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Chapter 5Chap

ter 5

FRAME RELAYFRAME RELAY

Semester 4

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ObjectiveObjective

Learn about another type of WAN technology,Frame Relay, that can be implemented to solve

connectivity issues for users who need access togeographically distant locations.

Learn about Frame Relay services, standards,components, and operation.

Describes the configuration tasks for Frame Relayservice, along with the commands for monitoringand maintaining a Frame Relay connection.

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ContentContent

#Day Topic Duration

Frame Relay Technology

LMI: Ciscos Implementation of FR

LMI Features

Frame Relay Sub-Interfaces

Configuration of Basic Frame Relay

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FRAME RELAY TECHNOLOGYFRAME RELAY TECHNOLOGY

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IntroductionIntroduction

Frame Relay is a Consultative Committee forCCITT and ANSI standard.

Defines a process for sending data over apublic data network (PDN).

A way of sending information over a WAN bydividing data into packets.

It operates at the physical and data linklayers of the OSI reference model.

It relies on upper-layer protocols such as TCPfor error correction.

Frame Relay uses virtual circuits to make

connections.

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OverviewOverview

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Frame Relay devicesFrame Relay devices

The network providing the Frame Relay

interface can be either: A carrier-provided public network

A network of privately owned equipment, servinga single enterprise.

Frame Relay network devices such asswitches, routers, CSU/DSUs, or multiplexers.

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Connection oriented servicesConnection oriented services

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Frame Relay terminologiesFrame Relay terminologies

Access rate

Local management interface (LMI)

Committed information rate (CIR) Committed burst (Bc)

Committed rate measurement interval (Tc)

Excess burst (Be)

Forward explicit congestion noti. (FECN)

Backward explicit congestion noti.(BECN)

Discard eligibility (DE) indicator

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Frame Relay technologyFrame Relay technology

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Terminologies:Terminologies: Access RateAccess Rate

The clock speed of the connection (local loop) to theFrame Relay cloud.

It is the rate at which data travels into or out of thenetwork

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Terminologies:Terminologies: DLCIDLCI

Data-link connection identifier.

A number that identifies the end point in a FrameRelay network.

Significance only to the local network.

The Frame Relay switch maps the DLCIs between apair of routers to create a permanent virtual circuit.

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Terminologies:Terminologies: LMILMI

Local management interface.

A signaling standard between the CPE device and theFrame Relay switch

Responsible for managing the connection andmaintaining status btw the devices.

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Terminologies:Terminologies: CIRCIR

Committed information rate.

The CIR is the guaranteed rate, that the serviceprovider commits to providing.

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Terminologies:Terminologies: BcBc

Committed Burst

The maximum number of bits that the switch agreesto transfer during a interval.

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Terminologies:Terminologies: TcTc

Committed Rate Measurement Interval.

The time interval shouldnt exceed 125 ms, almostalways 125 ms

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Terminologies:Terminologies: Excess burstExcess burst

The maximum number of uncommitted bits that theswitch attempts to transfer beyond the CIR.

Dependent on the service offerings available by the

vendor, but is typically limited to the port speed ofthe local access loop.

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Terminologies:Terminologies: FECNFECN

Forward explicit congestion notification.

When a switch recognizes congestion in the network,it sends a FECN packet to the destination device.

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Terminologies:Terminologies: BECNBECN

Backward explicit congestion notification.

When a switch recognizes congestion in the network,it sends a BECN packet to the source router,instructing the router to reduce the rate at which it issending packets.

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Frame Relay congestionFrame Relay congestion

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Terminologies:Terminologies: DEDE

Discard eligibility indicator.

A set bit that indicates the frame may be discardedin preference to other frames if congestion occurs

The DE bit is set on the oversubscribed traffic.

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Frame Relay operationFrame Relay operation

A public FR service is deployed by putting FRswitching equipment in the central office of acarrier.

Economic benefits are got by from trafficsensitive charging rates and lack ofequipment and service maintenance.

The lines that connect user devices to theprovider can operate at a speed selected

from a broad range of data rates. Speeds between 56 kbps and 2 Mbps are

typical, although Frame Relay can supportlower and higher speeds.

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Frame Relay operationFrame Relay operation

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Frame Relay multiplexingFrame Relay multiplexing

Frame Relay provides a means formultiplexing many logical data

conversations, as virtual circuits, through ashared physical medium

Frame Relay's multiplexing provides moreflexible and efficient use of available

bandwidth. Frame Relay allows users to share bandwidth

at a reduced cost.

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Frame Relay multiplexingFrame Relay multiplexing

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Frame Relay DLCIFrame Relay DLCI

FR standards address PVCs that areconfigured and managed in a FR network.

FR PVCs are identified by DLCIs, that have

LOCAL significance. Multiplexing many virtual circuit through a

physical medium.

FR switches constructs a table mapping DLCIvalues to outbound ports.

The complete path to the destination isestablished before the first frame is sent.

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Frame Relay DLCI (cont.)Frame Relay DLCI (cont.)

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Frame Relay frame formatFrame Relay frame format

DLCI: Indicates the DLCI value. Consists of

the first 10 bits of the Address field. Congestion Control: The last 3 bits in the

address field. These are the FECN, BECN, anddiscard eligible (DE) bits.

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Frame Relay addressingFrame Relay addressing

DLCI address space is limited to 10 bits. possible 1024 DLCI addresses.

The usable portion of these addresses aredetermined by the LMI type: The Cisco LMI type supports a range of DLCI

addresses from DLCI 16-1007.

The ANSI/ITU LMI type supports the range ofaddresses from DLCI 16-992.

The remaining DLCI addresses are reservedfor vendor implementation.

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Frame Relay addressingFrame Relay addressing

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LMI: CISCOS IMPLEMENTATION OFLMI: CISCOS IMPLEMENTATION OF

FRAME RELAYFRAME RELAY

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LMI functionsLMI functions

To determine the operational status of thevarious PVCs that the router knows about

To transmit keepalive packets to ensure thatthe PVC stays up and does not shut downdue to inactivity

To tell the router what PVCs are available

Three LMI types can be invoked by therouter: ansi, cisco, and q933a

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LMI operationLMI operation

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LMI extension functionsLMI extension functions

In addition to the basic Frame Relay protocolfunctions for transferring data, the Frame

Relay specification includes LMI extensionsthat make supporting large, complexinternetworks easier. Virtual circuit status messages

Multicasting Global addressing

Simple flow control

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LMI FEATURESLMI FEATURES

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Frame Relay SignalingFrame Relay Signaling

Cisco supports three LMI standards: Cisco

ANSI T1.617 Annex D

ITU-T Q.933 Annex A

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Frame Relay mapFrame Relay map

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Frame Relay mappingFrame Relay mapping

Network address DLCI

The routing table is then used to supply thenext-hop protocol address or the DLCI foroutgoing traffic.

The resolution is done through a datastructure called a Frame Relay map.

This data structure can be staticallyconfigured in the router, or the Inverse ARPfeature can be used for automatic setup ofthe map.

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Frame Relay mappingFrame Relay mapping

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Inverse ARPInverse ARP

The Inverse ARP mechanism allows therouter to automatically build the Frame

Relay map.1. The router learns the DLCIs that are in use fromthe switch during the initial LMI exchange.

2. The router then sends an Inverse ARP request toeach DLCI for each protocol configured on theinterface.

3. The return information from the Inverse ARP isthen used to build the Frame Relay map.

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Frame Relay Inverse ARP and LMI SignalingFrame Relay Inverse ARP and LMI Signaling

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Stages of Inverse ARP and LMI OperationStages of Inverse ARP and LMI Operation

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Frame Relay switching tableFrame Relay switching table

The Frame Relay switching table consists offour entries: two for incoming port and DLCI,and two for outgoing port and DLCI.

The DLCI could, therefore, be remapped as itpasses through each switch; the fact that theport reference can be changed is why theDLCI does not change even though the portreference might change.

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Frame Relay switching tableFrame Relay switching table

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Frame Relay default: nonbroadcast, multiaccess (NBMA)

Selecting a Frame Relay TopologySelecting a Frame Relay Topology

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FRAME RELAY SUBINTERFACESFRAME RELAY SUBINTERFACES

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What are Frame Relay subinterfacesWhat are Frame Relay subinterfaces

Subinterfaces are logical subdivisions of aphysical interface.

In a subinterface configuration, each PVC canbe configured as a point-to-point connection,which allows the subinterface to act as adedicated line.

By using multiple virtual subinterfaces, theoverall cost of implementing a Frame Relaynetwork can be reduced.

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FR without subinterfaceFR without subinterface

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FR with subinterfaceFR with subinterface

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SubinterfaceSubinterface

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Split horizon routing environmentsSplit horizon routing environments

Split horizon reduces routing loops by notallowing a routing update received on onephysical interface to be sent back out that

same interface. As a result, if a remote router sends an

update to the headquarters router that isconnecting multiple PVCs over a singlephysical interface, the headquarters routercannot advertise that route through the samephysical interface to other remote routers.

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Split horizon and reachability problemSplit horizon and reachability problem

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Reachability issues:Reachability issues: Point-to-pointPoint-to-point

A single subinterface is used to establish one

PVC connection to another physical interfaceor subinterface on a remote router.

Each point-to-point connection is its ownsubnet. In this environment, broadcasts are

not a problem because the routers are point-to-point and act like a leased line.

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Reachability issues:Reachability issues: MultipointMultipoint

A single subinterface is used to establishmultiple PVC connections to multiple physical

interfaces or subinterfaces on remote routers. All the participating interfaces would be inthe same subnet, and each interface wouldhave its own local DLCI.

Because the subinterface is acting like aregular Frame Relay network, routing updatesare subject to split horizon.

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FRAME RELAY CONFIGURATIONFRAME RELAY CONFIGURATION

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Basic Frame Relay configurationBasic Frame Relay configuration

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Basic Frame Relay configurationBasic Frame Relay configuration

A basic Frame Relay configuration assumesthat:

you want to configure Frame Relay on onephysical interface and that LMI and Inverse ARPare supported by the remote routers.

The LMI notifies the router about the

available DLCIs. Inverse ARP is enabled by default, so it does

not appear in configuration output.

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Basic configuration:Basic configuration: Step 1Step 1

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Verifying Frame Relay operationVerifying Frame Relay operation

if i i

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Verifying Frame Relay operationVerifying Frame Relay operation

C fi F R l S i h

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Configure Frame Relay SwitchConfigure Frame Relay Switch

Enable Frame Relay Switching

FRSW(conf)# frame-relay switching

Configure interface

FRSW(conf-if)#Encapsulation frame-relay

FRSW(conf-if)#frame-relay intf-type dce|dte

FRSW(conf-if)#clock rate 56000

FRSW(conf-if)#frame-relay lmi-type cisco|ansi|q933a

FR route (create PVC - Switching Table)

FRSW(conf-if)#frame-relay route interface

C fi i i fC fi ti bi t f

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Configuration subinterfacesConfiguration subinterfaces

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Configuring SubinterfacesConfiguring Subinterfaces

Point-to-point Subinterfaces act like leased lines.

Each point-to-point subinterface requires its own subnet.

Point-to-point is applicable to hub and spoke topologies.

Multipoint Subinterfaces act like NBMA networks, so they do not resolve the

split-horizon issues.

Multipoint can save address space because it uses a single subnet.

Multipoint is applicable to partial mesh and full mesh topologies.

C fi ti bi t fC fi ti bi t f

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C fi ti bi t fC fi ti bi t f

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M lti i t bi t f lM lti i t bi t f l

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Multipoint subinterfaces exampleMultipoint subinterfaces example

P i t t i t bi t f lP i t t i t bi t f l

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Point-to-point subinterfaces examplePoint-to-point subinterfaces example

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FRAME RELAY CONFIGURATIONFRAME RELAY CONFIGURATIONEXAMPLESEXAMPLES

C fi tiC fi ti ith t bi t fith t bi t f

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Configuration:Configuration: without subinterfacewithout subinterface

#interface serial 0

#encapsulation frame-relay

#ip address 9.0.0.1 255.0.0.0

#router igrp 1

#network 1.0.0.0

#network 9.0.0.0

LMI type is automatically sensed

The encapsulation is Cisco

DLCI is learned via LMI status messages

Inverse ARP is enable (by default)

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FR Cloud

C fi tiConfiguration: S ifi d l tiSpecified encapsulation

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Configuration:Configuration: Specified encapsulationSpecified encapsulation

#interface serial 0

#ip address 9.0.0.1 255.0.0.0#encapsulation frame-relay ietf

#frame-relay lmi-type ansi

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FR CloudDLCI 41 DLCI 42

C fi tiConfiguration: ith bi t fwith subinterface

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Configuration:Configuration: with subinterfacewith subinterface

#interface serial 0


#frame-relay lmi-type ansi

#interface serial 0.1 point-to-point

#frame-relay interface-dlci 41

#ip address 9.0.0.1 255.0.0.0

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FR CloudDLCI 41 DLCI 42


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#interface serial 0


#interface serial 0.1 multipoint#ip address 9.0.0.1 255.0.0.0

#frame-relay interface-dlci 41

#frame-relay interface-dlci 43 ietf

DLCI 41 DLCI 42

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DLCI 44

DLCI 43

Configuration:Configuration: disabled inverse ARPdisabled inverse ARP

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Configuration:Configuration: disabled inverse ARPdisabled inverse ARP

#interface serial 0.1 multipoint

#ip address 9.0.0.1 255.0.0.0

#frame-relay interface-dlci 41#frame-relay interface-dlci 43 ietf

#frame-relay map ip 9.0.0.2 41 broadcast

#frame-relay map ip 9.0.0.3 43 broadcast

DLCI 41 DLCI 42

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DLCI 44

DLCI 43

Configuring a Static Frame Relay MapConfiguring a Static Frame Relay Map

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Configuring a Static Frame Relay MapConfiguring a Static Frame Relay Map

Configuring Point to Point SubinterfacesConfiguring Point to Point Subinterfaces

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Configuring Point-to-Point SubinterfacesConfiguring Point-to-Point Subinterfaces

Multipoint Subinterfaces ConfigurationMultipoint Subinterfaces Configuration

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p gp g

ExampleExample

Q&AQ&A

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