Cisco® CCNP Route Exam Notes : Operations,Frame Relay Devices

2. Layer2 Technologies

2.2 Explain Frame Relay

2.2.a Operations

Frame Relay is a WAN protocol that operates at the physical and data link layers of the OSI reference model. Frame relay is a type of WAN connection used to connect one site to many remote sites through a single physical circuit.

Frame Relay Devices: Devices attached to a Frame Relay WAN fall into the following two general categories: Data Terminal Equipment (DTE) and Data Circuit-Terminating Equipment (DCE)

DTEs are considered to be terminating equipment for a specific network and typically are located on the premises of a customer. They may be owned by the customer. DTE devices are terminals, personal computers, routers, and bridges.

DCEs are carrier-owned internetworking devices. The purpose of DCE equipment is to provide clocking and switching services in a network, which are the devices that actually transmit data through the WAN. In most cases, these are packet switches located on ISPs.

The following are true about Frame Relay

1. Frame Relay is primarily, a Layer 2 standard.

2. Frame Relay DLCIs have local significance.

3. Cisco supports three types of LMIs (Link Management Interface): cisco, ansi, and q933a

4. Cisco supports two types of Frame Relay encapsulation: cisco, and ietf. When you are connecting a Cisco router with a non-Cisco router, use ietf as the encapsulation method.

Given below are salient features of Frame Relay DLCIs:

1. DLCIs (Data Link Connection Identifier) have only local significance, It means, the end devices over FR network can have different DLCI numbers.

2. DLCI number is provided by the FR service provider. DLCI number is mapped to Layer

3 protocol address using "frame-relay map" statement.

4. DLCI numbers must be unique on a router.

Frame Relay supports two types of virtual circuits (VCs):

1. Permanent Virtual Circuits (PVCs) - These are permanently established connection that are used for frequent and consistent data transfers between DTEs across a Frame Relay cloud.

2. Switched Virtual Circuits (SVCs) - These are temporary connections used in situations requiring only occassional data transfers between DTEs across Frame Relay cloud.

The terms "Call Setup", "Data Transfer", "Idle", and "Call Termination" are associated with SVCs. Frame Relay SVCs are not widely supported by manufacturers.

When the sub-interfaces on a serial interface are to be configured for Frame Relay, each sub interface needs to be assigned individual DLCI. The following command assigns a dlci of 100 to any sub-interface is:

R(config-if)# frame-relay interface-dlci 100

Note that prior to issuing the above command; issue the following command to get into proper sub interface configuration mode:

R(config)# interface serial0.1 point-to-point

The command "show frame-relay lmi" displays the LMI status,

The following is the sample output of "show frame-rely lmi"

show frame-rely lmi command output

where as the command "show frame-relay pvc" displays the frame-relay pvc status.

Frame Relay Generic Configuration Example

The following sample output shows a generic Frame Relay configuration on DLCI 100:

show frame-relay pvc 100 command output

Frame Relay offers NBMA (Non Broadcast Multi Access) connectivity to various destinations. There might be several PVCs residing on one serial interface. A result of this would be, no broadcasts are forwarded among these PVCs due to implementation of split horizon rule Split horizon rule prevents a route from being advertised onto the same interface (through which the router was learned).

One way to allow broadcasts to propagate among these PVCs is to disable split horizon. But, this may again result in routing loops. The recommended solution to this problem is sub-interfaces. A sub-interfaces are logical subdivisions of a physical interface. Routing updates received on one sub interface can be sent to another sub interface. This enables the FR network administrator to implement the split horizon, and at the same time use multiple PVCs on one physical interface.

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