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Frame Relay. CCNA4 Chapter 6. Frame Relay Overview. Layer 2 Connection-oriented Packet switched Virtual Circuits: Permanent (and Switched) Links two DTE devices through DCE Originally designed for operation over ISDN Faster than X.25 Encapsulation can be Cisco or IETF.

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Frame relay

Frame Relay

CCNA4 Chapter 6

Frame relay overview
Frame Relay Overview

  • Layer 2

  • Connection-oriented

  • Packet switched

  • Virtual Circuits:

    Permanent (and Switched)

  • Links two DTE devices through DCE

  • Originally designed for operation over ISDN

  • Faster than X.25

  • Encapsulation can be Cisco or IETF

RouterA(config-if)#encapsulation frame-relay ?

ietf Use RFC1490 encapsulation

Frame relay dlci
Frame Relay - DLCI

  • Data Link Connection Identifier

  • IP addresses must be resolved to DLCI

  • Logical Virtual Circuits are distinguished by DLCI

  • Only locally significant

  • DLCI do not need to match end to end, though the may do if connected to different switches

  • DLCI are assigned by the service provider.

  • FR switch maps DLCI numbers to outgoing ports















Dlci mapping
DLCI Mapping

  • The FR switch uses DLCI numbers to communicate

  • Network layer addresses must be mapped

Static mapping:

RouterA(config-if)#frame-relay map ip 18

Dynamic mapping uses Inverse ARP:

  • Router learns DLCI from switch during initial LMI exchange

  • Router sends IARP request for each DLCI

  • 3. The return information is then used to build the map

DLCI = 100

What’s the next hop address for DLCI 100?

My IP address:

Frame relay rates
Frame Relay Rates

Network performance at the router level is affected by:

Access Rate

Line speed of connection to the provider


Guaranteed bandwidth


Faster than CIR for a short time period

(Tc) Committed Rate measurement interval

(Bc) Committed Burst

(Be) Excess Burst – maximum transfer attempted

DE = Discard Eligibility bit. Set on oversubscribed traffic

Lmi local management interface
LMI -Local Management Interface

  • Signalling between router and frame switch

  • Determine which PVCs are available and their status

  • LMI DLCI=1023

  • Extensions to basic frame relay protocol:

    Common: Status of virtual circuits (Keepalives)

    Optional: Multicasting (DLCI 1019-1022)

    Global Addressing

    Simple Flow Control

  • Cisco IOS 11.2 and later uses auto-detection

  • Cisco routers support three versions of LMI:

RouterA(config-if)#frame-relay lmi-type ?





Frame Relay Format

LMI Frame

Mandatory bytes


Status-enquiry message (from user device)

Status message:keepalives and PVC status

Frame relay subinterfaces
Frame Relay Subinterfaces

FR can divide a single physical WAN interface into multiple subinterfaces.

Subinterfaces are logical subdivisions of the physical interface.

Split-Horizon issues can be resolved.

RouterA(config)#int s0

RouterA(config-if)#encapsulation frame-relay

RouterA(config-if)#int s0.1 ?

multipoint Treat as a multipoint link

point-to-point Treat as a point-to-point link

Point to Point is used when each PVC is a separate subnet.

Multipoint is used when the PVCs use the same subnet.

Frame Relay Subinterfaces

Configure the local DLCI for the subinterface to distinguish it from the physical interface:

RouterA(config-if)#frame-relay interface-dlci ?

<16-1007> Define a DLCI as part of the current


RouterA(config-if)#frame-relay interface-dlci 16

Example configuration:

RouterA(config)#interface Serial0

RouterA(config-if)# no ip address

RouterA(config-if)# encapsulation frame-relay

RouterA(config-if)# interface Serial0.102 point-to-point

RouterA(config-if)# ip address

RouterA(config-if)# frame-relay interface-dlci 102

Verifying frame relay
Verifying Frame Relay

show interface s0

show frame-relay pvc

show frame-relay map

show frame-relay lmi


Q- What is the first step in configuring Frame-Relay on an interface?

A- Setting the encapsulation

Q- What are the possible types of LMI?

A- Cisco, ANSI and Q.933a

Q- What is the solution to the issue of split-horizon when configuring multiple VCs on a single physical interface?

A- Sub-interfaces

Q- What command can you issue to see your PVCs?

A- show frame-relay pvc

Q- What three bits does frame-relay use for congestion control?


Q- What DLCI value defines an LMI frame?

A- 1023