chapter 12 frame relay210.70.254.122/chingyu/ch12 frame relay.pdf9 frame relay standards and eq....

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Chapter 12Frame Relay

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Frame Relay Stand. & Eq.Virtual CircuitsLMI

Contents

Monitoring Frame Relay

Performance ParametersFrame FormatFrame Relay TopologiesFrame Relay Config.

Review Questions

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Frame Relay Standards and Equipments

Frame Relay is a packet-switching andencapsulation technology that function at thePhysical and Data Linklayers of the OSI model.

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Frame Relay Standards and Eq. (cont.)

The data rate for the Frame Relay can be from56 Kbps to 45.736 Mbps, or higher.

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ITU-T and ANSI defineFrame Relay as a connection between the data terminal equipment(DTE) and data communications equipment (DCE) .

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DCE is switching equipment supplied by a telecom. provider that serves as a connection to the public data network (PDN).

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DTE is also known ascustom premises equipment (CPE), because it is the equipment that belongs to, and is maintained by, the PDN customer.

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For example, if youconnect your Cisco routerto a Frame Relay switch, the Cisco router is the CPEand the Frame Relay switchis the DCE. (see Fig. 12-1)

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In Fig. 12-2, you can see a CSU/DSU that is used with a Cisco 2501 router to make the connection. The CSU/DSU is atthe customer location. The unit is used for encoding, filtering, and translating communications to and from the digital line.

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Some routers have built-in cards that allow them to make WAN connections. Fig. 12-3 indicates a T1 CSU/DSU cards built into the router.

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A typical example Frame Relay connection was shown in the figure.

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Review Questions

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Virtual Circuits

You can use Frame Relay with nearly any serial interface. It operates by multiplexing, which means that it combines multiple data streams onto onephysical link.

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Virtual Circuits (cont.)

Frame Relay separate each data streams into logical (software-maintained)connections called virtual circuits, which carry thedata transferred on the connection.

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Two types of virtual circuits, switched virtual circuits(SVC) and permanent virtualcircuits (PVC), connectFrame Relay ports.

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The SVC softwareautomatically dials the WAN, establishing and terminating the connectionas required to transfer dataover Frame Relay service.

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PVCs remain permanentlyconnected to the WAN. The administrator manually defines the PVC; it remains until the network administrator removes it.

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DLCIFrame Relay connections identify virtual circuits by Data Link Connection Identifier (DLCI) number.

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DLCI (cont.)

For example, a DLCInumber associates an IP address with a specificvirtual circuits.

The DLCI number mapvirtual circuits to layer 3protocol addresses.

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Frame Relay MapDLCI numbers are

mapped, or assigned, to a specific interface. Each router that supports Frame Relay will have aFrame Relay map, which is a table in RAM.

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Fig. 12-4 shows a sample Frame Relay configuration.

Notice that the Frame Relay switching table is configured to map its ports to the correct DLCI numbers for the virtual connection.

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The switching table in this example is simplified. Remember that DLCInumbers are only locally significant, so it would be possible both routers use the same DLCI numbers to specify a virtual circuit.

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SubinterfacesA single router serial

interface can service multiple PVCs through a single physical serialinterface.

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Subinterfaces (cont.)

To allow a single serialinterface to support multiple PVCs, the IOS divides the interface into logical subinterface.

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Subinterfaces (cont.)For example, if Serial 0(s0) had three subinterfaces, they would be referenced ass0.1, s0.2, and s0.3.

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Subinterfaces (cont.)Subinterfaces are not

real physical interfaces; they are virtual interfacesassociated with a physical interface.

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Subinterfaces (cont.)With subinterfaces, the

cost of implementing multiple Frame Relayvirtual circuits is reducedbecause one port is required on the router.

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Contents



Review Questions

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LMI

The Local Management Interface (LMI) is used only locally, between the Frame Relay DTE (e.g., a router) and Frame Relay DCE (e.g.,a switch), as shown in the figure.

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The LMI basically extend the functionality of Frame Relay by:

Making the DLCIsglobally significant rather then locally significant

LMI (cont.)

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LMI (cont.)

Creating a signaling mechanism between the routers and Frame Relayswitch, which could reportthe status of the link.Supporting multicasting

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LMI (cont.)

Providing DLCI numbers that are globally significant makesautomatic configuration of Frame Relay map possible.

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LMI (cont.)

LMI uses keepalivepackets to verify theFrame Relay link and to ensure the flow of data.The Frame Relay switch in turn provides the status of all VCs and their respective DLCI numbers.

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LMI (cont.)

Each virtual circuit, represented by its DLCI number, can have one of three connection states:

ActiveInactiveDeleted

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LMI (cont.)

Without LMI, the Frame Relay map must be built statically in the router.By making DLCIs globally significant, LMI facilitates dynamic Frame Relay map tables through the use of inverse ARP protocol.

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LMI (cont.)

Inverse ARP In multipoint config.,

routers use the protocol inverse ARP to send a query using DLCI number to find a remote IPaddress.

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LMI (cont.)

Inverse ARP (cont.)As other routers respond to the inverse ARP queries, the local routers can build its Frame Relaymap automatically.

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LMI (cont.)

Inverse ARP (cont.)To maintain Frame Relaymap, routers exchange inverse ARP messagesevery 60 sec by default.

LMI is required for inverse ARP to work.

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LMI (cont.)

Inverse ARP (cont.)Do not confuse inverse

ARP with reverse ARP (RARP); RARP is used primarily on LANs to provide hosts that only have MAC address with IPaddress.

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LMI (cont.)

Encapsulation TypesDifferent Frame Relay

switches, CPE, and Frame Relay connectivity equipment support different types of LMI encapsulation.

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LMI (cont.)

Encapsulation Types (cont.)Cisco routers support

theses types of LMI encapsulation:

ciscoansiq933a

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LMI (cont.)

Split HorizonSplit horizon is a routing technique that reduces the chance of routing loops on a network.

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Split Horizon (cont.)

LMI (cont.)

A split horizon implementation technique preventing routing updateinformation received on one physical interface from being rebroadcast to other devicesthrough that same physical interface.

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LMI (cont.)

People also refer to this rule as nonbroadcastmultiaccess (NBMA).NBMA is a term used to describe WAN networks that use VCs for connectivity.

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Fig. 12-5 shows a split horizon problem. RouterAwould not be able to sendroute updates received from RouterB to RouterC, and vice versa.


LMI (cont.)

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LMI (cont.)

Split Horizon (cont.)The best solution is to

configure separate point-to-point subinterfaces for each virtual connection. (see Fig. 12-6)

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Split Horizon (cont.)Point-to-point connections allow you to divide a single serial interface into multiple subinterfaces, each supporting a separate virtual connection.

LMI (cont.)

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Split Horizon (cont.)In a multipoint configurations, the network administrator can configure a single serial interface or subinterface to support multiple connections to physical or logical interfaces on other routers.

LMI (cont.)

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LMI (cont.)

Split Horizon (cont.)A single subinterface

configured for multiple connection is still subject to the spit horizon rule.

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LMI (cont.)

Split Horizon (cont.)The only benefit to the

multiple configuration is that it allows you to use a single network for all of your routers, as shown inFig. 12-7.

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Contents



Review Questions

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Performance Parameters

Terms that appear in thecontract may include:

Access rate: The speedof the line, which indicates transfer rate. Access rate is also known as the local access rate.

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Performance Parameters (cont.)

Committed Information Rate (CIR): The minimumtransfer rate that the Frame Relay customernegotiates with Frame Relay service provider.

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CIR (cont.): The service provider agrees to alwaysallow the customer to transfer information at no less than the transfer rate specified by the CIR.


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Committed Burst Size (CBS): The maximum amount of data bits that the service provider agrees to transfer in a set time period under normal conditions.

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Excess Burst Size (EBS): The amount of excess traffic (over the CBS) that the network will attempt to transfer during a set time period. The network can discard EBS data, if necessary.

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Oversubscription: When the sum of the data arriving overall virtual circuits exceeds the access rate, the situation is called oversubscription. Oversubscription results in dropped packets.

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Two features of oversubscription are:

Fewer T1 lines: less infrastructure;

Win-win concept: huge saving for both customers and providers.

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The figure shows an example of how the Frame Relay traffic parametersaffect the data rate of a VC.(excess

burst rate)(committedburst rate)

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As long as the data rate ofVC is below the CIR/BCvalues, the Frame Relay switch allows the frames into the Frame Relay network.

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However, those frames (4 and 5) that exceed the VCvalue will have their Discard Eligible bits set, which allows the carrier to drop these frames in times of internal congestion.

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Also, any frames that exceed the BE are dropped, in this example, frames 6 and 7 are dropped.

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CongestionWhen the Frame Relay

switch recognizes congestion, it send a forward explicit congestion notification(FECN) message to the destination router.


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Congestion (cont.)


In addition, the switch sends a backward explicit congestion notification(BECN) message to the transmitting, or source router.

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Review Questions

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Frame Format

Frame Relay devices can use different Frame Relayframe format. Fig. 12-8 shows the Ciscoproprietary Frame Relayframe format.

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Contents



Review Questions

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Frame Relay Topologies

Frame Relay can use all of the WAN technologies discussed in Ch. 5: peer(point-to-point), star (hub-and-spoke), partial mesh, or full mesh physical topology. (see Fig. 12-9),

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Contents



Review Questions

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Frame Relay ConfigurationBasic Multipoint Config. With 2 Routers

In Fig. 12-10, RouterA is a Cisco router running IOS ver. 11.2, so it has ability to autosense LMI type. In addition, it automatically receives the DLCI informationby querying the network.

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Frame Relay Configuration (cont.)

Config. With 2 Routers (cont.)Table 12-1 lists the Ciscorouter prompts and commands that you need to complete this configuration.

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If RouterB is running IOSrelease ver. 11.0, the configuration commands for the router are shown in Fig. 12-11.


Config. With 2 Routers (cont.)

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Another example of configuring with 2 routers

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To configure a multipoint subinterfaces, you map it to multiple remote routers using the same subnet mask, butdifferent DLCI number.

Multipoint ConfigurationUsing Subinterfaces

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Table 12-2 outlinesthe steps to configureRouterA.

Multipoint ConfigurationUsing Subinterfaces (cont.)

須修改此部份

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修改成：

(config-subif)# frame-relay interface-dlci 102



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Point-to-Point ConfigurationUsing Subinterfaces

In Fig. 12-13, RouterA will be configured for three point-to-point connections to remote routers B, C, and D.

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Point-to-point connections Frame Relay configurations do not support Inverse ARP.

Point-to-Point ConfigurationUsing Subinterfaces (cont.)

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You have to configure each subnet separately and use the frame-relay interface-dlci command to associate the DLCI number with a specific subinterface.

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These remote routersconfigurations will resemble the earlier basic multipoint configuration with two routers.

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The commands to configure RouterA for the point-to-point exampleare shown in Fig. 12-14.

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104

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Frame Relay Static MappingYou statically configure

your DLCI entries in the following situation:

The remote router doesn’t support Inverse ARP.

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Frame Relay Static Mapping (cont.)

You need to assignspecific subinterfaces to a specific DLCI connections.

You want to reduce broadcast traffic.

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Frame Relay Static Mapping (cont.)

You are configuringOSPF over Frame Relay.

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Review Questions

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You can check your Frame Relay configurations by using show commands.

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Monitoring Frame Relay (cont.)

show frame-relay mapshow frame-relay lmi

show interfaceshow frame-relay pvc

The most common showcommands for monitoring Frame Relay operations are:

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Monitoring Frame Relay (cont.)

Figs. 12-15~18 show these commands with their output.

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Contents



Review Questions

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Review QuestionsWhich protocol is used to automatically build the Frame Relay map along with LMI?

Ans: C

A. ARPB. RARPC. Inverse ARPD. DLCIE. None of the above

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Review Questions (cont.)What is the purpose of

keepalive packets?

Ans: B

A. To reduce data transfer ratesB. To keep PVCs activeC. To increase data transfer rateD. To negotiate connection speedE. None of the above

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Review Questions (cont.)

Which of the following layers do WAN specifications typically define? (Choose all that apply.)

Ans: A, B

A. PhysicalB. Data LinkC. NetworkD. TransportE. Presentation

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In Frame Relay, what would be considered the DCE?

Ans: D

A. Customer’s routerB. Terminal adapterC. PPPD. Frame Relay switch

Review Questions (cont.)

E. None of the above

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Review Questions (cont.)Which of the following is a type of virtual circuit? (Choose all that apply.)

Ans: B, C

A. MVCB. PVCC. SVCD. QVCE. RVC

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Review Questions (cont.)Which of the following are LMI encapsulation types supported by Cisco routers? (Choose all that apply.)

Ans: B, C, E

A. LMI 2B. ciscoC. ansiD. q923iE. q923a

chapter 12 frame relay210.70.254.122/chingyu/ch12 frame relay.pdf9 frame relay standards and eq....

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