CCNA4_M5_Frame_Relay (1130798)

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CCNA – Semester4Module 5Frame RelayObjectives• Components of a Frame Relay network• The technology of Frame Relay and topology of aFrame Relay network• Configuring Frame Relay• Issues of a non-broadcast multi-access networkFrame Relay ConceptsIntroducing Frame Relay• An ITU-T and ANSI standard.• A packet-switched, connection-oriented, WANservice.• It operates at the data link layer of the OSIreference model.• Uses a subset of HDLC protocol LAPF.Frame Relay operation• Frames carry data between user devices called DTE, andthe DCE at the edge of the WAN.FRAD• Computing equipment that is not on a LAN mayalso send data across a Frame Relay network.• The computing equipment will use a FrameRelay access device (FRAD) as the DTE.Frame Relay Toll network••May be privately owned, but it is more commonly provided asa service by a public carrier.Typically consists of many geographically scattered FrameRelay switches interconnected by trunk lines.Frame Relay terminology• Connection through the Frame Relay networkbetween two DTEs is called a virtual circuit (VC).• VCs established dynamically by sendingsignaling messages to the network are SVCs.• PVCs are preconfigured by the carrierTerminologies: Access Rate• The clock speed of the connection (local loop) tothe Frame Relay cloud.• It is the rate at which data travels into or out of thenetwork.Terminologies: DLCI• Data-link connection identifier.• A number that identifies the end point in a FrameRelay network.• Significant only to the local network.• The Frame Relay switch maps the DLCIs between apair of routers to create a permanent virtual circuit.Terminologies: LMI• Local management interface.• A signaling standard between the CPE device andthe Frame Relay switch.• Responsible for managing the connection andmaintaining status between the devices.Terminologies: CIR• Committed information rate (in bps).The averagerate at which you want to transmit in periods ofnoncongestion.• The CIR is the guaranteed rate, that the serviceprovider commits to providing.• While a frame is being transmitted, each bit will besent at the port speed.Terminologies: Tc• Committed Rate Measurement Interval.

The timeinterval over which the rates are calculated is calledthe committed time.• The time interval shouldn’t exceed 125 ms, almostalways 125 ms.Terminologies: Bc• The number of committed bits in Tc is thecommitted burst.• Bc=CIR x TcTerminologies: Excess burst• The maximum number of uncommitted bits that theswitch attempts to transfer beyond the CIR.• Dependent on the service offerings available by thevendor, but is typically limited to the port speed ofthe local access loop.TerminologiesMinCIRFrame Relay flow control• The switch maintains a bit counter for each VC.• An incoming frame is marked DE if it puts thecounter over Bc.• An incoming frame is discarded if it pushes thecounter over Bc + Be.• At the end of each Tc seconds the counter isreduced by Bc.Terminologies: FECN• Forward explicit congestion notification.• When a switch recognizes congestion in thenetwork, it sends a FECN packet to the destinationdevice.Terminologies: BECN• Backward explicit congestion notification.• When a switch recognizes congestion in thenetwork, it sends a BECN packet to the sourcerouter, instructing the router to reduce the rate atwhich it is sending packets.Frame Relay congestionTerminologies: DE• 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.Frame Relay bandwidth• The serial connection or access link to theFrame Relay network is normally a leased line.• The speed of the line is the access speed or portspeed.• Port speeds are typically between 64 kbps and 4Mbps.

Some providers offer speeds up to 45Mbps.Frame Relay frame format• DLCI: Indicates the DLCI value. Consists of thefirst 10 bits of the Address field.• Congestion Control: The last 3 bits in theaddress field. These are the FECN, BECN, anddiscard eligible (DE) bits.Frame Relay addressing• DLCI address space is limited to 10 bits. Æ possible 1024DLCI addresses.• The usable portion of these addresses are determined bythe LMI type:– The Cisco LMI type supports a range of DLCI addresses from DLCI16-1007.– The ANSI/ITU LMI type supports the range of addresses from DLCI16-992.• The remaining DLCI addresses are reserved for vendorimplementation.Frame Relay TopologyFrame Relay LMI functions• The heartbeat mechanism, which verifies that aVC is operational• The multicast mechanism• The flow control• The ability to give DLCIs global significance• The VC status mechanismLMI types• The LMI type configured on the router mustmatch the type used by the service provider.• Three types of LMIs are supported by Ciscorouters:– Cisco – The original LMI extensions– Ansi – Corresponding to the ANSI standard T1.617Annex D– q933a – Corresponding to the ITU standard Q933 AnnexALMI frame format• LMI messages are sent in frames distinguished by anLMI-specific DLCI.• Cisco specification as DLCI 1023.• The LMI frame contains 4 mandatory bytes:– The 1st bytes has the same format as the LAPB unnumberedinformation (UI) frame indicator, with the poll/final bit set to zero.– The 2nd byte is referred to as the protocol discriminator, which is setto a value that indicates LMI.– The 3rd byte (call reference) is always filled with zeros.– The final byte is the message type field:• Status messages• Status enquiry messagesFrame Relay mapping••••Network address ÅÆ DLCIThe routing table is then used to supply the next-hop protocoladdress or the DLCI for outgoing traffic.The resolution is done through a data structure called a Frame Relaymap.This data structure can be statically configured in the router, or theInverse ARP feature can be used for automatic setup of the map.LMI operation• LMI status messages combined with Inverse ARPmessages allow a router to associate network layer anddata link layer addresses.• When a router that is connected to a Frame Relaynetwork is started, it sends an LMI status inquirymessage to the network.• The network replies with an LMI status messagecontaining details of every VC configured on the accesslink.• Subsequent responses include only status changes.Frame Relay mappingFrame Relay switching table• The Frame Relay switching table consists of four entries:incoming port and DLCI, and outgoing port and DLCI.• The DLCI may be remapped as it passes through eachswitch.Configuring Frame RelayFrame Relay encapsulation• Frame Relay is configured on a serial interfaceand the default encapsulation type is the Ciscoproprietary version of HDLC.• To change the encapsulation to Frame Relayuse theencapsulation frame-relay [cisco | ietf]LMI type• The LMI connection is established andconfigured by the command:frame-relay lmi-type [ansi | cisco | q933a]• IOS Release 11.2 or later, the LMI-type isautosensed and no configuration is needed.• The default LMI type is cisco.• The LMI type is set on a per-interface basis and isshown in the output of the show interfacescommand.Configuring basic Frame RelayConfiguring a static Frame Relay map• The local DLCI must be statically mapped to the networklayer address when:– Remote router does not support Inverse ARP.– Broadcast traffic and multicast traffic over the PVC must becontrolled.– Paritial-mesh Frame Relay topology.router(conf-if)#frame-relay map protocol protocol-addressdlci [broadcast]• Static frame-relay map disables InverseARP, to turn itback on use the command:router(conf-if)#frame-relay inverse-arp [protocol] [dlci]Configuring a static Frame Relay mapSplit Horizon••When a single interface is used to interconnect multiple sites,there may be reachability issues as nonbroadcast multiaccess(NBMA) nature of Frame Relay .Split horizon does not allow routing updates to be sent out thesame interface that was the source of the route information.Frame Relay subinterfaces• Use subinterfaces in Frame Relay:– To enable the forwarding of broadcast in a hub-and-spoke FrameRelay topology.– To subject problem regarding split-horizon– To reduce overall cost of many physical interfaces• Frame Relay subinterfaces can be configured in eitherpoint-to-point or multipoint mode:– Point-to-point– MultipointFrame Relay subinterfaces• The encapsulation frame-relay command is assigned tothe physical interface.

All other configuration items,such as the network layer address and DLCIs, areassigned to the subinterface.Configuring Frame Relay subinterfaces• Configure encapsulation and no shut the physicalinterface without ip address.router(config-if)#interface serial number.subinterface-number{multipoint | point-to-point}• Using major interface as point-to-point, DLCIconfiguration is not required as it can be learned via LMIfrom Frame Relay switch.• With subinterfaces, use this command configure thelocal DLCI:router(config-subif)#frame-relay interface-dlci dlci-number[cisco|ietf]Subinterface configuration sampleConfigure Cisco router as Frame Relay switch• Cisco router can be configured as Frame Relay switchusing command:Router(config)#frame-relay switching• Then all connections should be DCE type and bespecified with the command:Router(conf-if)#frame-relay intf-type dce|dte• Configure LMI type on Frame Relay switch:Router(conf-if)#frame-relay lmi-type {ansi | cisco | q933a}• Configure Frame Relay routes to create switching table:Router(conf-if)#frame-relay route in_dlci interface out_interfaceout_dlciFrame Relay routesVerifying the Frame Relay configuration• show interfaces command displays LMI type, LMI DLCI,Frame Relay DTE/DCE type• show frame-relay lmi command to display LMI trafficstatistics.• show frame-relay pvc [interface interface] [dlci] commandto display the status of each configured PVC as well astraffic statistics.• show frame-relay map command to display the currentmap entries and information about the connections.• show frame-relay route command on frame relay switch todisplay switching table.Show interfaceShow frame-relay lmiShow frame-relay pvcShow frame-relay mapTroubleshooting the Frame Relay configuration• debug frame-relay lmi command to determinewhether the router and the Frame Relay switchare sending and receiving LMI packets properly.• debug frame-relay events command to displayframe-relay packets.Debug frame-relay lmiSummary••••••••The components of a Frame Relay networkThe technology of Frame RelayPoint-to-point and point-to-multipoint topologiesThe topology of a Frame Relay network and potentialproblemsHow to configure a Frame Relay Permanent Virtual Circuit(PVC)How to create a Frame Relay Map on a remote networkWhy subinterfaces are needed and how they are configuredHow to verify and troubleshoot a Frame Relay connectionLab1 Topology1.2.3.PVC(21-42) and PVC(32-41) belong to 1 subnet (partial-mesh)All interfaces are multipoint subinterface (full-mesh)IP address is 192.168.1.0/24 .

Each Loopback interface requires 30IPs. Rouing protocol is EIGRP with AS 100Lab2 TopologyLab2 Requirements• PVC(111-121), PVC(123-143), PVC(142-112) form1 subnet• PVC(122-131) form 1 subnet• PVC(132-141) form 1 subnet• Routing protocol is EIGRP, AS 200• Network address is 172.30.0.0/16, each loopbackinterface requires 31 IPs.CCNA4 – Module5.

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