Frame Relay




Frame Relay ( an ITU-T and ANSI) is a packet-switched technology that operates using virtual circuits to connect LANs across a cloud. It uses SVC and PVC. PVC are pre-configured by the ISP. It is designed to run on high speed digital lines and offers no error checking capabilities. Each PVC connection to a different location is labelled with a Data-link Connection Identifier (DLCI) and has its own set amount of bandwidth known as the committed information rate (CIR). Frame Relay topologies are:

Star (Hub & Spoke)
Full Mesh
Partial Mesh.

Local Management Interfaces (LMI) is the signalling standard and maintains the connection between two locations. LMI types used are

LMI used with Inverse ARP allows a router to map a DLCI with a particular network address and can detect the PVC dynamically.
Frame Relay networks are multi-access networks similar to Ethernet except that they do not forward broadcast traffic. Frame Relay is a nonbroadcast multi-access network (NBMA).

Frame Relay uses packet switching technology with variable length packets. It also makes use of STDM for optimum use of the available bandwidth.
Because FR originally does not send broadcast , each virtual circuit requires a Layer 2 address for identification. In Frame Relay, this address is the data-link connection identifier (DLCI).

The DLCI identifies the VC that data uses to reach a particular destination. The DLCI is stored in the address field of every frame transmitted. The DLCI usually has only local significance and may be different at each end of a VC.

The Layer 2 DLCI is associated with the Layer 3 address of the device at the other end of the VC. Mapping the DLCI to a remote IP address can occur manually or dynamically using a process known as Inverse ARP.

Establishing a mapping of DLCI to remote IP address occurs in the following steps:

1. The local device announces its presence by sending its Layer 3 address out on the VC.

2. The remote device receives this information and maps the Layer 3 IP address to the local Layer 2 DLCI.

3. The remote device announces its IP address on the VC.

4. The local device maps the Layer 3 address of the remote device to the local DLCI on which it received the information.

Frames carry data between user devices called DTE, and the DCE at the edge of the WAN.
Connection through the Frame Relay network between two DTEs is called a virtual circuit (VC). SVCs established dynamically by sending signaling messages to the network are. PVCs are preconfigured by the carrier
The clock speed of the connection (local loop) to the Frame Relay cloud is the rate at which data travels into or out of the network.

(Config)# frame-relay switching

Under the serial interface
(config-if)#encapsulation frame-relay


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