1. Semester 4, Chapter 6 Allan Johnson
Frame Relay
Semester 4, Chapter 6
Allan Johnson

2. Table of Contents Frame Relay Technology Local Management Interface
The Use of Subinterfaces
Configuring Frame Relay
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3. Frame Relay Technology
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4. Frame Relay History
Frame Relay was designed as a stream-lined version of X.25.
X.25, a 1970s ITU-T standard, ensured reliable transport at the data link layer with error detection and error correction.
With the introduction of DoD’s TCP/IP in the early 1980s, TCP took over error correction.
Although Frame Relay detects errors at the data link layer, it does not correct. That’s now TCP’s job.

5. Frame Relay History
In 1990, Cisco Systems, StrataCom, Nothern Telecom and Digital Equipment (sometimes referred to as the Group of Four) worked to standardize the Frame Relay protocol and add what they dubbed LMI extensions.
Today, Frame Relay is the most popular WAN protocol because it is:
Faster than X.25, it uses TCP for error correction
Cost-effective - you no longer have to pay for a dedicated point-to-point link
Versatile - can operate over a variety of interfaces (ISDN, Serial, Dial-up, etc.)

6. Frame Relay Operation
Devices in the Frame Relay network are the DTE (customer equipment) and DCE (provider’s frame relay switch)
Often cheaper than other technologies because many times the service provider also owns the DTE.
The Frame Relay connection between the DTE and DCE operates at the data link and physical layers of the OSI model.
Data Link
Physical

7. Frame Relay Operation
Frame Relay operates over a permanent virtual circuit (PVC), which means that a permanent connection exists between the source DCE and destination DCE over the frame relay network.
Therefore, there is no need for call setup and termination like in ISDN. Frame Relay has two states:
Data transfer--between the DCE and the provider’s DTE
Idle--the line is active, but no data is being transferred.

8. Frame Relay Operation
The connection between the local DTE and the DCE in a Frame Relay network is logically identified with a Data-link Connection Identifier (DLCI).
A word about Switched Virtual Circuits (SVC)
Frame Relay over ISDN must use a SVC with ISDN’s call setup and termination procedures.
However, currently few manufacturers of DCE equipment support Frame Relay SVCs, so implementation is minimal.
Therefore, we will assume a PVC when discussing Frame Relay.

9. Frame Relay Lexicon
DLCI - Identifies logical connections to the Frame Relay network and has local significance only
FECN - Forward explicit congestion notification; tell receiving DTE to implement congestion avoidance procedures
BECN - Backwards explicit congestion notification; tells the sending DTE to slow down the transfer rate by 25%.
DE - Discard eligibility; bit set in the frame to say “frame is not business critical” and can be discarded
CIR - Committed information rate guaranteed by the service provider.
LMI - Local Management Interface; determines the operational status of PVCs

10. FECN-tells receiving DTE device to implement congestion avoidance procedures
FRAMES
BECN
FECN
BECN-tells sending DTE device to reduce the rate of sending data.
DLCI-identifies logical connections on the Frame Relay switch to which the customer is attached

11. Frame Relay Frame Format
Flag - like most frame formats, the flag indicates the beginning and end of the frame
The DLCI makes up the first 10 bits of the address field, while the FECN, BECN, and DE bits are the last 3 bits.

12. Local Management Interface
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13. LMI Overview
LMI, similar to LCP in PPP, is a set of extensions to the basic Frame Relay protocol.
LMI’s main functions are to:
determine the operational status of the PVC between source and destination.
transmit keepalives to ensure PVC stays up
inform router what PVCs are available
LMI extensions were added by the Group of Four.
A common extension that must be used by all who implement Frame Relay is virtual circuit status messages.
Optional extensions include...
Multicasting
Global addressing
Simple Flow Control

14. LMI Frame Format
The LMI Frame has four mandatory bytes. They are outlined in red below. Know them!!

15. Frame Relay Map
The term map means to “map” or bind a Layer 2 address to a Layer 3 address.
An ARP table maps MACs to IPs in a LAN
In ISDN, we use the dailer-map command to map SPIDs to IP addresses
In Frame Relay, we need to map the data link layer’s DLCI to the IP address
We use the frame-relay map command

16. Frame Relay Map
The Frame Relay switch builds a table of incoming/outgoing ports and DLCIs.
The router builds a Frame Relay Map through Inverse ARP requests of the switch during the LMI exchange process.
The Frame Relay Map is used by the router for next-hop address resolution.

17. Use of Subinterfaces
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18. All Routers in Same Network
Using a Subinterface
In order to have an active Frame Relay link to all your routers in the network, you need either
All routers on the same network or subnet
or use subinterfaces
Frame Relay Network
/24
DLCI 17
DLCI 18
DLCI 16 A B C
All Routers in Same Network

19. Using a Subinterface
Rarely do you have all routers on the same network or subnet, so we use subinterfaces.
Each serial interface can logically be divided into as many subinterfaces as you need to establish PVCs with each destination.
Each destination’s DLCI needs a separate point-to-point subinterface.
Each side of the PVC must belong to the same network.

20. Subinterface Example #1
Each PVC as a point-to-point link in its own network or subnet A B C D S S S

21. Subinterface Example #2
Frame Relay Network S
DLCI 17
DLCI 18
DLCI 16 A B C S S S S S
AB-PVC
AC-PVC
BC-PVC

22. Configuring Frame Relay
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23. Frame Relay Encapsulation
To enable Frame Relay, simply go to the serial interface and enter the command
However, if you are connecting to a non-Cisco remote router, you must specify the option IETF
Router(config)#encap frame-relay [cisco|IETF]
Router(config)#int s0
Router(config-if)#encapsulation frame-relay

24. One Subnet/Network Configuration
The curriculum says to use a subinterface and specify multipoint.
router(config-if)#int s1.1 multipoint
However, subinterfaces are not necessary when configuring PVCs on the same subnet.
Also, we do not have to set the LMI type since our Cisco IOS is 11.2 or later. LMI type is autosensed.
However, we enter a map command to link the remote rouer’s DLCI to its IP address.

25. One Subnet/Network Configuration
Configure frame relay encapsulation and map the destinations’ DLCIs to their IPs
Repeat the commands on each router in the network
Frame Relay Network
/24
DLCI 17
DLCI 18
DLCI 16 A B C
RouterA(config)#int s0
RouterA(config-if)#encapsulation frame-relay
RouterA(config-if)#frame-relay map ip broadcast
RouterA(config-if)#frame-relay map ip broadcast

26. Multiple Subnet/Network Configuration
When your routers are parts of different networks or subnets, then you must either physically attach them on different interfaces or use subinterfaces on a single interface.
First step is to set the encapsulation type of the serial interface and state no IP address.
RouterA(config)#int s0
RouterA(config-if)#encap frame-relay
RouterA(config-if)#no ip address
RouterA(config-if)#no shut
Then enter subinterface configuration mode to assign each point-to-point link its IP address and define the destination’s DLCI.
Next slide shows the subinterface commands.

27. Multiple Subnet/Network Configuration
Frame Relay Network S
DLCI 17
DLCI 18
DLCI 16 A B C S S S S S
AB-PVC
AC-PVC
BC-PVC
RouterA(config-if)#int s0.17 point-to-point
RouterA(config-subif)#ip address
RouterA(config-subif)#frame-relay interface-dlci 17
RouterA(config-subif)#int s0.18 (continue with configuration)

28. Additional Frame Relay Labs
Chapter 6 does not provide enough practice for configuring Frame Relay.
If you have an Adtran unit, use the first two labs in Chapter 8 of the Semester 6 curriculum to gain additional Frame Relay experience.
Lab 8.2.1: Configuring Frame Relay
A “One Subnet/Network” Frame Relay Configuration
Lab 8.3.4: Configuring Frame Relay with Subinterfaces
A “Multiple Subnet/Network” Frame Relay Configuration

29. Verifying Frame Relay show interface serial 0 show frame-relay map
is Frame Relay sending and receiving data?
displays both LMI and DLCI information
show frame-relay map
displays the frame relay table on the router
show frame-relay pvc
used to verify a frame relay configuration

30. Table of Contents
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