Configuring a Frame Relay Switch
Conceivably, the most useful device inside and outside the lab might be the Frame Relay switch. Much like the jump register, the configuration of a Frame Relay switch seems to be one of those untold secrets of router configuration. When I learned how to configure Frame Relay switching, I was able to model many network installations. Because I was the "service provider," I could assign my own DLCIs, matching those exactly as AT&T or MCI might provide. By accurately modeling the network in the lab, you increase your confidence level of the installation, along with lowering the probability of misconfiguration or the chance of sending out bad equipment. This section focuses on configuring the Cisco router as a Frame Relay switch. Further Frame Relay configuration details are covered in Chapter 5, "WAN Protocols and Technologies: Frame Relay."
Essentially, Frame Relay switching is a means of switching frames based upon the data-link connection identifier (DLCI). In the router's Frame Relay ARP table, a DLCI number is associated with an interface. Frame Relay uses its ARP table to examine DLCIs and interface pairings to make its decisions on whether to forward a frame out a specific interface.
The Frame Relay switch is predominately a DCE device, which means two things:
• Any modeling requires a minimum of three routers: one router for the switch and two routers to use the switch to communicate with each other.
• DCE cables are needed on the frame switch's serial interfaces.
At this point, it is important to define some common Frame Relay terms:
• Permanent virtual circuit (PVC)— The logical end-to-end circuit used for frame transport. A PVC's endpoints are addressed with DLCIs.
• Data-link connection identifier (DLCI)— A logical number between 16 and 1007 used to identify the PVC between the customer premises equipment (CPE) and the Frame Relay switch. In most cases, the DLCI is only locally significant, which implies that only the local devices know what the DLCI numbers are. It is possible to have two PVCs with the same DLCI number on the remote ends referring to the same central site.
• Local Management Interface (LMI)— Best defined as the signaling standard used between the router and the Frame Relay switch. LMI is used by the switch to learn which DLCIs are defined and their status. LMI also supports a 10-second keepalive mechanism that verifies that the PVC is active and that data is being exchanged. Three types of LMI are supported on Cisco routers: cisco, ansi, and q933a. The router will try an autonegotiation on all three LMI types.
- cisco— LMI type defined by the "big three," Cisco, Digital, and Northern Telecom. This is the default LMI type, after autonegotiation fails. LMI status information is sent on DLCI 0.
- ansi— LMI type defined by ANSI standard T1.617, commonly called Annex D.
This is the most common type of LMI found across all Frame Relay networks. LMI status information is sent on DLCI 1023.
- q933a— LMI type defined as ITU-T Q.933, or simply Annex A. LMI status information is sent on DLCI 0.
• Network-to-Network Interface (NNI)— NNI is the standard used for two switches to communicate. NNI is used in both Frame Relay and ATM. In ATM, it is referred to as network node interface.
To configure Frame Relay switching, it is necessary to perform the following tasks: Step 1. Enable Frame Relay switching.
Step 2. Configure the interface LMI and Frame Relay interface type. Step 3. Configure PVCs, with the frame-relay route command.
In this first example, you configure a Frame Relay switch with two end devices or routers. It is always good to make a PVC diagram of your model before you begin. On your diagram, include the DCE, PVC, and interface denotations. Figure 1-13 exhibits the diagram that you use for this example. The diagram highlights the network from a hardware and service provider perspective. The Frame Relay switch, in the middle, has two V.35 DCE cables to two other routers, R1 and R2. These two routers have V.35 DTE male cables connected to their Serial 0 port. You configure a PVC with DLCI 101 on Serial 0 mapping to DLCI 102 on Serial 5.
Figure 1-13. Basic Frame Relay Configuration Example
Figure 1-13. Basic Frame Relay Configuration Example
The first step in configuring the Frame Relay switch, excluding drawing your diagram, is to enable Frame Relay switching. This is done with the global configuration command frame-relay switching. Next, configure the serial interfaces for frame relay switching. You need to set the encapsulation to Frame Relay with the encapsulation frame-relay command, and you must set the LMI type with the frame-relay lmi-type [ansi | cisco | q993a ] command from the interface prompt. To continue to configure the Frame Relay interface, add the frame-relay intf-type dce command. Because the interface is DCE, you need to use the clock rate bps command. The bps values range from 1200 to 8,000,000. Finally, the frame-relay route [16-1007]inbound_DLCI interface outbound_serial_ interface [16-1007]outbout_DLCI command creates a PVC on the interface and maps it to another interface. Example 1-19 demonstrates the use of these commands and the basic configuration of a Frame Relay switch.
Continue reading here: Example 120 Displaying the Entire Frame Relay Configuration
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