Frame Relay

Outline Describe the Introduction Describe the history of Frame Relay Describe how Frame Relay works Describe the primary functionality traits of Frame Relay Describe the format of Frame Relay frames

Introduction Packet-Switching Networks Frame Relay Networks Switching Technique Routing X.25 Frame Relay Networks Architecture User Data Transfer Call Control

Introduction Frame Relay (FR) is a high-performance WAN protocol that operates at the physical and data link layers of the OSI reference model. FR originally was designed for use across Integrated Service Digital Network (ISDN) interfaces. Today, it is used over a variety of other network interfaces as well. FR is an example of a packet-switched technology. Packet-switched networks enable end stations to dynamically share the network medium and the available bandwidth.

What is Frame Relay? “A packet-switching protocol for connecting devices on a Wide Area Network (WAN)” quoted from Webopedia. FR networks in the U.S. support data transfer rates at T-1 (1.544 Mb/s) and T-3 (45 Mb/s) speeds. In fact, you can think of Frame Relay as a way of utilizing existing T-1 and T-3 lines owned by a service provider. Most telephone companies now provide FR service for customers who want connections at 56 Kb/s to T-1 speeds. (In Europe, FR’s speeds vary from 64 Kb/s to 2 Mb/s. In the U.S., Frame Relay is quite popular because it is relatively inexpensive. However, it is being replaced in some areas by faster technologies, such as ATM.

Introduction FR often is described as a streamlined version of X.25, offering fewer of the robust capabilities, such as windowing and retransmission of last data that are offered in X.25. This is because FR typically operates over WAN facilities that offer more reliable connection services and a higher degree of reliability than the facilities available during the late 1970s and early 1980s that served as the common platform for X.25 WANs. FR is strictly a Layer 2 protocol suite, whereas X.25 provides services at Layer 3 (the network layer, we will discuss it later) as well. This enables FR to offer higher performance and greater transmission efficiency than X.25, and makes FR suitable for current WAN applications, such as LAN interconnection.

Outline Describe the Introduction Describe the history of Frame Relay Describe how Frame Relay works Describe the primary functionality traits of Frame Relay Describe the format of Frame Relay frames

Frame Relay Standardization Initial proposals for the standardization of FR were presented to the Consultative Committee on International Telephone and Telegraph (CCITT) in 1984. Because of lack of interoperability and lack of complete standardization, however, FR did not experience significant deployment during the late 1980s. A major development in Frame Relay’s history occurred in 1990 when Cisco, Digital Equipment Corporation (DEC), Northern Telecom, and StrataCom formed a consortium to focus on Frame Relay technology development.

Frame Relay Standardization (Cont.) This consortium developed a specification that conformed to the basic Frame Relay protocol that was being discussed in CCITT, but it extended the protocol with features that provide additional capabilities for complex internetworking environments. These Frame Relay extensions are referred to collectively as the Local Management Interface (LMI). ANSI and CCITT have subsequently standardized their own variations of the original LMI specification, and these standardized specifications now are more commonly used than the original version. Internationally, Frame Relay was standardized by the International Telecommunication Union—Telecommunications Standards Section (ITU-T). In the United States, Frame Relay is an American National Standards Institute (ANSI) standard.

Frame Relay Devices Devices attached to a Frame Relay WAN fall into the following two general categories: Data terminal equipment (DTE) DTEs generally are considered to be terminating equipment for a specific network and typically are located on the premises of a customer. Example of DTE devices are terminals, personal computers, routers, and bridges. Data circuit-terminating equipment (DCE) DCEs are carrier-owned internetworking devices. The purpose of DCE equipments is to provide clocking and switching services in a network, which are the devices that actually transmit data through the WAN.

Frame Relay Devices (cont.)

Frame Relay Devices (cont.) The connection between a DTE device and a DCE device consists of both a physical layer component (L1) and a link layer component (L2). The physical component defines the mechanical, electrical, functional, and procedural specifications for the connection between the devices. One of the commonly used physical layer interface specifications is the recommended standard (RS)-232.

Serial Point-to-Point Connection Router connections End user device DTE DCE Service Provider EIA/TIA-232 EIA/TIA-449 V.35 X.21 EIA-530 Network connections at the CSU/DSU

Packet-Switching Networks Basic technology the same as in the 1970s One of the few effective technologies for long distance data communications Frame relay and ATM are variants of packet-switching Advantages: Flexibility, resource sharing, robust, responsive Disadvantages: Time delays in distributed networks, overhead penalties Need for routing and congestion control

Definition of Packet Switching Refers to protocols in which messages are divided into packets before they are sent. Each packet is then transmitted individually and can even follow different routes to its destination. Once all the packets forming a message arrive at the destination, they are recompiled into the original message. Most modern Wide Area Network (WAN) protocols, including TCP/IP, X.25, and Frame Relay, are based on packet-switching technologies. In contrast, normal telephone service is based on a circuit-switching technology, in which a dedicated line is allocated for transmission between two parties. Circuit-switching is ideal when data must be transmitted quickly and must arrive in the same order in which it's sent. This is the case with most real-time data, such as live audio and video. Packet switching is more efficient and robust for data that can withstand some delays in transmission, such as e-mail messages and Web pages.

Circuit-Switching Long-haul telecom network designed for voice Network resources dedicated to one call Shortcomings when used for data: Inefficient (high idle time) Constant data rate

Packet-Switching Data transmitted in short blocks, or packets Packet length < 1000 octets Each packet contains user data plus control info (routing) Store and forward

The Use of Packets

Packet Switching: Datagram Approach

Advantages with compared to Circuit-Switching Greater line efficiency (many packets can go over shared link) Data rate conversions Non-blocking under heavy traffic (but increased delays). When traffic becomes heavy on a circuit-switching network, some calls are blocked. Priorities can be used.

Disadvantages relative to Circuit-Switching Packets incur additional delay with every node they pass through Jitter: variation in packet delay Data overhead in every packet for routing information, etc Processing overhead for every packet at every node traversed

Simple Switching Network

Switching Technique Large messages broken up into smaller packets Datagram Each packet sent independently of the others No call setup More reliable (can route around failed nodes or congestion) Virtual circuit Fixed route established before any packets sent No need for routing decision for each packet at each node

Packet Switching: Virtual-Circuit Approach

An Introduction to X.25 The first commercial packet-switching network interface standard was X.25. X.25 is now seldom used in developed countries but is still available in many parts of the world (see next page). A popular standard for packet-switching networks. The X.25 standard was approved by the CCITT (now the ITU-T) in 1976. It defines layers 1, 2, and 3 in the OSI Reference Model. 3 levels Physical level (X.21) Link level: LAPB (Link Access Protocol-Balanced), a subset of HDLC (High-level Data Link Control) Packet level (provides virtual circuit service)

Regional Meteorological Telecommunication Network for Region II (Asia) RTH in Region II NMC in Region II Moscow 64K Washington Centre in other region 64K MTN circuit Regional circuit Interregional circuit Additional circuit Novosibirsk 14.4-33.6K (V.34) Khabarovsk IMTN-MDCN CIR<32/768K> 14.4-33.6K (V.34) NI No implementation 14.4-33.6K (V.34) Almaty 14.4-33.6K (V.34) 14.4-33.6K (V.34) Via Moscow 9.6K Non-IP link IP link 14.4-33.6K (V.34) 14.4-33.6K (V.34) NI 14.4-33.6K V.34 Bishkek 14.4-33.6K (V.34) Ulaanbaatar PyongYang Tokyo Id V.34 Ashgabad Tashkent Offenbach Id V.34 IMTN-MDCN Frame Relay CIR<8/8K> CMA-VSAT IMTN-MDCN Frame Relay CIR<48/48K> CMA-VSAT NI 75 Baghdad 75 NI Tehran Dushanbe IMTN-MDCN Frame Relay CIR<48/48K> NI Beijing IMTN-MDCN Frame Relay CIR<16/8K> NI NI Frame Relay CIR<32/32K> 64K Offenbach NI 75 Frame Relay CIR<16/16K> 50 Kabul Kuwait Thimpu 9.6K 64K 64K NI Karachi 64K NI IMTN-MDCN Frame Relay CIR<16/16K> Seoul 64K 64K Bahrain New Delhi Jeddah Frame Relay CIR<16/16K> 200 128K 50 64K Doha 64K Kathmandu Moscow NI Internet Hong Kong 50 Internet Abu-Dhabi CMA-VSAT 64K Internet 100 2.4K 100 ISDN 128K 64K 50 Hanoi Algiers Macao Cairo Dhaka Frame Relay CIR<16/16K> Muscat Internet Internet Internet Vientiane 9.6K Cairo 50 Frame Relay CIR<16/16K> Internet Yangon IMTN-MDCN CIR<32/32K> Sanaa 64K Colombo 1200 Male Melbourne 50 200 64K Washington Bangkok Manila 75 NI Phnom Penh Melbourne 2.4K Frame Relay CIR<16/16K> Regional Meteorological Telecommunication Network for Region II (Asia) Current status as of December 2004 Singapore Kuala Lumpur

The Use of Virtual Circuits

User Data and X.25 Protocol Control Information Layer 3 header X.25 packet LAPB header LAPB trailer LAPB frame

Frame Relay Networks Designed to eliminate much of the overhead in X.25 Call control signaling on separate logical connection from user data Multiplexing/switching of logical connections at layer 2 (not layer 3) No hop-by-hop flow control and error control Throughput an order of magnitude higher than X.25

Comparison of X.25 and Frame Relay Protocol Stacks

Virtual Circuits and Frame Relay Virtual Connections

Frame Relay Architecture X.25 has 3 layers: physical, link, network Frame Relay has 2 layers: physical and data link (or LAPF, Link Access Procedure for Frame Mode Bearer Services) LAPF core: minimal data link control Preservation of order for frames Small probability of frame loss LAPF control: additional data link or network layer end-to-end functions

Outline Describe the Introduction Describe the history of Frame Relay Describe how Frame Relay works Describe the primary functionality traits of Frame Relay Describe the format of Frame Relay frames

Frame Relay Virtual Circuits Frame Relay provides connection-oriented data link layer communications. This means that a defined communication exists between each pair of devices and that these connections are associated with a connection identifier (ID). This service is implemented by using a FR virtual circuit, which is a logical connection created between two DTE devices across a Frame Relay packet-switched network (PSN). Virtual circuits provide a bidirectional communication path from one DTE device to another and are uniquely identified by a data-link connection identifier (DLCI).

Frame Relay Virtual Circuits (cont.) A number of virtual circuits can be multiplexed into a single physical circuit for transmission across the network. This capability often can reduce the equipment and network complexity required to connect multiple DTE devices. A virtual circuit can pass through any number of intermediate DCE devices (switches) located within the Frame Relay PSN. Frame Relay virtual circuits fall into two categories: switched virtual circuits (SVCs) and permanent virtual circuits (PVCs).

Switched Virtual Circuits (SVCs) Switched virtual circuits (SVCs) are temporary connections used in situations requiring only sporadic data transfer between DTE devices across the Frame Relay network. A communication session across an SVC consists of the following four operational states: Call setup—The virtual circuit between two Frame Relay DTE devices is established. Data transfer—Data is transmitted between the DTE devices over the virtual circuit. Idle—The connection between DTE devices is still active, but no data is transferred. If an SVC remains in an idle state for a defined period of time, the call can be terminated. Call termination—The virtual circuit between DTE devices is terminated.

Permanent Virtual Circuits (PVCs) Permanent virtual circuits (PVCs) are permanently established connections that are used for frequent and consistent data transfers between DTE devices across the Frame Relay network. Communication across a PVC does not require the call setup and termination states that are used with SVCs. PVCs always operate in one of the following two operational states: Data transfer—Data is transmitted between the DTE devices over the virtual circuit. Idle—The connection between DTE devices is active, but no data is transferred. Unlike SVCs, PVCs will not be terminated under any circumstances when in an idle state. DTE devices can begin transferring data whenever they are ready because the circuit is permanently established.

Data-Link Connection Identifier Frame Relay virtual circuits are identified by data-link connection identifiers (DLCIs). DLCI values typically are assigned by the Frame Relay service provider (for example, the telephone company). Frame Relay DLCIs have local significance, which means that their values are unique in the LAN, but not necessarily in the Frame Relay WAN.

Congestion-Control Mechanisms Frame Relay reduces network overhead by implementing simple congestion-notification mechanisms rather than explicit, per-virtual-circuit flow control. Frame Relay typically is implemented on reliable network media, so data integrity is not sacrificed because flow control can be left to higher-layer protocols. Frame Relay implements two congestion-notification mechanisms: Forward-explicit congestion notification (FECN) Backward-explicit congestion notification (BECN)

Congestion-Control Mechanisms FECN and BECN each is controlled by a single bit contained in the Frame Relay frame header. The Frame Relay frame header also contains a Discard Eligibility (DE) bit, which is used to identify less important traffic that can be dropped during periods of congestion. The FECN bit is part of the Address field in the Frame Relay frame header. The FECN mechanism is initiated when a DTE device sends Frame Relay frames into the network. If the network is congested, DCE devices (switches) set the value of the frames’ FECN bit to 1. When the frames reach the destination DTE device, the Address field (with the FECN bit set) indicates that the frame experienced congestion in the path from source to destination. The DTE device can relay this information to a higher-layer protocol for processing. Depending on the implementation, flow control may be initiated, or the indication may be ignored.

Congestion-Control Mechanisms The BECN bit is part of the Address field in the Frame Relay frame header. DCE devices set the value of the BECN bit to 1 in frames traveling in the opposite direction of frames with their FECN bit set. This informs the receiving DTE device that a particular path through the network is congested. The DTE device then can relay this information to a higher-layer protocol for processing. Depending on the implementation, flow-control may be initiated, or the indication may be ignored.

Frame Relay Discard Eligibility The Discard Eligibility (DE) bit is used to indicate that a frame has lower importance than other frames. The DE bit is part of the Address field in the Frame Relay frame header. DTE devices can set the value of the DE bit of a frame to 1 to indicate that the frame has lower importance than other frames. When the network becomes congested, DCE devices will discard frames with the DE bit set before discarding those that do not. This reduces the likelihood of critical data being dropped by Frame Relay DCE devices during periods of congestion.

Frame Relay Error Checking Frame Relay uses a common error-checking mechanism known as the cyclic redundancy check (CRC). The CRC compares two calculated values to determine whether errors occurred during the transmission from source to destination. Frame Relay reduces network overhead by implementing error checking rather than error correction. Frame Relay typically is implemented on reliable network media, so data integrity is not sacrificed because error correction can be left to higher-layer protocols running on top of Frame Relay.

Frame Relay Local Management Interface The Local Management Interface (LMI) is a set of enhancements to the basic Frame Relay specification. The LMI was developed in 1990 by Cisco Systems, StrataCom, Northern Telecom, and Digital Equipment Corporation. It offers a number of features (called extensions) for managing complex internetworks. Key Frame Relay LMI extensions include global addressing, virtual circuit status messages, and multicasting. The LMI global addressing extension gives Frame Relay data-link connection identifier (DLCI) values global rather than local significance. DLCI values become DTE addresses that are unique in the Frame Relay WAN. The global addressing extension adds functionality and manageability to Frame Relay internetworks.

Frame Relay Local Management Interface (cont.) Individual network interfaces and the end nodes attached to them, for example, can be identified by using standard address-resolution and discovery techniques. In addition, the entire Frame Relay network appears to be a typical LAN to routers on its periphery. LMI virtual circuit status messages provide communication and synchronization between Frame Relay DTE and DCE devices. These messages are used to periodically report on the status of PVCs, which prevents data from being sent into black holes (that is, over PVCs that no longer exist). The LMI multicasting extension allows multicast groups to be assigned. Multicasting saves bandwidth by allowing routing updates and address-resolution messages to be sent only to specific groups of routers. The extension also transmits reports on the status of multicast groups in update messages.

Frame Relay Network Implementation A common private Frame Relay network implementation is to equip a T1 multiplexer with both Frame Relay and non-Frame Relay interfaces. Frame Relay traffic is forwarded out the Frame Relay interface and onto the data network. Non-Frame Relay traffic is forwarded to the appropriate application or service, such as a private branch exchange (PBX) for telephone service or to a video-teleconferencing application.

Frame Relay Network Implementation Frame Relay is implemented in both public carrier-provided networks and in private enterprise networks. Public Carrier-Provided Networks In public carrier-provided Frame Relay networks, the Frame Relay switching equipment is located in the central offices of a telecommunications carrier. Subscribers are charged based on their network use but are relieved from administering and maintaining the Frame Relay network equipment and service. Generally, the DCE equipment also is owned by the telecommunications provider. DTE equipment either will be customer-owned or perhaps will be owned by the telecommunications provider as a service to the customer. Private Enterprise Networks More frequently, organizations worldwide are deploying private Frame Relay networks. In private Frame Relay networks, the administration and maintenance of the network are the responsibilities of the enterprise (a private company). All the equipment, including the switching equipment, is owned by the customer.

Outline Describe the Introduction Describe the history of Frame Relay Describe how Frame Relay works Describe the primary functionality traits of Frame Relay Describe the format of Frame Relay frames

Frame Relay Frame Formats Flags—Delimits the beginning and end of the frame. The value of this field is always the same and is represented either as the hexadecimal number 7E or as the binary number 01111110. Address—Contains the following information: (in bits) DLCI: ‘0’ for Call Control message Extended Address (EA): Address field extension bit C/R: the C/R bit is not currently defined. Congestion Control:

LAPF Core LAPF (Link Access Procedure for Frame Mode Bearer Services) Frame delimiting, alignment and transparency Frame multiplexing/demultiplexing Inspection of frame for length constraints Detection of transmission errors Congestion control

LAPF-core Formats

User Data Transfer No control field, which is normally used for: Identify frame type (data or control) Sequence numbers Implication: Connection setup/teardown carried on separate channel Cannot do flow and error control

Outline Describe the Introduction Describe the history of Frame Relay Describe how Frame Relay works Describe the primary functionality traits of Frame Relay Describe the format of Frame Relay frames Frame Call Control and Example

Frame Relay Call Control Data transfer involves: Establish logical connection and DLCI Exchange data frames Release logical connection

Frame Relay Call Control 4 message types needed SETUP CONNECT RELEASE RELEASE COMPLETE

Frame Relay Overview Virtual circuits make connections DCE or Frame Relay Switch CSU/DSU Purpose: This figure provides a big-picture definition of Frame Relay. Emphasize: Frame Relay is used between the CPE device and the Frame Relay switch. It does NOT affect how packets get routed within the Frame Relay cloud. Frame Relay is a purely Layer 2 protocol. The network providing the Frame Relay service can be either a carrier-provided public network or a network of privately owned equipment serving a single enterprise. Make a clear distinction between DCE, DTE, and CPE. Emphasize that Frame Relay over SVCs is not discussed in this chapter because it is not widely supported by service providers at this time. The service provider must also support SVCs in order for Frame Relay over SVCs to operate. Note: In Cisco IOS Release 11.2, two traffic shaping features were introduced: Generic (adaptive) traffic shaping Frame Relay traffic shaping Both of these features can be used to adjust the rate at which traffic is sent by the router. In addition, these features allow the router to throttle the traffic rate based on BECNs received from the Frame Relay switch. Neither of these features are discussed in this course. Frame Relay traffic shaping is discussed in the Building Cisco Remote Access Networks (BCRAN) course. Information on both can be found in Cisco documentation. Frame Relay works here. Virtual circuits make connections Connection-oriented service

EIA/TIA-232, EIA/TIA-449, V.35, X.21, EIA/TIA-530 Frame Relay Stack OSI Reference Model Frame Relay Application Presentation Session Transport Purpose: This figure compares Frame Relay to the OSI model. Emphasize: The same serial standards that support point-to-point serial connections also support Frame Relay serial connections. Frame Relay operates at the data link layer. Frame Relay supports multiple upper-layer protocols. Network IP/IPX/AppleTalk, etc. Data Link Frame Relay EIA/TIA-232, EIA/TIA-449, V.35, X.21, EIA/TIA-530 Physical

Frame Relay Terminology PVC DLCI: 100 DLCI: 200 LMI 100=Active 400=Active DLCI: 400 Local Access Loop=64 kbps Local Access Loop=T1 Purpose: This figure provides an overview of terminology so that the student is prepared to understand the Frame Relay operation discussion. The terminology used with Frame Relay varies by service provider. These are the commonly used terms. Point out the local access loop and note that the local access rate is different than the rate used within the Frame Relay cloud. The DLCI is of local significance, therefore, point out that the same DLCI can be used in multiple places in the network. The autosensing LMI is a Release 11.2 or later feature. Frame Relay connections are made using PVCs. The circuits are identified by the DLCI assigned by the service provider. Reference: For more information on Frame Relay, including a Frame Relay glossary, refer to the Frame Relay Forum World Wide Web page: http://www.frforum.com/4000/4003.html This course does not discuss Frame Relay traffic flow issues. So terms like BECN, FECN and discard eligible are not discussed in this course. These terms are some of the terms that can be found in the Frame Relay Forum’s glossary. The BCRAN discusses Frame Relay traffic flow issues. PVC Local Access Loop=64 kbps DLCI: 500

Frame Relay Address Mapping DLCI: 500 PVC 10.1.1.1 CSU/DSU Inverse ARP or Frame Relay map Purpose: This figure illustrates mapping the data-link connection identifier (DLCI) to the network layer address such as IP. Emphasize: The DLCI is of local significance, therefore, point out that the same DLCI can be used in multiple places in the network. Frame Relay connections are made using PVCs. The circuits are identified by the DLCI assigned by the service provider. Explain what Inverse ARP is used for. Static mapping can be configured instead of inverse ARP. Frame Relay IP (10.1.1.1) DLCI (500) Get locally significant DLCIs from provider Map your network addresses to DLCIs

x Frame Relay Signaling Cisco supports three LMI standards: Cisco DLCI: 500 PVC 10.1.1.1 CSU/DSU x LMI 500=Active 400=Inactive DLCI: 400 PVC Keepalive Purpose: This figure describes the Local management Interface (LMI) and shows the key standards. Emphasize: Explain LMI. Note: Other key American National Standards Institute (ANSI) standards are T1.606, which defines the Frame Relay architecture, and T1.618, which describes data transfer. Other key International Telecommunication Union Telecommunication Standardization sector (ITU-T) specifications include I.122, which defines ITU-T Frame Relay architecture, and Q.922, which standardizes data transfer. Use of these LMI standards is especially widespread in Europe. The original “gang of four” no longer exists; StrataCom merged with Cisco and Digital Equipment Corporation was acquired by Compaq Computers. Cisco supports three LMI standards: Cisco ANSI T1.617 ITU-T Q.933

Frame Relay Inverse ARP and LMI Operation 1 Frame Relay Cloud DLCI=100 DLCI=400 172.168.5.5 172.168.5.7 Layer 1 of 4: Purpose: This figure describes the Inverse ARP and LMI process. Emphasize: Step 1—Indicates that each router must connect to the Frame Relay switch. Note: The status inquiry messages are part of LMI operation. Explain what Inverse ARP is used for.

Frame Relay Inverse ARP and LMI Operation 1 Frame Relay Cloud DLCI=100 DLCI=400 172.168.5.5 172.168.5.7 Status Inquiry Status Inquiry 2 2 Layer 2 of 4: Purpose: This figure describes the Inverse ARP and LMI process. Emphasize: Step 1—Indicates that each router must connect to the Frame Relay switch. Step 2—Discusses what information is sent from the router to the Frame Relay switch.

Frame Relay Inverse ARP and LMI Operation 1 Frame Relay Cloud DLCI=100 DLCI=400 172.168.5.5 172.168.5.7 Status Inquiry Status Inquiry 2 2 Local DLCI 100=Active Local DLCI 400=Active Layer 3 of 4: Purpose: This figure describes the Inverse ARP and LMI process. Emphasize: Step 1—Indicates that each router must connect to the Frame Relay switch. Step 2—Discusses what information is sent from the router to the Frame Relay switch. Step 3—Discusses what the Frame Relay switch does with the received information. 4 3 3

Frame Relay Inverse ARP and LMI Operation 1 Frame Relay Cloud DLCI=100 DLCI=400 172.168.5.5 172.168.5.7 Status Inquiry Status Inquiry 2 2 Local DLCI 100=Active Local DLCI 400=Active Layer 4 of 4: Purpose: This figure describes the Inverse ARP and LMI process. Emphasize: Step 1—Indicates that each router must connect to the Frame Relay switch. Step 2—Discusses what information is sent from the router to the Frame Relay switch. Step 3—Discusses what the Frame Relay switch does with the received information. Step 4—Discusses the sending of Inverse ARP messages. 4 3 3 Hello, I am 172.168.5.5. 4

Frame Relay Inverse ARP and LMI Operation (cont.) Cloud DLCI=400 DLCI=100 172.168.5.5 172.168.5.7 Frame Relay Map 172.168.5.5 DLCI 400 Active 5 Hello, I am 172.168.5.7. 4 Layer 1 of 3: Purpose: This figure describes the Inverse ARP and LMI process (cont...). Emphasize: Step 5—Discusses how the Inverse ARP message is used to create the Frame Relay map table dynamically. Frame Relay Map 172.168.5.7 DLCI 100 Active 5

Frame Relay Inverse ARP and LMI Operation (cont.) Cloud DLCI=400 DLCI=100 172.168.5.5 172.168.5.7 Frame Relay Map 172.168.5.5 DLCI 400 Active 5 Hello, I am 172.168.5.7. 4 Layer 2 of 3: Purpose: This figure describes the Inverse ARP and LMI process (cont...). Emphasize: Step 5—Discusses how the Inverse ARP message is used to create the Frame Relay map table dynamically. Step 6—Shows how Inverse ARP has a periodic interval. Frame Relay Map 172.168.5.7 DLCI 100 Active 5 Hello, I am 172.168.5.5. 6

Frame Relay Inverse ARP and LMI Operation (cont.) Cloud DLCI=400 DLCI=100 172.168.5.5 172.168.5.7 Frame Relay Map 172.168.5.5 DLCI 400 Active 5 Hello, I am 172.168.5.7. 4 Layer 3 of 3: Purpose: This figure describes the Inverse ARP and LMI process (cont...). Emphasize: Step 5—Discusses how the Inverse ARP message is used to create the Frame Relay map table dynamically. Step 6—Shows how Inverse ARP has a periodic interval. Step 7—Discusses the periodic interval for keepalive messages. It’s an LMI function. Transition: The next section explains how to configure Frame Relay. Frame Relay Map 172.168.5.7 DLCI 100 Active 5 Hello, I am 172.168.5.5. 6 Keepalives Keepalives 7 7

Configuring Basic Frame Relay Rel. 11.2 Router Rel. 10.3 Router HQ Branch interface Serial1 ip address 10.16.0.1 255.255.255.0 encapsulation frame-relay bandwidth 64 Slide 1 of 2: Purpose: This figure introduces basic Frame Relay configuration over a physical interface. It is important that students understand how configuration occurs in order for them to understand the subinterfaces discussion later in the chapter. These steps assume that LMI and Inverse ARP are supported, therefore no static maps are needed. Regarding step 3: Cisco’s Frame Relay encapsulation uses a 4-byte header, with 2 bytes to identify the DLCI and 2 bytes to identify the packet type. Use the ieft encapsulation to connect to other vendors. The IETF standard is defined in RFCs 1294 and 1490. Regarding step 4: The LMI connection is established by the frame-relay lmi-type [ansi | cisco | q933a] command. The default values established during initial setup are usually sufficient to maintain connectivity with the Frame Relay network. Altering these values would only be required in case of intermittent failures. Changing the default values of the LMI should only be attempted after consulting with your service provider. These configuration steps are the same, regardless of the network-layer protocols operating across the network. interface Serial1 ip address 10.16.0.2 255.255.255.0 encapsulation frame-relay bandwidth 64 frame-relay lmi-type ansi

Configuring Basic Frame Relay (cont.) Rel. 11.2 Router Rel. 10.3 Router HQ Branch interface Serial1 ip address 10.16.0.1 255.255.255.0 encapsulation frame-relay bandwidth 64 interface Serial1 ip address 10.16.0.2 255.255.255.0 encapsulation frame-relay bandwidth 64 frame-relay lmi-type ansi Slide 2 of 2: Purpose: This figure continues the basic Frame Relay configuration over a physical interface. Emphasize: Regarding step 5: This command is used to notify the routing protocol that bandwidth is configured on the link. It is used by IGRP to determine the metric of the link. IGRP uses bandwidth as one of the factors to determine the metric. This command also affects statistics, in particularly statistics in the show interface command. Inverse ARP Enabled by default Does not appear in configuration output

Configuring a Static Frame Relay Map DLCI=110 IP address=10.16.0.1/24 p1r1 HQ Branch DLCI=100 IP address=10.16.0.2/24 Purpose: This figure discusses the static map command option: Emphasize: You can use the frame-relay map command to configure multiple DLCIs to be multiplexed over one physical link. Instead of using Inverse ARP, the Frame Relay map tells the Cisco IOS software how to get from a specific protocol and address pair to the correct DLCI. Point out that this command is similar to building a static route. The simplest way to generate a static map is to let the router learn the information dynamically first. Some users let the router learn the information dynamically, then enable static maps for easier network administration. These configuration steps are the same, regardless of the network-layer protocols operating across the network. Although static maps are not needed when Inverse ARP is enabled, it is a good idea to configure them for each connection for easier network administration. interface Serial1 ip address 10.16.0.1 255.255.255.0 encapsulation frame-relay bandwidth 64 frame-relay map ip 10.16.0.2 110 broadcast

Verifying Frame Relay Operation Router#show interface s0 Serial0 is up, line protocol is up Hardware is HD64570 Internet address is 10.140.1.2/24 MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 1/255 Encapsulation FRAME-RELAY, loopback not set, keepalive set (10 sec) LMI enq sent 19, LMI stat recvd 20, LMI upd recvd 0, DTE LMI up LMI enq recvd 0, LMI stat sent 0, LMI upd sent 0 LMI DLCI 1023 LMI type is CISCO frame relay DTE FR SVC disabled, LAPF state down Broadcast queue 0/64, broadcasts sent/dropped 8/0, interface broadcasts 5 Last input 00:00:02, output 00:00:02, output hang never Last clearing of "show interface" counters never Queueing strategy: fifo Output queue 0/40, 0 drops; input queue 0/75, 0 drops <Output omitted> Slide 1 of 6: Purpose: This figure shows how the show interface command is used to verify whether Frame Relay operation and router connectivity to remote routers are working. Emphasize: Describe the highlighted output to the students. Displays line, protocol, DLCI, and LMI information

Verifying Frame Relay Operation (cont.) Router#show frame-relay lmi LMI Statistics for interface Serial0 (Frame Relay DTE) LMI TYPE = CISCO Invalid Unnumbered info 0 Invalid Prot Disc 0 Invalid dummy Call Ref 0 Invalid Msg Type 0 Invalid Status Message 0 Invalid Lock Shift 0 Invalid Information ID 0 Invalid Report IE Len 0 Invalid Report Request 0 Invalid Keep IE Len 0 Num Status Enq. Sent 113100 Num Status msgs Rcvd 113100 Num Update Status Rcvd 0 Num Status Timeouts 0 Slide 2 of 6: Purpose: This figure shows how the show frame-relay LMI command is used to verify the LMI type used for signaling. Emphasize: Describe the highlighted output to the students. Displays LMI information

Verifying Frame Relay Operation (cont.) Router#show frame-relay pvc 100 PVC Statistics for interface Serial0 (Frame Relay DTE) DLCI = 100, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial0 input pkts 28 output pkts 10 in bytes 8398 out bytes 1198 dropped pkts 0 in FECN pkts 0 in BECN pkts 0 out FECN pkts 0 out BECN pkts 0 in DE pkts 0 out DE pkts 0 out bcast pkts 10 out bcast bytes 1198 pvc create time 00:03:46, last time pvc status changed 00:03:47 Slide 3 of 6: Purpose: This figure shows how the show frame-relay pvc command is used to verify whether Frame Relay operation and router connectivity to remote routers are working. Emphasize: Describe the highlighted output to the students. Displays PVC traffic statistics

Verifying Frame Relay Operation (cont.) Router#show frame-relay map Serial0 (up): ip 10.140.1.1 dlci 100(0x64,0x1840), dynamic, broadcast,, status defined, active Displays the route maps, either static or dynamic Slide 4 of 6: Purpose: This figure shows how the show frame-relay map command is used to verify that Frame Relay has a map entry in the Frame Relay map table. Emphasize: Describe the highlighted output to the students.

Verifying Frame Relay Operation (cont.) Router#show frame-relay map Serial0 (up): ip 10.140.1.1 dlci 100(0x64,0x1840), dynamic, broadcast,, status defined, active Router#clear frame-relay-inarp Router#sh frame map Router# Slide 5 of 6: Purpose: This figure shows how the clear frame-relay-inarp command is used to clear dynamically created Frame Relay maps. Clears dynamically created Frame Relay maps

Verifying Frame Relay Operation (cont.) Router#debug Frame lmi Frame Relay LMI debugging is on Displaying all Frame Relay LMI data Router# 1w2d: Serial0(out): StEnq, myseq 140, yourseen 139, DTE up 1w2d: datagramstart = 0xE008EC, datagramsize = 13 1w2d: FR encap = 0xFCF10309 1w2d: 00 75 01 01 01 03 02 8C 8B 1w2d: 1w2d: Serial0(in): Status, myseq 140 1w2d: RT IE 1, length 1, type 1 1w2d: KA IE 3, length 2, yourseq 140, myseq 140 1w2d: Serial0(out): StEnq, myseq 141, yourseen 140, DTE up 1w2d: 00 75 01 01 01 03 02 8D 8C 1w2d: Serial0(in): Status, myseq 142 1w2d: RT IE 1, length 1, type 0 1w2d: KA IE 3, length 2, yourseq 142, myseq 142 1w2d: PVC IE 0x7 , length 0x6 , dlci 100, status 0x2 , bw 0 Slide 6 of 6: Purpose: This figure shows how the debug frame-relay lmi command is used to debug your Frame Relay signaling. Displays LMI debug information

Selecting a Frame Relay Topology Full Mesh Purpose: This figure is a transition discussion to illustrate the need for subinterfaces. Now that students are familiar with the concept and configuring of Frame Relay, they are ready to consider the issues and solutions related to broadcast updates in an NBMA Frame Relay network. Emphasize: Compare the different topologies described. Explain that by default interfaces that support Frame Relay are multipoint connection types. This type of connection is not a problem when only one PVC is supported by a single interface; but it is when multiple PVCs are supported by a single interface. In this situation, broadcast routing updates received by the central router cannot be broadcast to the other remote sites. Broadcast routing updates are issued by distance vector protocols. Link-state and hybrid protocols use multicast and unicast addresses. Partial Mesh Star (Hub and Spoke) Frame Relay default: nonbroadcast, multiaccess (NBMA)

Reachability Issues with Routing Updates 1 B 2 A A C C 3 Purpose: This figure continues the discussion that leads into the need for subinterfaces. Emphasize: Partial mesh Frame Relay networks must deal with the case of split horizon not allowing routing updates to be retransmitted on the same interface from which they were received. Split horizon cannot be disabled for certain protocols such as AppleTalk. Split horizon issues are overcome through the use of logical subinterfaces assigned to the physical interface connecting to the Frame Relay network. A physical interface can be divided into multiple, logical interfaces. Each logical interface is individually configured and is named after the physical interface. A decimal number is included to distinguish it. The logical port names contain a decimal point and another number indicating these are subinterfaces of interface serial 0 (S0). Subinterfaces are configured by the same configuration commands used on physical interfaces. A broadcast environment can be Frame Relay-created by transmitting the data on each individual circuit. This simulated broadcast requires significant buffering and CPU resources in the transmitting router, and can result in lost user data because of contention for the circuits. Reference: Interconnections by Radia Perlman is also a good reference on split horizon. Note: Subinterfaces are particularly useful in a Frame Relay partial-mesh NBMA model that uses a distance vector routing protocol. Instead of migrating to a routing protocol that supports turning off split horizon, subinterfaces can be used to overcome the split horizon problem. D Problem: Broadcast traffic must be replicated for each active connection

Resolving Reachability Issues Logical Interface Physical Interface Subnet A S0.1 S0.2 S0.3 S0 Subnet B Subnet C Purpose: This figure defines subinterfaces and how they can resolve NBMA issues. Emphasize: You can have connectivity problems in a Frame Relay network if these conditions exist: You are using an NBMA model. Your configuration is in a partial mesh. You are using a distance vector routing protocol. Split horizon is enabled on the routing protocol. If the routing protocol is configured with split horizon, routing updates from one router connected on the multipoint subinterface are not propagated to other routers connected on this multipoint subinterface. For example, if router C sends a routing update, this split horizon will keep this update from being sent back out the subinterface to router D. To resolve this problem you can: Use Frame Relay subinterfaces to overcome the split horizon problem. Use a routing protocol that supports disabling split horizon. Use this configuration if you want to save IP address space. You can also use this type of configuration with several fully meshed groups. Routing updates will be exchanged between the fully meshed routers. Note: When an interface is assigned “encapsulation frame-relay,” split horizon is disabled for IP and enabled for IPX and AppleTalk, by default. Solution: Split horizon can cause problems in NBMA environments Subinterfaces can resolve split horizon issues A single physical interface simulates multiple logical interfaces

Configuring Subinterfaces Point-to-Point Subinterfaces act as leased line Each point-to-point subinterface requires its own subnet Applicable to hub and spoke topologies Multipoint Subinterfaces act as NBMA network so they do not resolve the split horizon issue Can save address space because uses single subnet Applicable to partial-mesh and full-mesh topology Purpose: This figure begins the discussion on configuring subinterfaces. Emphasize: The encapsulation frame-relay command is assigned to the physical interface. All other configuration items, such as the network-layer address and DLCIs, are assigned to the subinterface. Multipoint may not save you addresses if you are using VLSMs. Further, it may not work properly given the broadcast traffic and split horizon considerations. The point-to-point subinterface option was created to avoid these issues. Note: Subinterfaces are also used with ATM networks and IPX LAN environments where multiple encapsulations exist on the same medium.

Configuring Point-to-Point Subinterfaces DLCI=110 10.17.0.2 A s0.3 10.18.0.1 DLCI=120 B interface Serial0 no ip address encapsulation frame-relay ! interface Serial0.2 point-to-point ip address 10.17.0.1 255.255.255.0  bandwidth 64 frame-relay interface-dlci 110 interface Serial0.3 point-to-point ip address 10.18.0.1 255.255.255.0 frame-relay interface-dlci 120 10.18.0.2 Purpose: This figure continues the discussion of configuring subinterfaces. Emphasize: The Frame Relay service provider will assign the DLCI numbers. These numbers range from 16 to 992. This range will vary depending on the LMI used. Use the frame-relay interface-dlci command on subinterfaces only. Use of the command on an interface, rather than a subinterface, will prevent the device from forwarding packets intended for the DLCI. It is also required for multipoint subinterfaces for which dynamic address resolution is enabled. It is not used for multipoint subinterfaces configured with the frame-relay map command for static address mapping. Using the frame-relay interface-dlci command with subinterfaces provides greater flexibility when configuring Frame Relay networks. On multipoint subinterfaces, the frame-relay interface-dlci command enables Inverse ARP on the subinterface. When this command is used with point-to-point subinterfaces, all traffic for the subinterface’s subnetwork are sent out this subinterface. The ability to change a subinterface from point-to-point to multipoint, or vice versa, is limited by the software architecture. The router must be rebooted for a change of this type to take effect. An alternative exists to rebooting the router and creating a network outage. Create another subinterface in the software and migrate the configuration parameters to the new subinterface using the proper point-to-point or multipoint setting, as required. C

Multipoint Subinterfaces Configuration Example DLCI=120 s2.2=10.17.0.1/24 s2.1=10.17.0.2/24 DLCI=130 RTR1 RTR3 DLCI=140 s2.1=10.17.0.3/24 interface Serial2 no ip address encapsulation frame-relay ! interface Serial2.2 multipoint ip address 10.17.0.1 255.255.255.0  bandwidth 64 frame-relay map ip 10.17.0.2 120 broadcast frame-relay map ip 10.17.0.3 130 broadcast frame-relay map ip 10.17.0.4 140 broadcast RTR4 Purpose: This graphic illustrates a multipoint subinterface example. Emphasize: In this example, the subinterface is configured to behave as a normal NBMA Frame Relay interface. No IP address is configured on the physical interface. It is important that the physical interface NOT have an address, otherwise routing will not work. The frame-relay map command is used to create the multiple PVC connections from a single interface. All connections are in the same subnet. The DLCIs are provided by your service provider. s2.1=10.17.0.4/24

Visual Objective pod ro’s s0 A 10.140.1.2 B 10.140.2.2 C 10.140.3.2 wg_pc_a 10.2.2.12 pod ro’s s0 A 10.140.1.2 B 10.140.2.2 C 10.140.3.2 D 10.140.4.2 E 10.140.5.2 F 10.140.6.2 G 10.140.7.2 H 10.140.8.2 I 10.140.9.2 J 10.140.10.2 K 10.140.11.2 L 10.140.12.2 e0/1 e0/2 e0 wg_ro_a 10.2.2.3 s0 10.140.1.2/24 wg_sw_a 10.2.2.11 Frame Relay wg_pc_l 10.13.13.12 wg_ro_l Objectives: Enable the Frame Relay on a serial link. Purpose: Teach students how to enable Frame Relay. Laboratory Instructions: Refer to the Lab Setup Guide. PPP with CHAP e0/1 e0/2 e0 s0 10.140.12.2/24 FR 10.13.13.3 wg_sw_l 10.13.13.11 ... s2/7.x 10.140.1.1/24 … 10.140.12.1/24 fa0/24 fa0/23 fa0/0 core_ server 10.1.1.1 core_sw_a 10.1.1.2 core_ro 10.1.1.3

Review Questions 1. What is a DLCI? 2. What are two methods to map a network layer address to a DLCI on a Cisco router? 3. What are the advantages of configuring Frame Relay subinterfaces? Purpose: Review the chapter with open ended questions. Note: The questions in this section are open ended questions designed to foster further discussion. Answers the the review questions are in the “Answers” appendix.

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