip 10.1.1.1 301 broadcastR3(config)# frame-relay map ip 10.1.1.2 301 broadcast

R1# show running-config!Hostname R1!Interface Serial 0/0/0ip address 10.1.1.1 255.255.255.0encapsulation frame-relaybandwidth 64frame-relay map ip 10.1.1.2 102 broadcastframe-relay map ip 10.1.1.3 103 broadcast

!

10.1.1.0/24



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Describe Advanced Concepts of Frame RelayTechnology

Note:If traffic from R1 to R3 hasto pass through R2 thenserial interfaces on R2have to be subinterfacesfor data to pass.



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Solving Reachabiltiy Issues

By default Frame Relay networks provide NBMAconnectivity between remote sites. Unfortunately, abasic routing operation can cause reach-ability issues.

Split Horizon

A technique used to prevent a routing loop in networks usingdistance vector routing protocols. Reduce routing loops bypreventing a routing update receive on one interface from beingforwarded out the same interface.

Frame Relay Subinteraces

Frame relay can partition a physical interface into multiplevirtual interfaces called subinterfaces. In a subinterfaceconfiguration, each VC can be configured as a point-to-pointconnection thus allowing updates to be passed.

This eliminates split horizon issues without increasing the

likelihood of routing loops.



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Paying for Frame Relay From a customers view, Frame Relay is an interface

and one or more PVCs. Customers simply buy FrameRelay services from the service provider.

Customer pays for three Frame Relay costcomponents:

Access rate or port speed – line charge based on the portspeed that has been negotiated and installed.

PVC – after a PVC is established, the additional cost toincrease Committed Information Rate (CIR) is small

CIR – Customers normally choose a CIR (Committed

Information Rate) lower than the port speed or access rate.

Bursting – Frame Relay takes advantage of any unusedcapacity, making it available to other customers and usually atno extra charge



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Paying for Frame Relay



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Frame Relay Flow Control

Frame Relay reduces network overhead byimplementing simple congestion-notificationmechanisms

Forward Explicit Congestion Notification (FECN)

Backward Explicit Congestion Notification (BECN)

- Both are controlled by a single bit in the frame header

These let the router know that congestion is occurring

and that the router may implement traffic shaping orthrottling.

Frame also contains Discard Eligibility (DE) bit thatidentifies less important traffic.



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A frame relay switch does the following three thingswhen it detects an excessive build-up of frames in itsqueue.

1. Drops frames from the queue that have the DE bit set.

2. Sets the FECN bit on all frames it receives on thecongested link

3. Sets the BECN bit on all frames it places on the congestedlink.

Note: when you look at the statistics for a particular interface and it showsthat the FECN bit and the BECN bit has been set, it means there is congestion on the interface.



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Router# show frame-relay map

Serial 1/2 (up): ip 172.16.1.4 dlci 401 (0x191,0x6410,

dynamic, broadcast.. status, active

Question:What two statements are true about the above output?

Answer:1. Local dlci number is 4012. Interface is in active state and in the process of

negotiating configuration parameters



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Configure an Advanced Frame Relay PVC

Point-to-Point Subinterfaces

Example:R1(config)# interface serial 0/0/1.102 point-to-pointR1(config)# frame-relay interface-dlcl 102



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Configure an Advanced Frame Relay PVC Look at Interfaces



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Configure an Advanced Frame Relay PVC

Troubleshooting



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Troubleshooting

R1 R2

DLCI 102 DLCI 201

S 0/0/0172.16.1.1

S 0/0/0172.16.1.2

R1(config)# int Serial 0/0/0R1(config)# ip address 172.16.1.1 255.2555.255.252R1(config)# encapsulation frame-relayR1(config)# frame-relay map ip 172.16.1.2 201R1(config)# no frame-relay inverse-arpR1(config)# no shutdown

R2(config)# int Serial 0/0/0R2(config)# ip address 172.16.1.2 255.2555.255.252

R2(config)# encapsulation frame-relayR2(config)# frame-relay map ip 172.16.1.1 102R2(config)# no frame-relay inverse-arpR2(config)# no shutdown

Question:Why is the connection between R1 and R2failing ?

Map commandusing incorrect

DLCIs



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Summary Frame relay is the most widely used WAN technology

because it:

–Provides greater bandwidth than leased line

–Reduces cost because it uses less equipment

–Easy to implement

Frame relay is associated with layer 2 of the OSI modeland encapsulates data packets in a frame relay frame

Frame relay is configured on virtual circuits

–These virtual circuits may be identified by a DLCI

Frame relay uses inverse ARP to map DLCI to IPaddresses



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Summary

Configuring frame relay requires

–Enable frame relay encapsulation

–Configuring either static or dynamic mapping

–Considering split horizon problems that develop when multipleVCs are placed on a single physical interface

Factor affecting frame relay configuration

–How service provider has their charging scheme set up

Frame relay flow control

–DE

–FECN

–BECN



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Summary

The following commands can be used to help verifyframe relay configuration

–Show interfaces

–Show frame-relay lmi –Show frame-relay pvc ###

–Show frame-relay map

Use the following command to help troubleshoot a

frame relay configuration –Debug frame-relay lmi

Chapter 3 Frame Relay-1

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