Frame Relay
Semester 4, Chapter 3
Table of Contents
Go There!
Frame Relay Technology
Go There!
Local Management Interface
Go There!
The Use of Subinterfaces
Go There!
Configuring Frame Relay
Frame Relay Technology
Table of Contents
End Slide Show
Frame Relay History
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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.
Frame Relay History
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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:
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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.)
Frame Relay Operation
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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
Frame Relay Operation
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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:
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Data transfer--between the DCE and the
provider’s DTE
Idle--the line is active, but no data is being
transferred.
Frame Relay Operation
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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)
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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.
Frame Relay Lexicon
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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
FECN-tells receiving DTE device to
implement congestion avoidance
procedures
FRAMES
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
Frame Relay Frame Format
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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.
Local Management Interface
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LMI Overview
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LMI, similar to LCP in PPP, is a set of extensions to
the basic Frame Relay protocol.
LMI’s main functions are to:
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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.
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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
LMI Frame Format
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The LMI Frame has four mandatory bytes.
They are outlined in red below. Know them!!
Frame Relay Map
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The term map means to “map” or bind a
Layer 2 address to a Layer 3 address.
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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
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We use the frame-relay map command
Frame Relay Map
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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 nexthop address resolution.
Use of Subinterfaces
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Using a Subinterface
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In order to have an active Frame Relay link to all
your routers in the network, you need either
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All routers on the same network or subnet
or use subinterfaces
A
DLCI 16
192.168.4.1
Frame Relay
Network
192.168.4.2
B
All Routers in
Same Network
192.168.4.0/24
DLCI 17
DLCI 18
192.168.4.3
C
Using a Subinterface
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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.
Subinterface Example #1
Each PVC as a
point-to-point link
in its own network
or subnet
A
B
192.168.1.2
192.168.2.2
192.168.3.2
S0.1 192.168.1.1
S0.2 192.168.2.1
S0.3 192.168.3.1
D
C
Subinterface Example #2
S0.17 192.168.4.1
A
S0.18 192.168.5.1
DLCI 16
AB-PVC
AC-PVC
Frame Relay
Network
S0.16 192.168.4.2
B
S0.18 192.168.6.1
DLCI 17
DLCI 18
BC-PVC
S0.16 192.168.5.2
C
S0.17 192.168.6.2
Configuring Frame Relay
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Frame Relay Encapsulation
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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
One Subnet/Network Configuration
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The curriculum says to use a subinterface
and specify multipoint.
router(config-if)#int s1.1 multipoint
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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.
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One Subnet/Network Configuration
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Configure frame relay
encapsulation and
map the destinations’
DLCIs to their IPs
Repeat the
commands on each
router in the network
192.168.4.2
B
A
DLCI 16
192.168.4.1
Frame Relay
Network
192.168.4.0/24
DLCI 17
DLCI 18
192.168.4.3
C
RouterA(config)#int s0
RouterA(config-if)#encapsulation frame-relay
RouterA(config-if)#frame-relay map ip 192.168.4.2 17 broadcast
RouterA(config-if)#frame-relay map ip 192.168.4.3 18 broadcast
Multiple Subnet/Network Configuration
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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
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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.
Multiple Subnet/Network Configuration
S0.17 192.168.4.1
A
S0.18 192.168.5.1
DLCI 16
AB-PVC
AC-PVC
Frame Relay
Network
S0.16 192.168.4.2
B
S0.18 192.168.6.1
DLCI 17
DLCI 18
BC-PVC
S0.16 192.168.5.2
C
S0.17 192.168.6.2
RouterA(config-if)#int s0.17 point-to-point
RouterA(config-subif)#ip address 192.168.4.1 255.255.255.0
RouterA(config-subif)#frame-relay interface-dlci 17
RouterA(config-subif)#int s0.18 (continue with configuration)
Verifying Frame Relay
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show interface serial 0
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show frame-relay map
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is Frame Relay sending and receiving data?
displays both LMI and DLCI information
displays the frame relay table on the router
show frame-relay pvc
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used to verify a frame relay configuration
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End Slide Show