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

RouterA(config-if)#encapsulation frame-relay ?ietf Use RFC1490 encapsulation

Frame Relay Operation

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

ADLCI=16

BDLCI=100

CDLCI=100

DDLCI=100

EDLCI=101DLCI=102

DLCI=201DLCI=101

DLCI Mapping• The FR switch uses DLCI numbers to communicate• Network layer addresses must be mapped

RouterA(config-if)#frame-relay map ip 172.16.30.19 18

Dynamic mapping uses Inverse ARP:

1. Router learns DLCI from switch during initial LMI exchange

2. 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:172.16.1.2

Static mapping:

Frame Relay RatesNetwork performance at the router level is affected by:

Access RateLine speed of connection to the provider

CIRGuaranteed bandwidth

BurstingFaster 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• 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 AddressingSimple 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 ?ciscoansiq933a

FramesFrame Relay Format

10 bit address (0-1023)8th bit of each byte indicates address length (currently 2 bytes)

Forward Explicit Congestion NotificationBackward Explicit Congestion NotificationDiscard Eligibility

LMI Frame

1023

Mandatory bytes

Status-enquiry message (from user device)Status message:keepalives and PVC status

Frame Relay SubinterfacesFR 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 s0RouterA(config-if)#encapsulation frame-relayRouterA(config-if)#int s0.1 ?multipoint Treat as a multipoint linkpoint-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.

RouterA(config-if)#frame-relay interface-dlci ?<16-1007> Define a DLCI as part of the currentsubinterfaceRouterA(config-if)#frame-relay interface-dlci 16

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

Frame Relay Subinterfaces

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 10.1.12.1 255.255.255.0

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

Example configuration:

Verifying Frame Relay

Command Description

Encapsulation, Layer 1 and 2 status, DLCIs in use on the interface, LMI DLCI and LMI type.

The status of each configured connection and traffic statistics,DLCI, BECN, FECN, DE

Displays the interface, network layer address and DLCI mapping

Statistics for the interface including LMI type and status enquiries sent and recieved

show interface s0

show frame-relay pvc

show frame-relay map

show frame-relay lmi

QuestionsQ- 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?

A- FECN, BECN, DE

Q- What DLCI value defines an LMI frame?

A- 1023