fame relay training course by eng.abdulrahman abutaleb in gti ,2005.doc
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FRAME-RELAY TECHNOLOGY
PreparedBy
Eng/Abdulrahman M. Abutaleb
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1-The History of Frame Relay Implementations
Initial proposals for the standardization of Frame Relay were presented to theConsultative Committee on International Telephone and Telegraph (CCITT)
in 1984. Because of lack of interoperability and lack of complete
standardization, however, Frame Relay did not experience significant
deployment during the late 1980s. A major development in Frame Relay's
history occurred in 1990 when Cisco, Digital Equipment Corporation (DEC),
Northern Telecom, and StrataCom formed a consortium to focus on Frame
Relay technology development. This consortium developed a specification
that conformed to the basic Frame Relay protocol that was being discussed in
CCITT, but it extended the protocol with features that provide additional
capabilities for complex internetworking environments. These Frame Relay
extensions are referred to collectively as the Local Management Interface
(LMI).
The first public frame relay service offering appeared in North America
during 1992 from companies such as AT&T, US Sprint, BT North America,
Wiltel and CompuServe. These companies installed frame relay nodes in
major cities throughout the United State and permitted access to these nodes
(at the POP - point of presence) by a local access line purchased from the
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local telecommunications company (see Figure below):
The customer then has to pay for the subscription to the frame relay service as
well as for the access line from the local telecommunications company
(normally at a speed between 56 kbps and 1.544 Mbps). For customers within
the major cities where the public frame relay operator's nodes are situated, thisis not a major problem since the cost of the access line is fairly small.
In Europe the only service providers to offer frame relay services during 1992
were BT, with extensions to their Global Network Services (GNS) managed
Packet Switched data network service, and the Finnish PTO, which installed a
limited frame relay network in Finland.
2-The Introduction of Frame Relay:
Frame Relay is a high-performance WAN protocol that operates at the
physical and data link layers of the OSI reference model. Frame Relay
originally was designed for use across Integrated Services Digital Network
(ISDN) interfaces. Today, it is used over a variety of other network interfaces
as well.
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Frame Relay is an example of a packet-switched technology. Packet-
switched networks enable end stations to dynamically share the network
medium and the available bandwidth. The following two techniques are used
in packet-switching technology:
• Variable-length packets
• Statistical multiplexing
Variable-length packets are used for more efficient and flexible data
transfers. These packets are switched between the various segments in the
network until the destination is reached.
Statistical multiplexing techniques control network access in a packet-
switched network. The advantage of this technique is that it accommodates
more flexibility and more efficient use of bandwidth. Most of today's popular
LANs, such as Ethernet and Token Ring, are packet-switched networks.
Frame Relay is a network access protocol similar in principle to X.25.
The main difference between Frame Relay and X.25 is data integrity (error
detection) and network error flow control (error correction).
X.25 does all of its data checking and correcting at the network level. Thatmeans the network devices correct the corrupt data or ask for the data to be
retransmitted. Thus, the cost of such checking and retransmission is network
delay.
Frame Relay performs error detection only, not error correction. It thereby
leaves the task of error correction to the protocols used by intelligent devices
at each end of the network. These intelligent devices will provide end-to-end
data integrity. As a result, since Frame Relay relies on the device at the end to
perform retransmission and error recovery, there is significantly less
processing required for the network and less overall delay.
Frame Relay is strictly a Layer 2 protocol suite, whereas X.25 providesservices at Layer 3 (the network layer) as well. This enables Frame Relay to
offer higher performance and greater transmission efficiency than X.25, and
makes Frame Relay suitable for current WAN applications, such as LAN
interconnection. See Table-1
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Network Characteristic Frame Relay X.25
Propagation Delay Low High
Error Correction
None, done by the
terminal equipment at
each end of the link
Node to Node
Protocol family HDLC HDLC
Good for interactive use? Yes
Barely acceptable. Rather
slow with one second or
more round trip delay.
Good for polling
protocols?
OK, sometimes, requires
"spoofing"Slow, even with spoofing
Good for LAN file
transfer Yes Slow
Good for voice?Good, standards
developing No
Ease of implementation Easy Difficult
Internationally, Frame Relay was standardized by the International
Telecommunication Union—Telecommunications Standards Section (ITU-T).
In the United States, Frame Relay is an American National Standards Institute
(ANSI) standard.
3-Frame Relay Devices
Devices attached to a Frame Relay WAN fall into the following two general
categories:
• Data terminal equipment (DTE)
• Data circuit-terminating equipment (DCE)
DTEs generally are considered to be terminating equipment for a specificnetwork and typically are located on the premises of a customer. In fact, they
may be owned by the customer. Examples of DTE devices are terminals,
personal computers, routers, and bridges.
DCEs are carrier-owned internetworking devices. The purpose of DCE
equipment is to provide clocking and switching services in a network, which
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are the devices that actually transmit data through the WAN. In most cases,
these are packet switches. Figure 2 shows the relationship between the two
categories of devices.
Figure 10-1 DCEs Generally Reside Within Carrier-Operated WANs
The connection between a DTE device and a DCE device consists of both a
physical layer component and a link layer component. The physical layer
component defines the mechanical, electrical, functional, and procedural
specifications for the connection between the devices. The link layer component defines the protocol that establishes the connection between the
DTE device, such as a router, and the DCE device, such as a switch.
4-Frame Relay Frame Formats:
To understand much of the functionality of Frame Relay, it is helpful to
understand the structure of the Frame Relay frame. Figure -3 depicts the basic
format of the Frame Relay frame, and Figure -4 illustrates the LMI version of the Frame Relay frame.
Flags indicate the beginning and end of the frame.
Three primary components make up the Frame Relay frame:
the header and address area, the user-data portion, and the frame check
sequence (FCS). The address area, which is 2 bytes in length, is comprised of
10 bits representing the actual circuit identifier and 6 bits of fields related to
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congestion management.
This identifier commonly is referred to as the data-link connection identifier
(DLCI). Each of these is discussed in the descriptions that follow.
Figure -3 Five Fields Comprise the Frame Relay Frame
Frame Relay normally modifies the HDLC header from a 1 byte address field
to a 2 byte address field, as seen above. You can have a 3 or 4 byte format as
well. In addition, there is no Control field!• Starting Delimiter Flag - 0x7E
• High order 6 bits of DLCI address
• Command/Response used by higher order applications for end-to-end
control
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• Extended Address bit which indicates whether this octet is the last one
in the header. 0 means more to follow and 1 means that this is the end.
• Low order 4 bits of DLCI address
• Forward Explicit Congestion Notification (FECN) bit• Backward Explicit Congestion Notification (FECN) bit
• Discard Eligibility (DE) bit
• Extended Address bit. In this case, this is a 1, but there can be more
address bits to follow to give 17 bits (3-byte address field) or 24 bits (4-
byte address field).
• Data - can be up to 16,000 octets, but the 16-bit FCS generally limits
the whole frame size to 4096 octets.• Frame Check Sequence (FCS)
• Ending Delimiter Flag - 0x7E
Valid 10 bit DLCI addresses are:
0Reserved for ANSI Annex D and CCITT Annex A link
management
1 - 15 Reserved
16 - 1007 Any PVC
1008 - 1018 Reserved
1019 - 1022 Reserved for LMI multicast
1023 Reserved for LMI link management
Additional address bytes allow for DLCI addresses greater than 1024,
however these are not common.
The following descriptions summarize the basic Frame Relay frame fields
illustrated in Figure -3.
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• Flags— Delimits the beginning and end of the frame. The value of this
field is always the same and is represented either as the hexadecimal
number 7E or as the binary number 01111110.
•
Address— Contains the following information:
• DLCI —The 10-bit DLCI is the essence of the Frame Relay header.
This value represents the virtual connection between the DTE device
and the switch. Each virtual connection that is multiplexed onto the
physical channel will be represented by a unique DLCI. The DLCI
values have local significance only, which means that they are unique
only to the physical channel on which they reside. Therefore, devices at
opposite ends of a connection can use different DLCI values to refer to
the same virtual connection.
• Extended Address (EA) —The EA is used to indicate whether the byte
in which the EA value is 1 is the last addressing field. If the value is 1,
then the current byte is determined to be the last DLCI octet. Although
current Frame Relay implementations all use a two-octet DLCI, this
capability does allow longer DLCIs to be used in the future. The eighth
bit of each byte of the Address field is used to indicate the EA.
• C/R —The C/R is the bit that follows the most significant DLCI byte in
the Address field. The C/R bit is not currently defined.
• Congestion Control —This consists of the 3 bits that control the Frame
Relay congestion-notification mechanisms. These are the FECN,
BECN, and DE bits, which are the last 3 bits in the Address field.
Forward-explicit congestion notification (FECN) is a single-bit
field that can be set to a value of 1 by a switch to indicate to an end
DTE device, such as a router, that congestion was experienced in the
direction of the frame transmission from source to destination. The
primary benefit of the use of the FECN and BECN fields is the
capability of higher-layer protocols to react intelligently to these
congestion indicators. Today, DECnet and OSI are the only higher-layer protocols that implement these capabilities.
Backward-explicit congestion notification (BECN) is a single-bit
field that, when set to a value of 1 by a switch, indicates that
congestion was experienced in the network in the direction opposite of
the frame transmission from source to destination.
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Discard eligibility (DE) is set by the DTE device, such as a router, to
indicate that the marked frame is of lesser importance relative to other
frames being transmitted. Frames that are marked as "discard eligible"
should be discarded before other frames in a congested network. This
allows for a basic prioritization mechanism in Frame Relay networks.
• Data —Contains encapsulated upper-layer data. Each frame in this
variable-length field includes a user data or payload field that will vary
in length up to 16,000 octets. This field serves to transport the higher-
layer protocol packet (PDU) through a Frame Relay network.
• Frame Check Sequence —Ensures the integrity of transmitted data.
This value is computed by the source device and verified by the
receiver to ensure integrity of transmission.
LMI Frame Format:
Frame Relay frames that conform to the LMI specifications consist of the
fields illustrated in Figure -4.
Figure -4 Nine Fields Comprise the Frame Relay That Conforms to the LMI
Format
The following descriptions summarize the fields illustrated in Figure -4.
• Flag —Delimits the beginning and end of the frame.
• LMI DLCI —Identifies the frame as an LMI frame instead of a basic
Frame Relay frame. The LMI-specific DLCI value defined in the LMI
consortium specification is DLCI = 1023.
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• Unnumbered Information Indicator —Sets the poll/final bit to zero.
• Protocol Discriminator —Always contains a value indicating that the
frame is an LMI frame.
• Call Reference —Always contains zeros. This field currently is not
used for any purpose.• Message Type—Labels the frame as one of the following message
types:
• Status-inquiry message —Allows a user device to inquire about the
status of the network.
• Status message —Responds to status-inquiry messages. Status
messages include keepalives and PVC status messages.
• Information Elements —Contains a variable number of individual
information elements (IEs). IEs consist of the following fields:
– IE Identifier —Uniquely identifies the IE. – IE Length —Indicates the length of the IE.
– Data —Consists of 1 or more bytes containing encapsulated
upper-layer data.
• Frame Check Sequence (FCS) —Ensures the integrity of transmitted
data.
5-Frame Relay Virtual Circuits:
Frame Relay provides connection-oriented data link layer communication.This means that a defined communication exists between each pair of devices
and that these connections are associated with a connection identifier. This
service is implemented by using a Frame Relay virtual circuit, which is a
logical connection created between two data terminal equipment (DTE)
devices across a Frame Relay packet-switched network (PSN).
Virtual circuits provide a bidirectional communication path from one DTE
device to another and are uniquely identified by a data-link connection
identifier (DLCI). A number of virtual circuits can be multiplexed into a
single physical circuit for transmission across the network. This capability
often can reduce the equipment and network complexity required to connect
multiple DTE devices.
A virtual circuit can pass through any number of intermediate DCE devices
(switches) located within the Frame Relay PSN.
Frame Relay virtual circuits fall into two categories: switched virtual circuits
(SVCs) and permanent virtual circuits (PVCs).
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Permanent Virtual Circuits
Permanent virtual circuits (PVCs) are permanently established connections
that are used for frequent and consistent data transfers between DTE devices
across the Frame Relay network. Communication across a PVC does not
require the call setup and termination states that are used with SVCs. PVCsalways operate in one of the following two operational states:
• Data transfer —Data is transmitted between the DTE devices over the
virtual circuit.
• Idle —The connection between DTE devices is active, but no data is
transferred. Unlike SVCs, PVCs will not be terminated under any
circumstances when in an idle state.
DTE devices can begin transferring data whenever they are ready because the
circuit is permanently established.
Data-Link Connection Identifier
Frame Relay virtual circuits are identified by data-link connection identifiers(DLCIs). DLCI values typically are assigned by the Frame Relay service
provider (for example, the telephone company).
Frame Relay DLCIs have local significance, which means that their values are
unique in the LAN, but not necessarily in the Frame Relay WAN.
Figure -5 illustrates how two different DTE devices can be assigned the same
DLCI value within one Frame Relay WAN.
Figure -5 A Single Frame Relay Virtual Circuit Can Be Assigned Different
DLCIs on Each End of a VC
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6-Congestion-Control Mechanisms:
Frame Relay reduces network overhead by implementing simple congestion-
notification mechanisms rather than explicit, per-virtual-circuit flow control.
Frame Relay typically is implemented on reliable network media, so data
integrity is not sacrificed because flow control can be left to higher-layer
protocols. Frame Relay implements two congestion-notification mechanisms:
• Forward-explicit congestion notification (FECN)
•
Backward-explicit congestion notification (BECN)
FECN and BECN each is controlled by a single bit contained in the Frame
Relay frame header. The Frame Relay frame header also contains a Discard
Eligibility (DE) bit, which is used to identify less important traffic that can be
dropped during periods of congestion.
The FECN bit is part of the Address field in the Frame Relay frame header.
The FECN mechanism is initiated when a DTE device sends Frame Relay
frames into the network. If the network is congested, DCE devices (switches)
set the value of the frames' FECN bit to 1. When the frames reach the
destination DTE device, the Address field (with the FECN bit set) indicatesthat the frame experienced congestion in the path from source to destination.
The DTE device can relay this information to a higher-layer protocol for
processing. Depending on the implementation, flow control may be initiated,
or the indication may be ignored.
The BECN bit is part of the Address field in the Frame Relay frame header.
DCE devices set the value of the BECN bit to 1 in frames traveling in the
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opposite direction of frames with their FECN bit set. This informs the
receiving DTE device that a particular path through the network is congested.
The DTE device then can relay this information to a higher-layer protocol for
processing. Depending on the implementation, flow-control may be initiated,
or the indication may be ignored.
Frame Relay Discard Eligibility
The Discard Eligibility (DE) bit is used to indicate that a frame has lower
importance than other frames. The DE bit is part of the Address field in the
Frame Relay frame header.
DTE devices can set the value of the DE bit of a frame to 1 to indicate that the
frame has lower importance than other frames. When the network becomes
congested, DCE devices will discard frames with the DE bit set before
discarding those that do not. This reduces the likelihood of critical data being
dropped by Frame Relay DCE devices during periods of congestion.
Frame Relay Error CheckingFrame Relay uses a common error-checking mechanism known as the cyclic
redundancy check (CRC). The CRC compares two calculated values to
determine whether errors occurred during the transmission from source to
destination. Frame Relay reduces network overhead by implementing error
checking rather than error correction. Frame Relay typically is implemented
on reliable network media, so data integrity is not sacrificed because error
correction can be left to higher-layer protocols running on top of Frame Relay.
Frame Relay Local Management Interface
The Local Management Interface (LMI) is a set of enhancements to the basic
Frame Relay specification. The LMI was developed in 1990 by Cisco
Systems, StrataCom, Northern Telecom, and Digital Equipment Corporation.
It offers a number of features (called extensions) for managing complex
internetworks. Key Frame Relay LMI extensions include global addressing,
virtual circuit status messages, and multicasting.
The LMI global addressing extension gives Frame Relay data-link connection
identifier (DLCI) values global rather than local significance. DLCI values
become DTE addresses that are unique in the Frame Relay WAN. The global
addressing extension adds functionality and manageability to Frame Relay
internetworks. Individual network interfaces and the end nodes attached tothem, for example, can be identified by using standard address-resolution and
discovery techniques. In addition, the entire Frame Relay network appears to
be a typical LAN to routers on its periphery.
LMI virtual circuit status messages provide communication and
synchronization between Frame Relay DTE and DCE devices. These
messages are used to periodically report on the status of PVCs, which
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prevents data from being sent into black holes (that is, over PVCs that no
longer exist).
The LMI multicasting extension allows multicast groups to be assigned.
Multicasting saves bandwidth by allowing routing updates and address-
resolution messages to be sent only to specific groups of routers. The
extension also transmits reports on the status of multicast groups in update
messages.
7-Frame Relay Traffic Flow:
The Local Access Rate is the clock speed of the port and is the rate that data
traffic travels in and out of the port.
The financial costs for Frame Relay are based on Access Line speeds, Linking
up the line and the Committed Information Rate (CIR).
The CIR is based on the expected volume of the traffic and can never exceed
the line speed. An example is a customer buying a 9.6K CIR on a 64K access
line. The customer will be guaranteed 9.6K speed but could burst up to 64K if
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the need arises, for which he may be charged or frames may be dropped. A
customer could go for a zero CIR which would mean that all traffic would be
marked as Discard Eligible (DE), so that in a period of congestion, these
frames would more likely be dropped. The CIR is calculated as an average
rate over a period of time called the Committed Rate Measurement Interval
(Tc) and given by the formula CIR = Bc/Tc.
The Committed Burst (Bc) is maximum number of bits that a switch is set to
transfer over any Tc. The formula Bc = Tc x CIR demonstrates that the higher
the Bc to CIR ratio is the longer the switch can handle a burst.
The Excess Burst (Be) is the maximum number of uncommitted bits over and
above the CIR, that the switch will try to forward.
The Excessive Information Rate (EIR) is the average rate over which bits
will be marked with DE and is given by the formula EIR = Be/Tc. If you do
not know Tc, then because Tc = Bc/CIR, you can substitute Tc into the EIR
formula to give EIR = CIR * Be/Bc.
The Peak Rate is CIR + EIR.
See Congestion in fig –6 below:
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The Forward Explicit Congestion Notification (FECN) notifies
downstream (destination) nodes of congestion when set to '1'; whereas
Backward Explicit Congestion Notification (BECN) notifies upstream
(source) nodes of congestion when set to '1'. Statistical Time Division
Multiplexing is used to provide a measure of this congestion. Discard
Eligibility (DE), when set to '1', indicates non-priority traffic and is therefore
eligible for discard in congested periods. Any traffic that goes above Bc is
marked as DE.
As an aside, when measuring bandwidth you need to realise that you are
measuring packets going into a buffer rather than packets going through a
physical link so as a percentage you may sometimes see values over 100%.
In a IP environment TCP is used to provide reliable transport of data. The
TCP window size slowly increases in size until a packet is dropped, then the
window size rapidly shrinks before slowly growing again dynamically. This is
often called Slow Start. In the diagram above, switch A gets congested due totraffic from all users that are supplied by the carrier, not just the client
illustrated. It is a good idea to configure the edge router to listen to the FECNs
and BECNs before the Frame Relay switches decide to drop packets, some of
which may be yours. If your packets are dropped then the IP traffic will go
through low Start and this adds to the congestion problem if Slow Start occurs
continually.
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Random Early Detect (RED) is a facility that picks on individual users at
different times and drops their packets so forcing them to go through slow
start at different times. This helps to spread out congestion and give control of
when packets are dropped back to the client.
Frame Relay Traffic Shaping (FRTS)
Using FECNs, BECNs, the CIR and the DE bit enables the Frame Relay
network to make intelligent decisions as to what to do when congestion occurs
in the network. Edge devices can set the DE bit when there is congestion so
that high precedence traffic does not get dropped, otherwise all traffic within a
PVC is treated the same by the FR switch.
You can shape outgoing traffic on a per VC basis and set up queues such that
different traffic types can be treated appropriately on the same VC.
As we saw earlier the CIR is the average data sent per Time measurement
interval ((Tc). Within that time interval the data rate could exceed the average
bit rate provided that the average is maintained over the time interval. The
committed burst size (Bc) is the maximum number of bits that is allowed
within the time interval. Consider the following scenario where a 128 kbps
line has a CIR of 64 kbps. If the committed burst size is 8000 bits then the
time measurement interval is given by Bc / CIR i.e. 8000 / 64000 = 0.125s.
See fig –7 diagram below:
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This diagram identifies the 125ms time intervals over a period of 1 second. In
any one time interval up to 8000 bits can be sent before the CIR is exceeded
(indicated as black) and the token bucket is empty. At the beginning of the
next time period, the token bucket is full again allowing up to 8000 more bits
to be sent. So at the line rate of 128kbps, 8000bits are sent after 62.5ms. The
above illustrates that the CIR is being fully utilised over the 1 second period.
Sometimes, less than 8000 bits will be sent in one time interval. In this case,
the remaining number of bits + Be can be sent in another time interval. If you
fully utilise the time interval i.e. send 8000 in 62.5ms, then you have to wait
until the next time interval before you can send any more data. This can
amount to a delay of 62.5ms in the worst case, which could be a problem for
delay sensitive traffic.
BECNs can be examined by FRTS and on a per VC basis, traffic can be
throttled back by temporarily buffering outbound packets.
Another diagram in fig –8 below:
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In the example we are using here, the router sends traffic between 48 Kbps
and 32 Kbps depending on congestion in the network. Networks may mark
frames above Bc with DE but have plenty of spare capacity to transport the
frame. The reverse is also possible: they can have limited capacity, yet discard
excessive frames immediately. Networks may mark frames above Bc + Be
with DE, and possibly transport it, or just drop the frames as suggested by the
International Telecommunication Union Telecommunication StandardizationSector specification ITU-T I.370. Traffic shaping throttles the traffic based on
backward-explicit congestion notification (BECN) tagged packets from the
switch network. If you receive 50 percent BECN, the router decreases the
traffic by one eighth of the current transmitted bandwidth for that particular
DLCI.
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Note: The access rate is the physical line speed of the interface connecting to
the Frame Relay. The guaranteed rate is the committed information rate (CIR)
the Telco has given for the PVC.
Link Management Interface (LMI)
Data Link Control Management Interface (DLCMI) software on the router
checks for Access Line Integrity, new or deleted DLCIs (PVCs) and status of
current DLCIs. There are three types of DLCMIs; ANSI T1 617D, Link
Management Interface (LMI) and CCITT Annex A. The DTE and DCE
need to be configured with the same one and therefore only needs to be the
same locally between the router and the switch.
You can configure a router as a partial Frame Relay switch for testing
purposes (hence the options, LMI switch, Annex D switch and Annex A
switch), using modem eliminators or X.21/V.35 crossover cables will enable
you to simulate a Frame Relay switch on one router connecting to a normally
configures router. Also, 'DLCMI none' could be chosen if you wished to
statically configure all the PVCs. With Rev 1 LMI the switch (DCE)
dynamically sends DLCI information and addressing to the DTE (router) and
we do not have to set up the DLCI numbers.
By default the router sends a Full Status Message every 6th poll and theswitch responds with a Full Status Response. These polling intervals must be
the same on the router as on the switch otherwise a no response to poll could
result in the line being brought down. DLCI 1023 is the LMI control DLCI at
the switch.
The A bit in the PVC status field of an LMI response is cleared when a
particular PVC becomes inactive, the local switch can then inform other
switches that this PVC is down.
If you have a Cisco-based Frame Relay network, then you have the capability
of utilising Enhanced LMI which is used by routers to request QoS
information from Frame Relay switches. This QoS information can then be
used for FRTS and ensures that you do not have mismatches in configuration
between the router and the switch.
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8-Frame Relay Topologies:
In a Fully meshed environment, every router has a PVC defined to every
other router and in a Non-fully meshed environment (or Hub and Spoke)
PVCs are only defined between routers that need to communicate.
Figure -9: Non-fully meshed topology.
Figure -10: Fully meshed topology.
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There are three access modes:
• Group Access: All PVCs on a particular interface are treated as one
network, and each DLCI number can be thought of as a MAC address
of a host on a LAN. IP addresses are assigned to the interface and
configuration applies to the interface and therefore all PVCs associated
with that interface.
• Direct Access: Each PVC is an individual network and the DLCI
number can be thought of as a MAC address of the host at the other end
of the WAN.
• Hybrid Access: For PDUs that need to be routed all PVCs are set in
Group Access mode whilst for bridged PDUs, each PVC is treated as a
separate network.
Address resolution occurs by mapping an IP address to a DLCI number, andthere are three options:
• ARP: Where the network address is known but the DLCI is not.
The ARP packet is sent to all active DLCIs and the network
address is mapped to the local DLCI through which the reply was
received.
• INARP: Inverse ARP is where the IP address of the host at the
other end of the PVC is unknown, so an INARP packet is sent to
all new DLCIs.
•
ARPINARP:This is a combination of both ARP and INARP.
In a fully meshed network Poison Reverse, Split Horizon and Actual can be
used to configure how route propagation can occur throughout the WAN. In a
non-fully meshed environment (Hub and Spoke) there is a requirement for the
hub to have its interfaces configured with Actual route propagation since the
spokes, although they may be on the same sub-network, will not be able to
directly talk to one another. The hub router will maintain a routing table with
the spoke IP addresses, however, any LANs sitting behind the spokes will be
advertised as unreachable (with Poison Reverse) or not advertised at all (with
Split Horizon) since the routes will come in on the same interface that the
destination packets, for these LANs, would need to be sent out on. So, the hub
router should have its route propagation set to Actual or static routes (with the
next hop address set as the hub router's interface) set for each spoke on the
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particular subnet concerned, or perhaps the best alternative, is to turn RIP
supply off on the hub router and enable the RIP interface parameter Generate
Default.
There is another problem in a hub to spoke environment. A ping from Spoke 1
to Spoke 2 on the same subnet will fail because Spoke 1 would look into its
routing table and see that Spoke 2 is on the same subnet. An ARP is sent from
Spoke 1 to Spoke 2 in order to resolve the IP address to a MAC address,
however the ARP goes to the Hub which drops it since it is not meant for it,
the IP address of Spoke 2 is never resolved and the ping cannot be sent. To
sort this out, adjacent hosts must be configured for each spoke within each
particular subnet. This creates static ARP table entries since you are assigning
a MAC address to the Frame Relay IP address of each spoke, this MAC
address will be the local DLCI number.
With Direct Access Mode there are no routing problems since each PVC is
treated as a separate network and therefore are simple point to point links,
often used to be dedicated to specific protocols. If using RIP, then there is the
potential for wasting address space since only two IP addresses will be used
and the subnet mask is always fixed within the same network. Obviously this
problem does not exist with variable subnet masking such as OSPF or RIP-2.
In Hybrid Access Mode, routing protocols view the interface as if it wereconfigured for Group Access Mode whilst bridging protocols view the
interface as if it were configured for Direct Access Mode.
RFC 1293 describes INARP in detail. RFC 1294 describes Multiprotocols over Frame Relay. RFC 1490 describes IP over Frame Relay.
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9-Frame Relay Forum:
The Frame Relay Forum (http://www.frforum.com/) came together in 1991
to promote the use of Frame Relay and to develop common ways of
implementing it. The forum is made up of many manufacturers anddevelopers. The Frame Relay Forum implementation agreements are listed
below:
FRF.1.1 - User-to-Network (UNI)
• FRF.2.1 - Network-to-Network (NNI)
• FRF.3.1 - Data Encapsulation
• FRF.3.2 - Multiprotocol Encapsulation Implementation (MEI)
• FRF.4.1 - UNI Switched Virtual Circuit (SVC)
• FRF.5 - Frame Relay/ATM PVC Network Interworking.
•
FRF.6 - Frame Relay Service Customer Network Management (MIB)• FRF.7 - Frame Relay PVC Multicast Service and Protocol Description
• FRF.8 - Frame Relay/ATM Service Interworking
• FRF.8.1 - Frame Relay/ATM PVC Service Interworking
• FRF.9 - Data Compression over Frame Relay
• FRF.10 - Frame Relay Network-to-Network interface SVC
• FRF.11 - Voice over Frame Relay. Defines multiplexed data, voice,
fax, DTMF digit-relay and CAS/Robbed-bit signaling frame formats.
Annex C is Cisco's derivative for fragmentation.
• FRF.12 - Frame Relay Fragmentation allows long data frames to be
fragmented into smaller frames and interleaved with real-time frames.Real-time voice and non real-time data frames can be carried together
on lower speed links without causing delay to the real-time traffic.
• FRF.13 - Service Level Definitions
• FRF.14 - Physical Layer Interface
• FRF.15 - End-to-End Multilink Frame Relay
• FRF.16 - Mulltilink Frame Relay UNI/NNI
• FRF.17 - Frame Relay Privacy
• FRF.18 - Network-to-Network FR/ATM SVC Service Interworking
10-Frame Relay/ATM InterworkingMany of today's wide area networks are based upon the Frame Relay protocol.
It is very efficient for the transport of data orientated traffic and is scaleable
from low speeds to reasonably high speeds. Today, there is a growing demand
to be able to inter-operate between Frame Relay and ATM and standards have
been produced to that end.
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• FRF.5: Network Interworking
• FRF.8: Service Interworking
• Mapping of ATM to Frame Relay and vice versa - VPI & VCI/DLCI,
CLP/DE, EFCI/FECN, Traffic parameters etc...
FRF.5 Frame Relay/ATM PVC Network Interworking
A single PVC can support voice and data from a Frame Relay DTE through to
an ATM UNI, however you cannot use FRF.12 if FRF.5 is operating on a
PVC. If you require FRF.12 then you need a separate PVC. Implementation of
FRF.5 introduces large delays within the internetworking switch which make
it less suitable for delay sensitive traffic such as voice.
FRF.8 Frame Relay/ATM Service Interworking
There are two modes for FRF.8:
• Translation Mode - this uses RFC 1490 (IP over Frame Relay) on the
Frame Relay side and RFC 1483 (multiprotocol encapsulation over
ATM) on the ATM side. This works well for data but not voice.
• Transparent Mode - the ATM side receives the payload untranslatedand works fine for voice but not for data.