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There was a major development in Frame Relay's history in 1990 when Cisco Systems, StrataCom, Northern Telecom, and Digital Equipment Corporation formed a consortium to focus Frame Relay technology development and accelerate the introduction of interoperable Frame Relay products. This consortium developed a specification conforming to the basic Frame Relay protocol being discussed in T1S1 and ITU-T, but extended it with features that provide additional capabilities for complex internetworking environments. These Frame Relay extensions are referred to collectively as the Local Management Interface (LMI).
As an interface to a network, Frame Relay is the same type of protocol as X.25 (see Chapter 19, "Troubleshooting X.25 Connections"). However, Frame Relay differs significantly from X.25 in its functionality and format. In particular, Frame Relay is a more streamlined protocol, facilitating higher performance and greater efficiency.
As an interface between user and network equipment, Frame Relay provides a means for statistically multiplexing many logical data conversations (referred to as virtual circuits) over a single physical transmission link. This contrasts with systems that use only time-division-multiplexing (TDM) techniques for supporting multiple data streams. Frame Relay's statistical multiplexing provides more flexible and efficient use of available bandwidth. It can be used without TDM techniques or on top of channels provided by TDM systems.
Another difference between Frame Relay and X.25 is the absence of explicit, per-virtual-circuit flow control in Frame Relay. Now that many upper-layer protocols are effectively executing their own flow control algorithms, the need for this functionality at the link layer has diminished. Frame Relay, therefore, does not include explicit flow control procedures that duplicate those in higher layers. Instead, very simple congestion notification mechanisms are provided to allow a network to inform a user device that the network resources are close to a congested state. This notification can alert higher-layer protocols that flow control may be needed.
Current Frame Relay standards address permanent virtual circuits (PVCs) that are administratively configured and managed in a Frame Relay network. Another type, switched virtual circuits (SVCs), has also been proposed. The Integrated Services Digital Network (ISDN) signaling protocol is proposed as the means by which DTE and DCE can communicate to establish, terminate, and manage SVCs dynamically.
The Frame Relay frame is shown in Figure 18-1. The flags fields delimit the beginning and end of the frame. Following the leading Flags field are 2 bytes of address information. Ten bits of these 2 bytes make up the actual circuit ID (called the DLCI, for data link connection identifier).
The 10-bit DLCI value is the heart of the Frame Relay header. It identifies the logical connection that is multiplexed into the physical channel. In the basic (not extended by the LMI) mode of addressing, DLCIs have local significance; that is, the end devices at two different ends of a connection may use a different DLCI to refer to that same connection. Figure 18-2 provides an example of the use of DLCIs in nonextended Frame Relay addressing.
In Figure 18-2, assume two PVCs, one between Atlanta and Los Angeles and one between San Jose and Pittsburgh. Los Angeles uses DLCI 12 to refer to its PVC with Atlanta, whereas Atlanta refers to the same PVC as DLCI 82. Similarly, San Jose uses DLCI 12 to refer to its PVC with Pittsburgh. The network uses internal proprietary mechanisms to keep the two locally significant PVC identifiers distinct.
At the end of each DLCI byte is an extended address (EA) bit. If this bit is 1, the current byte is the last DLCI byte. All implementations currently use a 2-byte DLCI, but the presence of the EA bits means that longer DLCIs may be agreed on and used in the future.
The bit marked C/R following the most significant DLCI byte is currently not used.
Finally, 3 bits in the 2-byte DLCI provide congestion control. The forward explicit congestion notification (FECN) bit is set by the Frame Relay network in a frame to tell the DTE receiving the frame that congestion was experienced in the path from source to destination. The backward explicit congestion notification (BECN) bit is set by the Frame Relay network in frames traveling in the opposite direction from frames encountering a congested path. The notion behind both of these bits is that the FECN or BECN indication can be promoted to a higher-level protocol that can take flow control action as appropriate. (FECN bits are useful to higher-layer protocols that use receiver- controlled flow control, whereas BECN bits are significant to those that depend on -"emitter-controlled" flow control.)
The discard eligibility (DE) bit is set by the DTE to tell the Frame Relay network that a frame has lower importance than other frames and should be discarded before other frames if the network becomes short of resources. Thus, it represents a very simple priority mechanism. This bit is usually set only when the network is congested.
In LMI messages, the basic protocol header is the same as in normal data frames. The actual LMI message begins with 4 mandatory bytes, followed by a variable number of information elements (IEs). The format and encoding of LMI messages is based on the ANSI T1S1 standard.
The first of the mandatory bytes (unnumbered information indicator) has the same format as the Link Access Procedure, Balanced (LAPB) unnumbered information (UI) frame indicator with the poll/final bit set to 0. (For more information about LAPB, see "Troubleshooting X.25 Connections.") The next byte is referred to as the protocol discriminator, which is set to a value that indicates LMI. The third mandatory byte (call reference) is always filled with zeros.
The final mandatory byte is the Message type field. Two message types have been defined. -Status-enquiry messages allow the user device to inquire about network status. Status messages respond to status-enquiry messages. Keepalives (messages sent through a connection to ensure that both sides will continue to regard the connection as active) and PVC status messages are examples of these messages and are the common LMI features that are expected to be a part of every implementation that conforms to the consortium specification.
Together, status and status-enquiry messages help verify the integrity of logical and physical links. This information is critical in a routing environment because routing algorithms make decisions based on link integrity.
Following the message type field is some number of IEs. Each IE consists of a single-byte IE identifier, an IE length field, and 1 or more bytes containing actual data.
The global addressing extension permits node identifiers. With this extension, the values inserted in the DLCI field of a frame are globally significant addresses of individual end-user devices (for example, routers). This is implemented as shown in Figure 18-4.
Global addressing provides significant benefits in a large, complex internetwork. The Frame Relay network now appears to the routers on its periphery like any LAN. No changes to higher-layer protocols are needed to take full advantage of their capabilities.
Multicasting is another valuable optional LMI feature. Multicast groups are designated by a series of four reserved DLCI values (1,019 to 1,022). Frames sent by a device using one of these reserved DLCIs are replicated by the network and sent to all exit points in the designated set. The multicasting extension also defines LMI messages that notify user devices of the addition, deletion, and presence of multicast groups.
In networks that take advantage of dynamic routing, routing information must be exchanged among many routers. Routing messages can be sent efficiently by using frames with a multicast DLCI. This allows messages to be sent to specific groups of routers.
Frame Relay can be used as an interface to either a publicly available carrier-provided service or to a network of privately owned equipment. A typical means of private network implementation is to equip traditional T1 multiplexers with Frame Relay interfaces for data devices, as well as non-Frame Relay interfaces for other applications such as voice and video-teleconferencing. Figure 18-5 shows this configuration.
A public Frame Relay service is deployed by putting Frame Relay switching equipment in the central offices of a telecommunications carrier. In this case, users can realize economic benefits from traffic-sensitive charging rates and are relieved from the work necessary to administer and maintain the network equipment and service.
In either type of network, the lines that connect user devices to the network equipment can operate at a speed selected from a broad range of data rates. Speeds between 56 kbps and 2 mbps are typical, although Frame Relay can support lower and higher speeds.
Whether in a public or private network, the support of Frame Relay interfaces to user devices does not necessarily dictate that the Frame Relay protocol is used between the network devices. No standards for interconnecting equipment inside a Frame Relay network currently exist. Thus, traditional circuit-switching, packet-switching, or a hybrid approach combining these technologies can be used.
This section discusses troubleshooting procedures for connectivity problems related to Frame Relay links. It describes specific Frame Relay symptoms, the problems that are likely to cause each symptom, and the solutions to those problems.
The following sections cover the most common network issues in Frame Relay networks:
Symptom: Connections over a Frame Relay link fail. The output of the show interfaces serial exec command shows that the interface and line protocol are down or that the interface is up and the line protocol is down.
Table 18-1 outlines the problems that might cause this symptom and describes solutions to those problems.
| Possible Problem | Solution |
|---|---|
Cabling, hardware, or carrier problem | Perform these steps for the local and remote router: Step 1 Use the show interfaces serial command to see whether the interface and line protocol are up. Step 2 If the interface and line protocol are down, check the cable to make sure it is a DTE1 serial cable. Make sure cables are securely attached. Step 3 If the cable is correct, try moving it to a different port. If that port works, then the first port is defective. Replace either the card or the router. Step 4 If the cable doesn't work on the second port, try replacing the cable. If it still doesn't work, there might be a problem with the DCE2. Contact your carrier about the problem. For detailed information on troubleshooting serial lines, refer to "Troubleshooting Serial Line Problems." |
LMI3 type mismatch | Step 1 Use the show interfaces serial command to check the state of the interface. Step 2 If the output shows that the interface is up but the line protocol is down, use the show frame-relay lmi exec command to see which LMI type is configured on the Frame Relay interface. Step 3 Make sure that the LMI type is the same for all devices in the path from source to destination. Use the frame-relay lmi-type {ansi | cisco | q933a} interface configuration command to change the LMI type on the router. |
Keepalives not being sent | Step 1 Enter the show interfaces command to find out whether keepalives are configured. If you see a line that says "keepalives not set," keepalives are not configured. Step 2 Use the keepalive seconds interface configuration command to configure keepalives. The default value for this command is 10 seconds. |
Encapsulation mismatch | Step 1 When connecting Cisco devices with non-Cisco devices, you must use IETF4 encapsulation on both devices. Check the encapsulation type on the Cisco device with the show frame-relay map exec command. Step 2 If the Cisco device is not using IETF encapsulation, use the encapsulation frame-relay ietf interface configuration command to configure IETF encapsulation on the Cisco Frame Relay interface. For information on viewing or changing the configuration of the non-Cisco device, refer to the vendor documentation. |
DLCI5 inactive or deleted | Step 1 Use the show frame-relay pvc exec command to view the status of the interface's PVC. Step 2 If the output shows that the PVC6 is inactive or deleted, there is a problem along the path to the remote router. Check the remote router or contact your carrier to check the status of the PVC. |
Step 1 Use the show frame-relay pvc privileged exec command to check the assigned DLCIs. Make sure that the correct DLCIs are assigned to the correct subinterface. If the DLCI is incorrect, use the no frame-relay map command to delete the incorrect DLCI number entry under the interface. Use the frame-relay map interface configuration command to define the mapping between an address and the correct DLCI used to connect to the address. Syntax: frame-relay map protocol protocol-address dlci [broadcast] [ietf | cisco] Syntax Description:
Example: The following example maps IP address 131.108.123.1 to DLCI 100:
Step 2 If the DLCIs appear to be correct, shut down the main interface using the shutdown interface configuration command, and then bring the interface back up using the no shutdown command. |
Symptom: Attempts to ping the remote router across a Frame Relay connection fail.
Table 18-2 outlines the problems that might cause this symptom and describes solutions to those problems.
| Possible Problem | Solution |
|---|---|
Encapsulation mismatch | Step 1 When connecting Cisco devices with non-Cisco devices, you must use IETF encapsulation on both devices. Check the encapsulation type on the Cisco device with the show frame-relay map exec command. Step 2 If the Cisco device is not using IETF encapsulation, use the encapsulation frame-relay ietf interface configuration command to configure IETF encapsulation on the Cisco Frame Relay interface. For information on viewing or changing the configuration of the non-Cisco device, refer to the vendor documentation. |
DLCI inactive or deleted | Step 1 Use the show frame-relay pvc exec command to view the status of the interface's PVC. Step 2 If the output shows that the PVC is inactive or deleted, there is a problem along the path to the remote router. Check the remote router or contact your carrier to check the status of the PVC. |
DLCI assigned to wrong subinterface | Step 1 Use the show frame-relay pvc privileged exec command to check the assigned DLCIs. Make sure that the correct DLCIs are assigned to the correct subinterfaces. Step 2 If the DLCIs appear to be correct, shut down the main interface using the shutdown interface configuration command, and then bring the interface back up using the no shutdown command. |
Misconfigured access list | Step 1 Use the show access-list privileged exec command to see whether there are access lists configured on the router. Step 2 If there are access lists configured, test connectivity by disabling access lists using the no access-group global configuration command. Check whether connectivity is restored. Step 3 If connections work, reenable access lists one at a time, checking connections after enabling each access list. Step 4 If enabling an access list blocks connections, make sure that the access list does not deny necessary traffic. Make sure to configure explicit permit statements for any traffic you want to pass. Step 5 Continue testing access lists until all access lists are restored and connections still work. |
frame-relay map command missing | Step 1 Use the show frame-relay map privileged exec command to see whether an address map is configured for the DLCI. Step 2 If you do not see an address map for the DLCI, enter the clear frame-relay-inarp privileged exec command and then use the show frame-relay map command again to see whether there is now a map to the DLCI. Step 3 If there is no map to the DLCI, add a static address map. Use the frame-relay map interface configuration command. Syntax: frame-relay map protocol protocol-address dlci [broadcast] [ietf | cisco] |
frame-relay map command missing | Syntax Description:
Example: The following example maps IP address 131.108.123.1 to DLCI 100:
Step 4 Make sure that the DLCIs and next-hop addresses specified in frame-relay map commands are correct. The specified protocol address should be in the same network as your local Frame Relay interface. For complete information on configuring Frame Relay address maps, refer to the Cisco IOS Wide-Area Networking Configuration Guide. |
No broadcast keyword in frame-relay map statements | Step 1 Use the show running-config privileged exec command on local and remote routers to view the router configuration. Check frame-relay map command entries to see whether the broadcast keyword is specified. Step 2 If the keyword is not specified, add the broadcast keyword to all frame-relay map commands. Syntax: frame-relay map protocol protocol-address dlci [broadcast] [ietf | cisco] Syntax Description:
Example: The following example maps IP address 131.108.123.1 to DLCI 100:
Note: By default, the broadcast keyword is added to dynamic maps learned via Inverse ARP1. |
| 1ARP = Address Resolution Protocol |
Symptom: Attempts to ping devices on a remote network across a Frame Relay connection fail.
Table 18-3 outlines the problems that might cause this symptom and describes solutions to those problems.
| Possible Problem | Solution |
|---|---|
Split horizon problem | In a hub-and-spoke Frame Relay environment, you must configure subinterfaces in order to avoid problems with split horizon. For detailed information on configuring subinterfaces, refer to the Cisco IOS Wide-Area Networking Configuration Guide and Wide-Area Networking Command Reference. Additionally, certain protocols such as AppleTalk and transparent bridging cannot be supported on partially meshed networks because they require split horizon, in which a packet received on an interface cannot be transmitted out the same interface even if the packet is received and transmitted on different virtual circuits. Configuring Frame Relay subinterfaces ensures that a single physical interface is treated as multiple virtual interfaces. This capability allows us to overcome split horizon rules. Packets received on one virtual interface can now be forwarded out another virtual interface, even if they are configured on the same physical interface. Subinterfaces address the limitations of Frame Relay networks by providing a way to subdivide a partially meshed Frame Relay network into a number of smaller, fully meshed (or point-to-point) subnetworks. Each subnetwork is assigned its own network number and appears to the protocols as if it is reachable through a separate interface. (Note that point-to- |
No default gateway on workstation | Step 1 From the local workstation or server, try to ping the remote workstation or server. Make several attempts to ping the remote device if the first ping is unsuccessful. Step 2 If all your attempts fail, check whether the local workstation or server can ping the local router's Frame Relay interface. Step 3 If you are unable to ping the local interface, check the local workstation or server to see whether it is configured with a default gateway specification. Step 4 If there is no default gateway specified, configure the device with a default gateway. The default gateway should be the address of the local router's LAN interface. For information on viewing or changing the workstation or server's default gateway specification, refer to the vendor documentation. |
Posted: Tue May 16 15:12:46 PDT 2000
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