Chapter 6- Semester4
Carl Marandola
CCRI
What is Frame Relay?
Frame Relay is an industrystandard, switched data
link-layer protocol that
handles multiple virtual
circuits using High-Level
Data Link Control (HDLC)
encapsulation between
connected devices.
Frame Relay uses virtual
circuits to make
connections through a
connection-oriented
service.
Frame Relay Features
CCITT and ANSI standard that defines a process for
sending data over a public data network (PDN).
high performance, efficient data technology used in
networks throughout the world.
Sends information over a WAN by dividing data into
packets.
Operates at the physical and data link layers of the OSI
reference model.
Relies on upper-layer protocols such as TCP for error
correction.
Frame Relay interface can be either a carrier-provided
public network or a network of privately owned
equipment, serving a single enterprise.
Terms used in Frame Relay
Access rate -- The clock
speed (port speed) of the
connection (local loop) to
the Frame Relay cloud. It
is the rate at which data
travels into or out of the
network.
Data-link connection
identifier (DLCI) – A DLCI
is a number that identifies
the end point in a Frame
Relay network. This
number has significance
only to the local network.
The Frame Relay switch
maps the DLCIs between
a pair of routers to create
a permanent virtual
circuit.
Terms used in Frame Relay- Continue
Local management interface (LMI) -- A signaling standard between
the customer premises equipment (CPE) device and the Frame
Relay switch that is responsible for managing the connection and
maintaining status between the devices.
LMIs can include support for
1. a keepalive mechanism, which verifies that data is flowing.
2. a multicast mechanism, which can provide the network server with its
local DLCI.
3. multicast addressing, providing a few DLCIs to be used as multicast
addresses and the ability to give DLCIs global (whole Frame Relay
network) significance, rather than just local significance (DLCIs used
only to the local switch);
4. a status mechanism, which provides an ongoing status on the DLCIs
known to the switch.
There are several LMI types, and routers need to be told which LMI type
is being used. Three types of LMIs are supported: cisco, ansi, and
q933a.
Terms used in Frame Relay- Continue
Committed information rate (CIR) -- The CIR is the
guaranteed rate, in bits per second, that the service
provider commits to providing.
Committed burst -- The maximum number of bits that the
switch agrees to transfer during a time interval. (It is
noted Bc)
Excess burst -- The maximum number of uncommitted
bits that the Frame Relay switch attempts to transfer
beyond the CIR. Excess burst is dependent on the
service offerings available by the vendor, but is typically
limited to the port speed of the local access loop.
Forward explicit congestion notification (FECN) -- A bit
set in a frame that notifies a DTE that congestion
avoidance procedures should be initiated by the
receiving device. When a Frame Relay switch
recognizes congestion in the network, it sends a FECN
packet to the destination device, indicating that
congestion has occurred.
Terms used in Frame Relay- Continue
Backward explicit congestion notification (BECN) -- A bit
set in a frame that notifies a DTE that congestion
avoidance procedures should be initiated by the receiving
device. As shown in Figure when a Frame Relay switch
recognizes congestion in the network, it sends a BECN
packet to the source router, instructing the router to reduce
the rate at which it is sending packets. If the router receives
any BECNs during the current time interval, it decreases
the transmit rate by 25%.
Terms used in Frame Relay- Continue
Forward explicit congestion
notification (FECN) -- A bit set in a
frame that notifies a DTE that
congestion avoidance procedures
should be initiated by the receiving
device. When a Frame Relay switch
recognizes congestion in the network,
it sends a FECN packet to the
destination device, indicating that
congestion has occurred.
Backward explicit congestion
notification (BECN) -- A bit set in a
frame that notifies a DTE that
congestion avoidance procedures
should be initiated by the receiving
device. when a Frame Relay switch
recognizes congestion in the network,
it sends a BECN packet to the source
router, instructing the router to reduce
the rate at which it is sending packets.
If the router receives any BECNs
during the current time interval, it
decreases the transmit rate by 25%.
Terms used in Frame Relay- Continue
Discard eligibility (DE) indicator -- A set bit that indicates
the frame may be discarded in preference to other
frames if congestion occurs. When the router detects
network congestion, the Frame Relay switch will drop
packets with the DE bit set first. The DE bit is set on the
oversubscribed traffic (that is, the traffic that was
received after the CIR was met).
Frame Relay operation
You deploy a public Frame Relay service by putting
Frame Relay switching equipment in the central office of
a telecommunications carrier. In this case, users get
economic benefits from traffic-sensitive charging rates,
and don't have to spend the time and effort to administer
and maintain the network equipment and service.
The lines that connect user devices to the network
equipment can operate at a speed selected from a broad
range of data rates. Speeds between 56 kbps and 2
Mbps are typical, although Frame Relay can support
lower and higher speeds
Frame Relay DLCI
Frame Relay provides a
means for multiplexing
many logical data
conversations, referred to
as virtual circuits, through a
shared physical medium by
assigning DLCIs to each
DTE/DCE pair of devices.
Frame Relay's multiplexing
provides more flexible and
efficient use of available
bandwidth. Therefore,
Frame Relay allows users
to share bandwidth at a
reduced cost.
Frame Relay DLCI
Frame Relay standards
address permanent virtual
circuits (PVCs) that are
administratively configured and
managed in a Frame Relay
network. Frame Relay PVCs
are identified by DLCIs
The service provider's
switching equipment
constructs a table mapping
DLCI values to outbound ports.
When a frame is received, the
switching device analyzes the
connection identifier and
delivers the frame to the
associated outbound port. The
complete path to the
destination is established
before the first frame is sent.
Frame Relay Frame Format
The Frame Relay frame format
is shown in the Figure. The
flag fields indicate the
beginning and end of the
frame. Following the leading
flag field are 2 bytes of
address information. 10 bits of
these 2 bytes make up the
actual circuit ID (that is, the
DLCI).
Congestion Control -- The last
3 bits in the address field,
which control the Frame Relay
congestion notification
mechanisms. These are the
FECN, BECN, and discard
eligible (DE) bits.
Frame Relay addressing
DLCI address space is limited to 10 bits. This creates a
possible 1024 DLCI addresses. The usable portion of
these addresses are determined by the LMI type used.
The remaining DLCI addresses are reserved for vendor
implementation. This includes LMI messages and
multicast addresses.
Examples:
The Cisco LMI type supports a range of DLCI addresses
from DLCI 16-1007 for carrying user-data.
The ANSI/ITU LMI type supports the range of addresses
from DLCI 16-992 for carrying user-data.
LMI operation & extensions
In addition to the basic Frame
Relay protocol functions for
transferring data, the Frame
Relay specification includes LMI
extensions that make supporting
large, complex internetworks
easier.
A summary of the LMI extensions
follows:
Virtual circuit status messages
(common) -- Provide
communication and
synchronization between the
network and the user device,
periodically reporting the
existence of new PVCs and the
deletion of already existing
PVCs, and providing general
information about PVC integrity.
LMI extensions -Continue
Multicasting (optional) -- Allows a sender to transmit a single
frame but have it delivered by the network to multiple
recipients.
Global addressing (optional) -- Gives connection identifiers
global rather than local significance, allowing them to be
used to identify a specific interface to the Frame Relay
network. Global addressing makes the Frame Relay network
resemble a local-area network (LAN) in terms of addressing;
Simple flow control (optional) -- Provides for an XON/XOFF
flow control mechanism that applies to the entire Frame
Relay interface. It is intended for devices whose higher
layers cannot use the congestion notification bits and that
need some level of flow control
LMI frame Format
The Frame Relay
specification
includes the LMI
procedures. LMI
messages are
sent in frames
distinguished by
an LMI-specific
DLCI (defined in
the consortium
specification as
DLCI = 1023).
LMI Feature- Global addressing
In normal Frame Relay there are no addresses that identify
network interfaces, or nodes attached to these
interfaces. Because these addresses do not exist, they
cannot be discovered by traditional address resolution
and discovery techniques.
So, static maps must be created to tell routers which DLCIs
to use to find a remote device and its associated
internetwork address.
Global addressing provides significant benefits in a large,
complex network. The Frame Relay network now
appears to the routers on its periphery like any LAN.
LMI Features- Multicasting
Multicast groups are designated by a series of four
reserved DLCI values (1019 to 1022). Frames
sent by a device using one of these reserved
DLCIs are replicated by the network and sent to
all exit points in the designated set. The
multicasting extension also defines LMI
messages that notify user devices of the
addition, deletion, and presence of multicast
groups.
LMI Features- Inverse ARP
The Inverse ARP mechanism
allows the router to
automatically build the Frame
Relay map, as shown in the
Figure. The router learns the
DLCIs that are in use from the
switch during the initial LMI
exchange. The router then
sends an Inverse ARP request
to each DLCI for each protocol
configured on the interface if
the protocol is supported. The
return information from the
Inverse ARP is then used to
build the Frame Relay map.
LMI Features- Frame Relay mapping
The router next-hop address
determined from the routing
table must be resolved to a
Frame Relay DLCI, as shown
in the Figure. The resolution is
done through a data structure
called a Frame Relay map.
The routing table is then used
to supply the next-hop protocol
address or the DLCI for
outgoing traffic. This data
structure can be statically
configured in the router, or the
Inverse ARP feature can be
used for automatic setup of the
map.
LMI Features- Frame Relay switching table
The router next-hop address
determined from the routing
table must be resolved to a
Frame Relay DLCI, as shown
in the Figure. The resolution is
done through a data structure
called a Frame Relay map.
The routing table is then used
to supply the next-hop protocol
address or the DLCI for
outgoing traffic. This data
structure can be statically
configured in the router, or the
Inverse ARP feature can be
used for automatic setup of the
map.
LMI Features- Frame Relay Switching
The Frame Relay switching
table consists of four entries:
two for incoming port and
DLCI, and two for outgoing
port and DLCI, as shown in the
Figure. The DLCI could,
therefore, be remapped as it
passes through each switch;
the fact that the port reference
can be changed is why the
DLCI does not change even
though the port reference
might change.
Frame Relay Subinterfaces
Subinterfaces are logical
subdivisions of a physical
interface.
By logically dividing a
single physical WAN
serial interface into
multiple virtual
subinterfaces, the overall
cost of implementing a
Frame Relay network can
be reduced.
Spilt Horizon Use in Frame Relay
if a remote router
sends an update to
the headquarters
router that is
connecting multiple
PVCs over a single
physical interface,
the headquarters
router cannot
advertise that route
through the same
physical interface to
other remote
routers
Subinterfaces
You can configure subinterfaces to support the
following connection types:
Point-to-point -- A single subinterface is used to
establish one PVC connection to another
physical interface or subinterface on a remote
router.
Multipoint -- A single subinterface is used to
establish multiple PVC connections to multiple
physical interfaces or subinterfaces on remote
routers.
Writing the IOS command sequence to
completely configure Frame Relay
Select the interface and go into interface configuration mode
Router(config) # interface serial 0
Configure a network-layer address, for examlple, an IP address
Router(config-if) # ip address 212.14.249.197 255.255.255.0
Select the encapsulation type used for data traffic end-to-end
Router(config-if) # encapsulation frame-relay [cisco|ietf]
For Cisco IOS 11.1 or earlier specify the LMI type used in the Frame
Relay Switch, Router(config-if) # frame-relay lmi-type {ansi|cisco|q933i}
for later versions of the Cisco IOS LMI type is autosensed
Configure the bandwidth of the link
Router(config-if) # bandwidth kilobits
If the inverse ARP was disabled on the router, re-enable it
Router(config-if) # frame-relay inverse-arp [protocol] [dlci]
Configuring subinterfaces
Select the interface
Remove any network layer address assigned to the physical
interface
Configure Frame Relay encapsulation as stated perviusly
Select the subinterface you want to configure
Router(config-if)# interface serial number.subinterface-number {multipoint|point-to-point}
 Interface number 1-4294967293
 Multipoint is used when you want the router to forward broadcasts and routing
updates it receives
 Point-to-point is used when you do not want the router to forward broadcasts and
routing updates it receives.
Configure the network layer address on the subinterface
If you configured the subinterface as multipoint or point-to-point you
must configure a local DLCI
Router(config-if)# frame-relay interface-dlci dlci-number
Router(config-if)# frame-relay map protocol protocol
address dlci [broadcast] [ietf|cisco|payload-compress
packet-by-packet]
GOOD LUCK Carl M