COnfiguring Basic Frame Relay


Frame Relay Configuration Tasks
Frame Relay is configured on a Cisco router from the Cisco IOS command-line interface (CLI). This section outlines the required steps to enable Frame Relay on your network, as well as some of the optional steps that you can use to enhance or customize your configuration.
In this section, you will configure the Cisco routers as Frame Relay access devices, or DTE, connected directly to a dedicated Frame Relay switch, or DCE.
Enable Frame Relay Encapsulation
This first figure, displays how Frame Relay has been configured on the serial interfaces. This involves assigning an IP address, setting the encapsulation type, and allocating bandwidth.
Step 1. Setting the IP Address on the Interface
On a Cisco router, Frame Relay is most commonly supported on synchronous serial interfaces. Use the ip address command to set the IP address of the interface. You can see that R1 has been assigned 10.1.1.1/24, and R2 has been assigned IP address 10.1.1.2/24.
Step 2. Configuring Encapsulation
The encapsulation frame-relay interface configuration command enables Frame Relay encapsulation and allows Frame Relay processing on the supported interface. There are two encapsulation options to choose from, and these are described below.
Step 3. Setting the Bandwidth
Use the bandwidth command to set the bandwidth of the serial interface. Specify bandwidth in kb/s. This command notifies the routing protocol that bandwidth is statically configured on the link. The EIGRP and OSPF routing protocols use the bandwidth value to calculate and determine the metric of the link.
Step 4. Setting the LMI Type (optional)
This is an optional step as Cisco routers autosense the LMI type. Recall that Cisco supports three LMI types: Cisco, ANSI Annex D, and Q933-A Annex A and that the default LMI type for Cisco routers is cisco.
Encapsulation Options
Recall that the default encapsulation type on a serial interface on a Cisco router is the Cisco proprietary version of HDLC. To change the encapsulation from HDLC to Frame Relay, use the encapsulation frame-relay [cisco | ietf] command. The no form of the encapsulation frame-relay command removes the Frame Relay encapsulation on the interface and returns the interface to the default HDLC encapsulation.
The default Frame Relay encapsulation enabled on supported interfaces is the Cisco encapsulation. Use this option if connecting to another Cisco router. Many non-Cisco devices also support this encapsulation type. It uses a 4-byte header, with 2 bytes to identify the DLCI and 2 bytes to identify the packet type.
The IETF encapsulation type complies with RFC 1490 and RFC 2427. Use this option if connecting to a non-Cisco router.
Configuring a Static Frame Relay Map
Cisco routers support all network layer protocols over Frame Relay, such as IP, IPX, and AppleTalk, and the address-to-DLCI mapping can be accomplished either by dynamic or static address mapping.
Dynamic mapping is performed by the Inverse ARP feature. Because Inverse ARP is enabled by default, no additional command is required to configure dynamic mapping on an interface.
Static mapping is manually configured on a router. Establishing static mapping depends on your network needs. To map between a next hop protocol address and a DLCI destination address, use the frame-relay map protocol protocol-address dlci [broadcast] command.
Using the Broadcast Keyword
Frame Relay, ATM, and X.25 are non-broadcast multiple access (NBMA) networks. NBMA networks allow only data transfer from one computer to another over a VC or across a switching device. NBMA networks do not support multicast or broadcast traffic, so a single packet cannot reach all destinations. This requires you to broadcast to replicate the packets manually to all destinations.
Some routing protocols may require additional additional configuration options. For example, RIP, EIGRP and OSPF require additional configurations to be supported on NBMA networks.
Because NBMA does not support broadcast traffic, using the broadcast keyword is a simplified way to forward routing updates. The broadcast keyword allows broadcasts and multicasts over the PVC and, in effect, turns the broadcast into a unicast so that the other node gets the routing updates.

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