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Frame Relay

Frame Relay

Frame Relay is a high-performance WAN protocol that operates at the physical and data link layers of the OSI reference model. Frame Relay originally was designed for use across Integrated Services Digital Network (ISDN) interfaces. Today, it is used over a variety of other network interfaces as well.

Frame relay is a type of WAN connection use to connect one site to many remote sites through a single physical circuit; this operation makes it easy to construct reliable and inexpensive networks.

Frame Relay
Frame Relay

Frame Relay network is very simple. Frame Relay connections are created by configuring network routers or other devices to communicate with a service provider Frame Relay switch. The service provider configures the Frame Relay switch, which helps keep end-user configuration tasks to a minimum.

Frame Relay has lower overhead than X.25 because it has fewer capabilities e.g, Frame Relay does not provide error correction; modern WAN facilities offer more reliable connection services and a higher degree of reliability than older facilities. The Frame Relay node simply drops packets without notification when it detects errors. Any necessary error correction, such as re-transmission of data, is left to the endpoints. This makes propagation from customer end to customer end through the network very fast.

Frame Relay network uses permanent virtual circuits (PVCs). PVC is the logical path along an originating Frame Relay link, through the network, and along a terminating Frame Relay link to its ultimate destination. Compare this to the physical path used by a dedicated connection. In a network with Frame Relay access, PVC uniquely defines the path between two endpoints.

Each virtual circuit is identified by a Data Link Connection Identifier (DLCI), which is simply a number between 0 and 1023. In fact, Cisco routers can only use DLCI numbers in the range 16 through 1007 to carry user data.

Frame Relay is an example of a packet-switched technology. Packet-switched networks enable end stations to dynamically share the network medium and the available bandwidth.

The following two techniques are used in Packet-Switching Technology:

  1. Variable-Length Packets are used for more efficient and flexible data transfers. These packets are switched between the various segments in the network until the destination is reached.
  2. Statistical Multiplexing techniques control network access in a packet-switched network. The advantage of this technique is that it accommodates more flexibility and more efficient use of bandwidth. Most of today’s popular LANs, such as Ethernet and Token Ring, are packet-switched networks.

Frame Relay Standardization

Initial proposals for the standardization of Frame Relay 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, Frame Relay 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. 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).

Since the consortium’s specification was developed and published, many vendors have announced their support of this extended Frame Relay definition. 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:

Relationship between the two categories of devices
Relationship between the two categories of devices
  1. DTEs (Data Terminal Equipment) generally are considered to be terminating equipment for a specific network and typically are located on the premises of a customer. In fact, they may be owned by the customer. Examples of DTE devices are terminals, personal computers, routers, and bridges.
  2. DCEs (Data Circuit-Terminating Equipment) are carrier-owned internetworking devices. The purpose of DCE equipment is to provide clocking and switching services in a network, which are the devices that actually transmit data through the WAN. In most cases, these are packet switches.

Frame Relay Structure

Standards for the Frame Relay protocol have been developed by ANSI and CCITT simultaneously. The separate LMI specification has basically been incorporated into the ANSI specification. The following discussion of the protocol structure includes the major points from these specifications.

The Frame Relay frame structure is based on the LAPD protocol. In the Frame Relay structure, the frame header is altered slightly to contain the Data Link Connection Identifier (DLCI) and congestion bits, in place of the normal address and control fields.

Frame Relay Header
Frame Relay Header

This new Frame Relay header is 2 bytes in length and has the following format:

  1. DLCI: 10-bit DLCI field represents the address of the frame and corresponds to a PVC.
  2. C/R: Designates whether the frame is a command or response.
  3. EA: Extended Address field signifies up to two additional bytes in the Frame Relay header, thus greatly expanding the number of possible addresses.
  4. FECN: Forward Explicit Congestion Notification (see ECN below).
  5. BECN: Backward Explicit Congestion Notification (see ECN below).
  6. DE (Discard Eligibility): When there is congestion on the line, the network must decide which frames to discard in order to free the line. Discard Eligibility provides the network with a signal to determine which frames to discard. The network will discard frames with a DE value of 1 before discarding other frames.
    The DE bit may be set by the user on some of its lower-priority frames. Alternatively, the network may set the DE bit to indicate to other nodes that a frame should be preferentially selected for discard, if necessary.
  7. Information: The Information field may include other protocols within it, such as an X.25, IP or SDLC (SNA) packet.

Frame Relay Virtual Circuits

Frame Relay provides connection-oriented data link layer communication. This means that a defined communication exists between each pair of devices and that these connections are associated with a connection identifier. This service is implemented by using a Frame Relay virtual circuit, which is a logical connection created between two data terminal equipment (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). 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

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:

  1. Call Setup: The virtual circuit between two Frame Relay DTE devices is established.
  2. Data Transfer: Data is transmitted between the DTE devices over the virtual circuit.
  3. 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.
  4. Call Termination: The virtual circuit between DTE devices is terminated.
    After the virtual circuit is terminated, the DTE devices must establish a new SVC if there is additional data to be exchanged. It is expected that SVCs will be established, maintained, and terminated using the same signaling protocols used in ISDN.

Permanent Virtual Circuits

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:

  1. Data Transfer: Data is transmitted between the DTE devices over the virtual circuit.
  2. 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).

A Single Frame Relay Virtual Circuit Can Be Assigned Different DLCIs on Each End of a VC
A Single Frame Relay Virtual Circuit Can Be Assigned Different DLCIs on Each End of a VC

Frame Relay DLCIs have local significance, which means that their values are unique in the LAN, but not necessarily in the Frame Relay WAN.

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.

A typical Frame Relay network consists of a number of DTE devices, such as routers, connected to remote ports on multiplexer equipment via traditional point-to-point services such as T1, fractional T1, or 56-Kb circuits.

The majority of Frame Relay networks deployed today are provisioned by service providers that intend to offer transmission services to customers. This is often referred to as a public Frame Relay service. Frame Relay is implemented in both public carrier-provided networks and in private enterprise networks. The following section examines the two methodologies for deploying Frame Relay.

Type of Frame Relay Network

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.

The majority of today’s Frame Relay networks are public carrier-provided networks.

A Simple Frame Relay Network Connects Various Devices to Different Services over a WAN
A Simple Frame Relay Network Connects Various Devices to Different Services over a WAN

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.

Benefits of Frame Relay

  • Cost Effectiveness:Frame Relay reduces network costs by using less equipment, less complexity, and an easier implementation. Frame Relay is a more cost-effective option for two reasons.
    1. First, with dedicated lines, customers pay for an end-to-end connection. That includes the local loop and the network link. With Frame Relay, customers only pay for the local loop, and for the bandwidth they purchase from the network provider. Distance between nodes is not important. While in a dedicated-line model, customers use dedicated lines provided in increments of 64 kb/s, Frame Relay customers can define their virtual circuit needs in far greater granularity, often in increments as small as 4 kb/s.
    2. Frame Relay’ shares bandwidth across a larger base of customers. Typically, a network provider can service 40 or more 56 kb/s customers over one T1 circuit. Using dedicated lines would require more DSU/CSUs (one for each line) and more complicated routing and switching. Network providers save because there is less equipment to purchase and maintain.
  • Frame Relay provides greater bandwidth, reliability, and resiliency than private or leased lines.
  • Flexibility: Virtual circuit provides considerable flexibility in network design. In Frame Relay, the end of each connection has a number to identify it called a Data Link Connection Identifier (DLCI). Any station can connect with any other simply by stating the address of that station and DLCI number of the line it needs to use.

How to configure Frame Relay

Configuring Frame Relay involves the following steps:

Change the Encapsulation

Go in interface mode and select the Frame Relay encapsulation on the interface. There are two types of Frame Relay encapsulations: Cisco and IETF. Cisco is the default. Syntax to set your encapsulation is

encapsulation frame-relay [ietf]

Configure LMI Type

The three LMI types are Cisco, Ansi, and Q933a. For IOS 11.2 and higher, the LMI type is automatically detected.

frame-relay lmi-type [cisco | ansi | 933a]

Configure Frame Relay Map

Configuring a static Frame Relay map, is optional unless you are using subinterfaces. The Frame Relay map will map a Layer 3 address to a local DLCI. This step is optional because inverse-arp will automatically perform this map for you. The syntax for a Frame Relay map is as follows:

frame-relay map protocol address dlci [broadcast] [cisco | ietf]

Configure Subinterfaces

If you are using a routing protocol in a hub-and-spoke topology, you will probably want to use subinterfaces to avoid the split-horizon problem. To configure a subinterface, remove the IP address off the main interface and put it under the subinterface. Configuring a subinterface involves assigning it a number and specifying the type. The following command creates point-to-point subinterface serial0/0.1

Router(config)#interface serial0/0.1 point-to-point

To create a multipoint subinterface, enter multipoint instead:

Router(config)#interface serial0/0.1 multipoint

Assign IP address to subinterface

After entering one of these commands you will be taken to the subinterface configuration mode where you can enter your IP address:

Router(config-subif)#ip address

If you are using a multipoint subinterface, you will need to configure frame-relay maps and you cannot rely on inverse-arp.

If you are using a point-to-point subinterface, you will need to assign a DLCI to the subinterface. This is only for point-to-point subinterfaces; this is not needed on the main interface or on multipoint subinterfaces. To assign a DLCI to a point-to-point subinterface, enter the following command under the subinterface:

frame-relay interface-dlci dlci

Configuration of Frame Relay

Frame Relay Configuration Topology
Frame Relay Configuration Topology

Now first configure R1. Fast Ethernet port and hostname is already configured. Double click on R1 and configure serial port for frame relay encapsulation and further create sub interface for connecting R2, R3, R4. Configure also static route for connecting remaining network.

Configure R1
R1#configure terminal
R1(config)#interface serial 0/0/0
R1(config-if)#encapsulation frame-relay
R1(config-if)#no shutdown
R1(config-subif)#interface serial 0/0/0.102 point-to-point
R1(config-subif)#ip address
R1(config-subif)#frame-relay interface-dlci 102
R1(config)#interface serial 0/0/0.103 point-to-point
R1(config-subif)#ip address
R1(config-subif)#frame-relay interface-dlci 103
R1(config)#interface serial 0/0/0.104 point-to-point
R1(config-subif)#ip address
R1(config-subif)#frame-relay interface-dlci 104
R1(config)#ip route
R1(config)#ip route
R1(config)#ip route

Configure R2
R2#configure terminal
R2(config)#interface serial 0/0/0
R2(config-if)#encapsulation frame-relay
R2(config-if)#no shutdown
R2(config)#interface serial 0/0/0.101 point-to-point
R2(config-subif)#ip address
R2(config-subif)#frame-relay interface-dlci 101
R2(config)#ip route

Configure R3
R3#configure terminal
R3(config)#interface serial 0/0/0
R3(config-if)#encapsulation frame-relay
R3(config-if)#no shutdown
R3(config)#interface serial 0/0/0.101 point-to-point
R3(config-subif)#ip address
R3(config-subif)#frame-relay interface-dlci 101
R3(config)#ip route

Configure R4
R4#configure terminal
R4(config)#interface serial 0/0/0
R4(config-if)#encapsulation frame-relay
R4(config-if)#no shutdown
R4(config)#interface serial 0/0/0.101 point-to-point
R4(config-subif)#ip address
R4(config-subif)#frame-relay interface-dlci 101
R4(config)#ip route

Now verify by doing ping from pc0 to all pc. It should be ping successfully. I have uploaded a configured topology but use it as the final resort first try yourself to configure it.

Commands Meanings
Router(config)#interface serial 0/0/0 Enter in interface mode.
Router(config-if)#encapsulation frame-relay Turns on Frame Relay encapsulation with the default encapsulation type of cisco.
Router(config-if)#frame-relay lmitype {ansi | cisco | q933a} Depending on the option you select, this command sets the LMI type to the ANSI standard, the Cisco standard, or the ITU-T Q.933 Annex A standard.
Router(config-if)#frame-relay interface-dlci 110 Sets the DLCI number of 110 on the local interface and enters Frame Relay DLCI configuration mode.
Router(config-fr-dlci)#exit Returns to interface configuration mode.
Router(config-if)#frame-relay map ip 110 broadcast Maps the remote IP address ( to the local DLCI number (110). The optional broadcast keyword specifies that broadcasts across IP should be forwarded to this address. This is necessary when using dynamic routing protocols.
Router(config-if)#no frame-relay inverse arp Turns off Inverse ARP.
Router#show frame-relay map Displays IP/DLCI map entries.
Router#show frame-relay pvc Displays the status of all PVCs configured.
Router#show frame-relay lmi Displays LMI statistics.
Router#clear frame-relay counters Clears and resets all Frame Relay counters.
Router#clear frame-relay inarp Clears all Inverse ARP entries from the map table.
Router#debug frame-relay lmi Used to help determine whether a router and Frame Relay switch are exchanging LMI packets properly.


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