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Frame Relay is a high performance WAN protocol that operates at the Physical and Data Link layers of the OSI model. Frame Relay originally was designed for use across ISDN interfaces. Today, it is used over a variety of other network interfaces as well.
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. Variable-length packets are used for more efficient and flexible transfers. These packets then are switched between the various network segments until the destination is reached. 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.
Frame Relay often is described as a streamlined version of X.25, offering fewer of the robust capabilities, such as windowing and retransmission of lost data, that are offered in X.25. This is because Frame Relay 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 platforms for X.25 WANs. Frame Relay is strictly a Layer 2 protocol suite, whereas X.25 provides services at Layer 3 as well. This enables Frame Relay to offer a higher performance and greater transmission efficiency that X.25 and makes Frame Relay suitable for current WAN applications such as LAN interconnection.
A major development in Frame Relay's history occurred in 1990 when Cisco Systems, StrataCom, Northern Telecom, and DEC 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 (now called ITU-T) but 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 ITU-T have standardized their own variations of the original LMI specification, and these standardized specifications now are more commonly used than the original version.Frame Relay Information
The physical equipment can vary between organizations. Some networks may use routers with separate CSU/DSUs (Channel Service Unit/ Data Service Unit) and some may use routers with built in CSU/DSUs. The CSU/DSU is located at the customer location of the digital connection, and is used for encoding, filtering, and translating communications to and from the digital line. In Frame Relay connections, the network device that connects to the Frame Relay switch is known as a Frame Relay access device (FRAD) also called a Frame Relay assembler/ disassembler. The Frame Relay switch is also called the Frame Relay network device (FRND pronounced "friend"). The network administrator typically handles the local connection up to the point that it enters the PDN. Items that are part of the PDN, including the Frame Relay switch, fall under control of and responsibility of the telecommunications provider. Frame Relay is used over a variety of network interfaces.Cisco Frame Relay supports the following protocols:
Frame Relay can be used with nearly any serial interface. Communications in a Frame Relay network are connection oriented and a defined communications path must exist between each pair of DTE devices. Virtual circuits provide a bi-directional communications path from one DTE device to another and are uniquely identified by a Data Link Connection Identifier(DLCI). The technology used in Frame Relay allows it to multiplex several data flows over the same physical media.
Frame Relay separates each data stream into logical (software maintained) connections called virtual circuits which carry the data transferred on the connection between two DTE devices. Two types of virtual circuits, SVCs (Switched Virtual Circuit) and PVCs (Permanent Virtual Circuit) connect Frame Relay ports. Switched Virtual Circuits (SVCs) allow access through a Frame Relay network by setting up a path to the destination endpoints only when the need arises and tearing down the path when it is no longer needed. 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. SVCs and PVCs can coexist on the same sites and routers. For example, routers at remote branch offices might set up PVCs to the central headquarters for frequent communications, but set up SVCs with each other as needed for intermittent communication.
DLCI (Data Link Connection Identifier)
Frame Relay virtual circuits are identified by Data Link Connection Identifiers (DLCIs). A DLCI serves as the addressing scheme within a Frame Relay network. DLCI values typically are assigned by the Frame Relay provider (e.g. telephone company). Frame Relay DLCIs have local significance, which means that the values themselves are not unique in the Frame Relay WAN. For example two DTE devices connected by a virtual circuit may use a different DLCI value to refer to the same connection. The service provider assigns a DLCI for each VC, which are used by Frame Relay to distinguish between different virtual circuits on the network. Since many virtual circuits can be terminated on one multipoint Frame Relay interface, many DLCIs are often affiliated with it.
For the IP devices on each end of a virtual circuit to communicate, their IP addresses need to be mapped to DLCIs. This mapping can function as a multipoint device --one that can identify to the Frame Relay network the appropriate destination virtual circuit for each packet that is sent over the single physical interface. The mappings can be done dynamically with IARP or manually with the Frame Relay map command.
Every DLCI can have local or global meaning everywhere within the Frame Relay network. DLCIs are usually assigned by the provider and start with 16. The following commands apply a DLCI number to an interface
Router3(config-if)#frame-relay interface-dlci ? <16-1007> Define a DLCI as part of the current subinterface Router3(config-if)#frame-relay interface-dlci 16 %FR-5-DLCICHANGE: Interface Serial0 - DLCI 16 state changed to ACTIVE Router3(config-fr-dlci)#
Local Management Interface
The Local Management Interface (LMI) is a set of enhancements to the Frame Relay protocol specifications. The LMI was developed in 1990 by four companies known as the "Gang of Four" (Cisco Systems, StrataCom, Northern Telecom, and DEC). 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. LMI was designed to exchange information about PVC status and to ensure that the link between two points was operating correctly. LMI is a standard signaling mechanism between CPE (usually a router) and the Frame Relay connection.
The LMI global addressing extensions gives Frame Relay DLCI values global rather than local significance. DLCI values become DTE addresses that are unique in the Frame Relay WAN. Providing DLCI numbers that are globally rather than just locally significant makes automatic configuration of the Frame Relay map possible. With LMI, DLCI values are unique within a Frame Relay network, and standard address resolution protocols such as ARP and reverse ARP and discovery protocols can be used to identify nodes within the network.
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 the update messages.
LMI uses keepalive packets (sent every 10 seconds by default) to verify the Frame Relay link and to ensure the flow of data. The Frame Relay switch in turn provides to the Frame Relay connectivity device the status of all virtual circuits that the device can utilize. Each virtual circuit, represented by its DLCI number, can have one of three connection states:
The Frame Relay switch reports this status information to the Frame Relay map on the local router. The status information is used by the Frame Relay connectivity device to determine whether data can be transmitted over the configured virtual circuit. The LMI messages can provide information about the following:
Beginning with Cisco IOS Release 11.2, the software supports Local Management Interface (LMI) autosense, which enables the interface to determine the LMI type supported by the switch. Support for LMI autosense means that you are no longer required to configure the Local Management Interface (LMI) explicitly. If the Frame Relay responds with more than one type, the Cisco router will automatically configure itself to use the last LMI type received. You can turn off LMI autosense by explicitly configuring an LMI type. The LMI type must be written into NVRAM so that the next time the router powers up, LMI autosense will be inactive. At the end of autoinstall, a frame-relay lmi-type xxx statement is included within the interface configuration. This configuration is not automatically written to NVRAM; you must explicitly write the configuration to NVRAM by using the copy system:running-config or copy nvram:startup-config commands.Explicitly configuring LMI type
The default type is cisco, but you can manually change it to ANSI or Q.933A. If you configure the LMI type manually, you will deactivate LMI autosense. If the router is attached to a public data network (PDN), the LMI type must match the type used on the public network. Otherwise, the LMI type can be set to suit the needs of your private Frame Relay network. The following command changes the LMI type to ANSI and explicitly sets the keepalive time interval:
Router3(config)#int s1 Router3(config-if)#ip address 192.168.1.55.2 255.255.255.0 Router3(config-if)#encapsulation frame-relay Router3(config-if)#bandwidth 64 Router3(config-if)#frame-relay lmi-type ? cisco ansi q933a Router3(config-if)#frame-relay lmi-type ansi Router3(config-if)#keepalive 8 Router3(config-if)#exitKeepalives
A keepalive interval must be set to configure the LMI. By default, this interval is 10 seconds (can be from 0 to 32,768 seconds) and, per the LMI protocol, must be less than the corresponding interval on the switch. To disable keepalives on networks that do not utilize LMI, use the no keepalive interface configuration command.LMI Types
Frame Relay frames that conform to the LMI specifications have one of the following message types:
The 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 over PVCs that no longer exist. Information in status messages include all or some of the following:
Not all Frame Relay providers support every piece of link status information. All current implementations provide the New and Active information, but support for other information varies by provider. Note that Frame Relay doesn't provide error checking, as do other network protocols such as Synchronous Data Link (SDLC). This makes Frame Relay connections more efficient, but it also means Frame Relay must rely on the upper-layer protocols such as TCP, to provide error correction.
Configure Frame Relay Maps
In configurations where Inverse ARP is not used to dynamically discover network protocol addresses on the virtual circuit, the frame-relay map command must be used to map layer 3 protocol addresses to the layer 2 DLCI.Syntax:
frame-relay map [protocol] [protocol addresss] [dlci #] [broadcast] [cisco|ietf]
[broadcast] -- Forwards broadcasts to this address.
[cisco|ietf] -- Used to specify Frame Relay encapsulation type. ietf for connecting to another vendor's equipment across a Frame Relay network. cisco is the Cisco encapsulation for Frame Relay.Example:
Router3(config-if)#frame-relay map ip 192.168.1.40 42 b ietfInverse ARP
Frame Relay Inverse ARP is a method of building dynamic address mappings in Frame Relay networks. Inverse ARP allows the router to discover the protocol address of a device associated with the virtual circuit. Inverse ARP creates dynamic address mappings, as contrasted with the frame-relay map command. It is enabled by default, but can be disabled explicitly for a given protocol and DLCI pair. You do not have to enable or disable Inverse ARP if you have a point-to-point interface, because this is only a single destination and discovery is not required. In order to maintain the Frame Relay map, routers exchange Inverse ARP messages every 60 seconds by default. To select Inverse ARP or disable it, use the following commands in interface configuration mode:Enable
frame-relay inverse-arp protocol dlciDisable
no frame-relay inverse-arp protocol dlci
Split Horizon is a routing technique that reduces the chance of routing loops on a network. A split horizon implementation prevents routing update information received on one physical interface from being rebroadcast to other devices through that same physical interface. Although split horizon is good for reducing routing loops, it can cause problems for Frame Relay routing updates.
Consider three routers called A, B, and C, that have one physical connection between them (e.g. A--B--C), routerA can communicate with routerB and routerB can communicate with routerC and routerB can talk to both A and C. On a LAN, A could talk to C, but not in Frame Relay, unless routerA had a PVC to routerC. The best solution is to configure subinterfaces for each virtual connection, because the individual virtual circuits can be maintained and split horizon can remain on. Routing update information that is received through one subinterface can be propagated to other subinterfaces. Dividing the Serial0 interface on routerB into S0.1 and S0.2 (subinterfaces) allows a different subnet identifier to be assigned to each virtual circuit. This allows router updates to go from routerC to routerA and vice versa.Two types of Subinterfaces
Configuring Subinterfaces on Frame RelayPoint-to-Point Connections
You first set the encapsulation type on the serial interface, then you can define the subinterfaces. This example sets three subinterfaces, and the DLCIs associated with the virtual circuits.
Notice how each subinterface is on a different subnet, this is for a point-to-point connection. The encapsulation frame-relay is using the default of cisco but can be ietf instead. The b is for enabling broadcast routing updates.
Router3(config)#interface serial 0 Router3(config-if)#no ip address Router3(config-if)#encapsulation frame-relay Router3(config-if)#exit Router3(config)#interface serial 0.1 point-to-point Router3(config-subif)#ip address 192.168.10.1 255.255.255.0 Router3(config-subif)#bandwidth 64 Router3(config-subif)#frame-relay interface-dlci 200 b Router3(config-if)#exit Router3(config)#interface serial 0.2 point-to-point Router3(config-subif)#ip address 192.168.20.1 255.255.255.0 Router3(config-subif)#bandwidth 64 Router3(config-subif)#frame-relay interface-dlci 300 b Router3(config-if)#exit Router3(config)#interface serial 0.3 point-to-point Router3(config-subif)#ip address 192.168.30.1 255.255.255.0 Router3(config-subif)#bandwidth 64 Router3(config-subif)#frame-relay interface-dlci 400 b Router3(config-if)#exit Router3(config)#router igrp 110 Router3(config-router)#network 192.168.10.0 Router3(config-router)#network 192.168.20.0 Router3(config-router)#network 192.168.30.0Multipoint Connections
For the following commands, notice how each DLCI mapping is on the same subnet, this is for a multipoint connection. The b is for enabling broadcast updates to the specific virtual circuit. Instead of using a frame-relay map command for every virtual circuit, you can use the frame-relay inverse-arp function to perform dynamic mapping of the IP address to the DLCI number. Frame Relay Inverse ARP is on by default, and is only disabled if you explicitly disable it. When the frame-relay map command is used, Inverse ARP is automatically disabled for the specified protocol on the specified DLCI.
Router3(config)#interface serial 1 Router3(config-if)#no ip address Router3(config-if)#encapsulation frame-relay Router3(config-if)#exit Router3(config)#interface serial 1.2 multipoint Router3(config-subif)#ip address 172.16.1.1 255.255.0.0 Router3(config-subif)#frame-relay interface-dlci 100 b Router3(config-subif)#bandwidth 64 Router3(config-subif)#frame-relay map ip 172.16.1.2 200 b Router3(config-subif)#frame-relay map ip 172.16.1.3 300 b Router3(config-subif)#frame-relay map ip 172.16.1.4 400 b Router3(config-subif)#exit Router3(config)#router igrp 222 Router3(config-router)#network 172.16.1.0
Frame Relay Performance Parameters
Some of the terms used by the telecommunications provider to specify performance parameters are:
Frame Relay Congestion Control
This is how Frame Relay handles congestion problems.
Frame Relay Topologies
Monitoring Frame Relay
You can use the show command to see if the commands you entered produced the desired effect on the router.
Router3>show frame ? ip show frame relay IP statistics lapf show frame relay lapf status/statistics lmi show frame relay lmi statistics map Frame-Relay map table pvc show frame relay pvc statistics route show frame relay route svc show frame relay SVC stuff traffic Frame-Relay protocol statistics
show frame-relay lmi
Enter the EXEC command show frame-relay lmi at the system prompt to display statistics about the Local Management Interface (LMI). The following is sample output from the show frame-relay lmi command when the interface is a DTE:
Router3#show frame-relay lmi LMI Statistics for interface Serial1 (Frame Relay DTE) LMI TYPE = ANSI 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 9 Num Status msgs Rcvd 0 Num Update Status Rcvd 0 Num Status Timeouts 9
show frame-relay map
Use the show frame-relay map EXEC command to display the current Frame Relay map entries and information about these connections. The following is sample output from the show frame-relay map command:
Router3#show frame-relay map Serial2 (up): IP 18.104.22.168 dlci 20(0x14,0x0440), dynamic CISCO, BW= 56000, status defined, active
show frame-relay pvc
Enter the show frame-relay pvc EXEC command at the system prompt to display statistics about permanent virtual circuits (PVCs) for Frame Relay interfaces. Enter no arguments to obtain statistics about all Frame Relay interfaces. The following is sample output from the show frame-relay pvc command:
Router3#show frame-relay pvc PVC Statistics for interface Serial1 (Frame Relay DCE) DLCI = 100, DLCI USAGE = SWITCHED, PVC STATUS = ACTIVE input pkts 0 output pkts 0 in bytes 0 out bytes 0 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 pvc create time 0:03:03 last time pvc status changed 0:03:03 Num Pkts Switched 0 DLCI = 101, DLCI USAGE = SWITCHED, PVC STATUS = INACTIVE input pkts 0 output pkts 0 in bytes 0 out bytes 0 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 pvc create time 0:02:58 last time pvc status changed 0:02:58 Num Pkts Switched 0 DLCI = 102, DLCI USAGE = SWITCHED, PVC STATUS = DELETED input pkts 0 output pkts 0 in bytes 0 out bytes 0 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 pvc create time 0:02:58 last time pvc status changed 0:02:58 Num Pkts Switched 0
show frame-relay route
Enter the show frame-relay route EXEC command at the system prompt to display all configured Frame Relay routes, along with their status. The following is sample output from the show frame-relay route command:
Router3#show frame-relay route Input Intf Input Dlci Output Intf Output Dlci Status Serial1 100 Serial2 200 active Serial1 101 Serial2 201 active Serial1 102 Serial2 202 active Serial1 103 Serial3 203 inactive Serial2 200 Serial1 100 active Serial2 201 Serial1 101 active Serial2 202 Serial1 102 active Serial3 203 Serial1 103 inactive
show frame-relay traffic
Use the show frame-relay traffic EXEC command to display the router's global Frame Relay statistics since the last reload. The following is sample output from the show frame-relay traffic command:
Router3#show frame-relay traffic Frame Relay statistics: ARP requests sent 14, ARP replies sent 0 ARP request recvd 0, ARP replies recvd 10
show interfaces serial
When using Frame Relay encapsulation, use the show interfaces serial command to display information on the multicast DLCI, the DLCI of the interface, and the LMI DLCI used for the Local Management Interface. The multicast DLCI and the local DLCI can be set using the frame-relay multicast-dlci and the frame-relay local-dlci commands, or provided through the Local Management Interface. The status information is taken from the LMI, when active. The following is sample output from the show interfaces serial command for a serial interface with the CISCO LMI enabled:
Router3#show interface serial 1 Serial1 is up, line protocol is down Hardware is MCI Serial Internet address is 22.214.171.124, subnet mask is 255.255.255.0 MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 246/255, load 1/255 Encapsulation FRAME-RELAY, loopback not set, keepalive set (10 sec) LMI enq sent 2, LMI stat recvd 0, LMI upd recvd 0, DTE LMI down LMI enq recvd 266, LMI stat sent 264, LMI upd sent 0 LMI DLCI 1023 LMI type is CISCO frame relay DTE Last input 0:00:04, output 0:00:02, output hang never Last clearing of "show interface" counters 0:44:32 Output queue 0/40, 0 drops; input queue 0/75, 0 drops Five minute input rate 0 bits/sec, 0 packets/sec Five minute output rate 0 bits/sec, 0 packets/sec 307 packets input, 6615 bytes, 0 no buffer Received 0 broadcasts, 0 runts, 0 giants 0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort 0 input packets with dribble condition detected 266 packets output, 3810 bytes, 0 underruns 0 output errors, 0 collisions, 2 interface resets, 0 restarts 178 carrier transitions
Debugging Frame LMI
To help you verify and troubleshoot the Frame Relay connection by seeing if the routers and switches are exchanging the correct LMI information, use the command debug frame-relay lmi.
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