There needs to be a way to link the layer 2 identifiers (DLCI) to layer 3 (Network layer) addresses.
Mapping
provides a mechanism to link one or more network addresses to a DLCI. Remember that
Frame Relay works only at the Data Link layer (layer 2 of the OSI model) and does not understand
IP addressing. In fact, to communicate via IP (because it could just as easily be IPX and AppleTalk
instead of IP), you need to convert the destination IP address to a destination DLCI (PVC) number.
The frame switch uses only DLCI numbers to communicate, not IP addresses.
Mappings can be done either statically by an administrator or dynamically via the router. If
you are mapping a static Network layer address to a DLCI number, you use the
frame-relay
map
command. It is necessary to create static mappings when the remote router does not support
dynamic addressing or when you’re using OSPF in some network configurations. It’s also necessary
even if you want to control broadcasts over your Frame Relay network.
To understand how to use static mappings, look at Figure 29.3. Figure 29.3 shows a corporate
office in Chicago connected to two other sites—one in Miami and one in New York. The
IP address of the serial interface in New York is 172.16.1.2/24, and the IP address of the Miami
serial interface is 172.16.1.3/24. It’s important to note that the Miami location is a Cisco router,
and the New York location is a non-Cisco router. A static mapping would have to be used for
different Frame Relay encapsulation methods to run under the same physical serial interface,
unless all routers used an open encapsulation type for interoperability, resulting in the ability to
use dynamic mapping.
FIGURE 2 9 . 3
Configuring Frame Relay static mappings
The following router output shows an example of how you would create static Frame Relay
mappings on the Chicago router:
Router(config)#
interface serial 0
Router(config-if)#
ip address 172.16.1.1 255.255.255.248
Router(config-if)#
frame-relay map ip 172.16.1.2 20 broadcast ietf
Router(config-if)#
frame-relay map ip 172.16.1.3 16 broadcast
Router(config-if)#
exit
The
frame-relay map
command maps the IP address of the remote location to a specific
PVC or DLCI. The first map statement tells the Chicago router that if it has an IP packet with
Chicago
Miami
Cisco router
New York
Non-Cisco router
PVC
PVC
DLCI to NY = 20
DLCI to Miami = 16
Corporate Office
172.16.1.2/24
a destination IP address of 172.16.1.2, it should use PVC 20 to get there. Also, because the New
York office is not a Cisco router (can you imagine that?), it should use the standard Internet
Engineering Task Force (IETF) encapsulation method. We’ll talk about encapsulation methods
used with Frame Relay in a minute.
Because Miami is a Cisco router, no specification of encapsulation is necessary because Cisco is
the default encapsulation method. The broadcast parameter at the end of each line specifies that
broadcasts should be forwarded over the PVC because they are not forwarded by default. The
frame-relay map
command supports many Network layer protocols, including IP, Connectionless
Network Services (CLNS), Digital Equipment Corporation’s Networking architecture (DECnet),
Xerox Network Services (XNS), and Virtual Integrated Network Service (VINES).
Dynamic addressing is turned on by default. It automatically maps Network layer addresses
to DLCI addresses rather well
.
Inverse ARP (IARP)
is used to automatically map a DLCI to a
network address (IP, IPX, and so on) without any user configuration. It provides Network
layer-to-DCLI-number translation and creates an entry in the DLCI mapping table. This table
is used by the router to correctly route outgoing traffic. No map configuration is necessary for
IARP to work.
IT Certification CCIE,CCNP,CCIP,CCNA,CCSP,Cisco Network Optimization and Security Tips
Data Link Connection Identifier (DLCI)
Frame Relay provides statistical time division multiplexing (Stat-TDM). Time division multiplexing
(TDM) is like going to Disneyland. It’s true. Remember how you have to stand in line to get
into Space Mountain? Well, after you get to the loading area, you’re placed in a section with rails
that separate you from the other passengers. You can then get into only the one car that is in front
of you and only when it is empty. Think of the holding area as the interface buffers of a router;
the cars are the time slots on the circuit. When a time slot drives up, you can get in, but not before
that, and not if someone is already in that slot.
Now Stat-TDM is an improvement over straight TDM. Stat-TDM enables you to jump into
a different line if it is not in use and to get into any car. This is a first-come, first-served technology.
Stat-TDM is used with Frame Relay to allow multiple logical data connections (virtual
circuits) over a single physical link. Basically, these circuits give time slots to first-come, firstserved
and priority-based frames over the physical link. Going back to our analogy, you can
think of Frame Relay as the capability to send multiple cars through space on one train, each
car holding a different person.
So, how is each person (data) identified in the car (time slot)? How does the frame switch
know where to send each frame? The answer to this is a
data link connection identifier (DLCI)
.
Because Frame Relay is based on virtual circuits instead of physical ones, DLCIs are used to
identify a virtual circuit and tie it to a physical circuit. This means each frame can be identified
as it traverses the Frame Relay switch and is then sent to the routers at the remote ends.
DLCIs are considered only locally significant, which means that they see the entire virtual circuit
but only up to the point of the Frame Relay switch. The provider is responsible for assigning
DLCIs and their significance to the network.
DLCIs identify the logical virtual circuit between the customer premises equipment (CPE)
and the Frame Relay switch. The switch then maps the DLCIs between each pair of routers in
order to create the PVC. The Frame Relay switch keeps a mapping table of DLCI numbers to
outgoing ports; it uses this table to forward frames out ports on the switch. (More information
about mapping follows in the next section, “DLCI Mapping.”)
When configuring your Cisco router to participate in a Frame Relay network, you must configure
a DLCI number for each connection. The Frame Relay provider supplies the DLCI numbers
for your router. If a DLCI is not defined on the link, the switch will discard the frame.
Figure 29.2 shows an example of how DLCIs are assigned to offices in Chicago and Miami.
The Chicago office will communicate through the Frame Relay switch to Miami by using
DLCI 17. Miami will communicate to Chicago by using DLCI 16. Remember that the valid
range of DLCIs is from 16 to 991.
FIGURE 2 9 . 2
Frame Relay PVC configuration
PVC
DLCI = 16 PVC
DLCI = 17
Chicago
Miami
NOTE:
Some providers assign a DLCI in such a way that it appears that the DLCI is
globally significant. For example, all circuits that terminate in Miami could be
assigned the local DLCI 17 at each site. But remember that even though all of
these DLCIs have the same number, they are not the same because DLCIs are
typically only locally significant.
(TDM) is like going to Disneyland. It’s true. Remember how you have to stand in line to get
into Space Mountain? Well, after you get to the loading area, you’re placed in a section with rails
that separate you from the other passengers. You can then get into only the one car that is in front
of you and only when it is empty. Think of the holding area as the interface buffers of a router;
the cars are the time slots on the circuit. When a time slot drives up, you can get in, but not before
that, and not if someone is already in that slot.
Now Stat-TDM is an improvement over straight TDM. Stat-TDM enables you to jump into
a different line if it is not in use and to get into any car. This is a first-come, first-served technology.
Stat-TDM is used with Frame Relay to allow multiple logical data connections (virtual
circuits) over a single physical link. Basically, these circuits give time slots to first-come, firstserved
and priority-based frames over the physical link. Going back to our analogy, you can
think of Frame Relay as the capability to send multiple cars through space on one train, each
car holding a different person.
So, how is each person (data) identified in the car (time slot)? How does the frame switch
know where to send each frame? The answer to this is a
data link connection identifier (DLCI)
.
Because Frame Relay is based on virtual circuits instead of physical ones, DLCIs are used to
identify a virtual circuit and tie it to a physical circuit. This means each frame can be identified
as it traverses the Frame Relay switch and is then sent to the routers at the remote ends.
DLCIs are considered only locally significant, which means that they see the entire virtual circuit
but only up to the point of the Frame Relay switch. The provider is responsible for assigning
DLCIs and their significance to the network.
DLCIs identify the logical virtual circuit between the customer premises equipment (CPE)
and the Frame Relay switch. The switch then maps the DLCIs between each pair of routers in
order to create the PVC. The Frame Relay switch keeps a mapping table of DLCI numbers to
outgoing ports; it uses this table to forward frames out ports on the switch. (More information
about mapping follows in the next section, “DLCI Mapping.”)
When configuring your Cisco router to participate in a Frame Relay network, you must configure
a DLCI number for each connection. The Frame Relay provider supplies the DLCI numbers
for your router. If a DLCI is not defined on the link, the switch will discard the frame.
Figure 29.2 shows an example of how DLCIs are assigned to offices in Chicago and Miami.
The Chicago office will communicate through the Frame Relay switch to Miami by using
DLCI 17. Miami will communicate to Chicago by using DLCI 16. Remember that the valid
range of DLCIs is from 16 to 991.
FIGURE 2 9 . 2
Frame Relay PVC configuration
PVC
DLCI = 16 PVC
DLCI = 17
Chicago
Miami
NOTE:
Some providers assign a DLCI in such a way that it appears that the DLCI is
globally significant. For example, all circuits that terminate in Miami could be
assigned the local DLCI 17 at each site. But remember that even though all of
these DLCIs have the same number, they are not the same because DLCIs are
typically only locally significant.
Permanent Virtual Circuits
Permanent virtual circuits (PVCs)
are dedicated virtual paths through the Frame Relay network
that are up and running 100 percent of the time (well, at least in theory!). Unlike an SVC, a PVC
does not require the call establishment and call teardown phases. However, when the circuit initially
comes up, some parameter negotiations do pass over the wire; these communications
should occur only when the dedicated circuit goes down.
The two phases for PVCs are as follows:
Data exchange
Data is transmitted between two devices, and each device can transmit data as
needed because it doesn’t need to wait for a call to be established to do so. The data exchange
can happen at any time because the virtual connection is permanent and always available.
Idle
The connection is still active, but data is not being transmitted. The idle time can be indefinite:
the circuits will not time out. The idle time keeps the VC up and keeps the line from timing
out when no data is present. This is done by the transmission of idle frames, the sole purpose
of which is to keep line synchronization in the absence of data.
PVCs have gained in popularity as the price for dedicated lines has decreased. They are the
types of links that we will configure later in this chapter.
are dedicated virtual paths through the Frame Relay network
that are up and running 100 percent of the time (well, at least in theory!). Unlike an SVC, a PVC
does not require the call establishment and call teardown phases. However, when the circuit initially
comes up, some parameter negotiations do pass over the wire; these communications
should occur only when the dedicated circuit goes down.
The two phases for PVCs are as follows:
Data exchange
Data is transmitted between two devices, and each device can transmit data as
needed because it doesn’t need to wait for a call to be established to do so. The data exchange
can happen at any time because the virtual connection is permanent and always available.
Idle
The connection is still active, but data is not being transmitted. The idle time can be indefinite:
the circuits will not time out. The idle time keeps the VC up and keeps the line from timing
out when no data is present. This is done by the transmission of idle frames, the sole purpose
of which is to keep line synchronization in the absence of data.
PVCs have gained in popularity as the price for dedicated lines has decreased. They are the
types of links that we will configure later in this chapter.
Switched Virtual Circuits
Switched virtual circuits (SVCs)
provide an economical way to connect to a Frame Relay network.
An SVC is a type of circuit that is brought up only when there is data to send. These circuits provide
temporary connectivity to the network on an as-needed basis. Switched virtual circuits are used with
many technologies—for example, a standard telephone call.
It is rare to find a Frame Relay SVC connection, and indeed, you might never see one. Typically,
PVCs are the only connections used with Frame Relay, although Cisco routers do support
Frame Relay with SVCs.
NOTE: Because they are rarely used, SVCs are not covered further in this book and are
not on the Remote Access exam.
provide an economical way to connect to a Frame Relay network.
An SVC is a type of circuit that is brought up only when there is data to send. These circuits provide
temporary connectivity to the network on an as-needed basis. Switched virtual circuits are used with
many technologies—for example, a standard telephone call.
It is rare to find a Frame Relay SVC connection, and indeed, you might never see one. Typically,
PVCs are the only connections used with Frame Relay, although Cisco routers do support
Frame Relay with SVCs.
NOTE: Because they are rarely used, SVCs are not covered further in this book and are
not on the Remote Access exam.
Frame Relay Virtual Circuits
Frame Relay Virtual Circuits
As mentioned earlier, Frame Relay is a layer 2 (Data Link layer) connection-oriented protocol.
After a connection has been established, end devices can transmit data across the network. This
layer 2 connection across the packet-switched network is called a
virtual circuit
.
The end devices (in this case, routers) will act as
data terminal equipment (DTE), and the
Frame Relay switch will be the data communications equipment (DCE). The difference between
the two is that the DCE device will most likely be responsible for the clocking of the line and
also initiates LMI messages, which we will discuss later in this chapter.
NOTE:Two quick terms you will encounter later:
ingress
refers to Frame Relay frames
from an access device toward the Frame Relay network, and
egress
refers to
Frame Relay frames leaving a Frame Relay network toward the destination device.
From the point-of-view of the router, the virtual circuit is somewhat transparent. This means
that the router sees the virtual circuit, but only up to the Frame Relay switch, which is where
the term
locally significant
originates with regard to the DLCI. In other words, the router speaks
LMI and understands what a DLCI is. This isn’t all that transparent. The transparency comes
in when you consider that the router uses the DLCI like a MAC address for the remote router
to bind the virtual circuit to the protocol address. The previous statement, however, is not a
pure Frame Relay function. In pure Frame Relay terms, the router still has to know how to recognize
the existence of the ingress Frame Relay switch. Even though the circuit might traverse
many switches en route to its destination, the router simply sees its connection to the local
Frame Relay switch, which again is part of the VC.
Figure 29.1 shows how routers see the Frame Relay network. In the figure, notice that Frame
Relay is configured between the routers and the switching office.
FIGURE 2 9 . 1
Frame Relay operation
Frame actually traverses this
CO
PVC
Routers see this
Users only see this User Server
Hub or
switch
Hub or
switch
Router
DLCI 16
CSU/DSU Demarc Demarc CSU/DSU
Router
DLCI 17
There are two ways for Frame Relay to establish this connectivity. Either you can set up
a circuit that is enabled only when needed by using a switched virtual circuit, or you can set
up a dedicated circuit between the local router and remote router by using a permanent virtual
circuit (PVC). Each of these is discussed in more detail next.
As mentioned earlier, Frame Relay is a layer 2 (Data Link layer) connection-oriented protocol.
After a connection has been established, end devices can transmit data across the network. This
layer 2 connection across the packet-switched network is called a
virtual circuit
.
The end devices (in this case, routers) will act as
data terminal equipment (DTE), and the
Frame Relay switch will be the data communications equipment (DCE). The difference between
the two is that the DCE device will most likely be responsible for the clocking of the line and
also initiates LMI messages, which we will discuss later in this chapter.
NOTE:Two quick terms you will encounter later:
ingress
refers to Frame Relay frames
from an access device toward the Frame Relay network, and
egress
refers to
Frame Relay frames leaving a Frame Relay network toward the destination device.
From the point-of-view of the router, the virtual circuit is somewhat transparent. This means
that the router sees the virtual circuit, but only up to the Frame Relay switch, which is where
the term
locally significant
originates with regard to the DLCI. In other words, the router speaks
LMI and understands what a DLCI is. This isn’t all that transparent. The transparency comes
in when you consider that the router uses the DLCI like a MAC address for the remote router
to bind the virtual circuit to the protocol address. The previous statement, however, is not a
pure Frame Relay function. In pure Frame Relay terms, the router still has to know how to recognize
the existence of the ingress Frame Relay switch. Even though the circuit might traverse
many switches en route to its destination, the router simply sees its connection to the local
Frame Relay switch, which again is part of the VC.
Figure 29.1 shows how routers see the Frame Relay network. In the figure, notice that Frame
Relay is configured between the routers and the switching office.
FIGURE 2 9 . 1
Frame Relay operation
Frame actually traverses this
CO
PVC
Routers see this
Users only see this User Server
Hub or
switch
Hub or
switch
Router
DLCI 16
CSU/DSU Demarc Demarc CSU/DSU
Router
DLCI 17
There are two ways for Frame Relay to establish this connectivity. Either you can set up
a circuit that is enabled only when needed by using a switched virtual circuit, or you can set
up a dedicated circuit between the local router and remote router by using a permanent virtual
circuit (PVC). Each of these is discussed in more detail next.
A Brief History of Frame Relay
Currently, Frame Relay is the most prevalent type of packet switching used in North America;
however, Frame Relay’s origin is very humble. Initially, Frame Relay was not even a standard
unto itself; instead, it was an extension of the Integrated Services Digital Network (ISDN) standard.
The International Telecommunications Union-Telecommunications, or ITU-T, (formerly
known as the
Comité Consultatif International de Téléphonique et Télégraphique
, or CCITT)
was the first to define the Frame Relay standard.
Many companies that saw the value of this technology quickly adopted the ITU-T standard
for Frame Relay. After these companies showed interest, ITU-T and other organizations proceeded
to develop the standard, but very slowly. Several corporations saw a need for a more
rapid development and implementation of a Frame Relay standard. Four companies—Digital
Equipment Corporation (DEC), Northern Telecom (Nortel), Cisco, and StrataCom—bound
together to form the
Group of Four
. This group began developing Frame Relay technology
more quickly, which enabled Frame Relay to work on disparate devices. In September 1990, the
Group of Four published
Frame Relay Specifications with Extensions
. This group eventually
became what is currently known as the
Frame Relay Forum
.
however, Frame Relay’s origin is very humble. Initially, Frame Relay was not even a standard
unto itself; instead, it was an extension of the Integrated Services Digital Network (ISDN) standard.
The International Telecommunications Union-Telecommunications, or ITU-T, (formerly
known as the
Comité Consultatif International de Téléphonique et Télégraphique
, or CCITT)
was the first to define the Frame Relay standard.
Many companies that saw the value of this technology quickly adopted the ITU-T standard
for Frame Relay. After these companies showed interest, ITU-T and other organizations proceeded
to develop the standard, but very slowly. Several corporations saw a need for a more
rapid development and implementation of a Frame Relay standard. Four companies—Digital
Equipment Corporation (DEC), Northern Telecom (Nortel), Cisco, and StrataCom—bound
together to form the
Group of Four
. This group began developing Frame Relay technology
more quickly, which enabled Frame Relay to work on disparate devices. In September 1990, the
Group of Four published
Frame Relay Specifications with Extensions
. This group eventually
became what is currently known as the
Frame Relay Forum
.
Understanding Frame Relay
Before we dive right into Frame Relay we need to have a better understanding of what Frame
Relay is, how it is used, and how it came about.
What Is Frame Relay?
Frame Relay
is a telecommunications service designed for cost-efficient data transmission
across a WAN. Frame Relay puts data in a variable-size unit called a
frame
and leaves any
necessary error correction up to the end points. This provides for a high-speed, low-overhead,
efficient network.
Frame Relay is a layer 2 (Data Link layer) connection-oriented protocol that creates virtual circuits
(VCs)—usually permanent virtual circuits (PVCs)—between two end devices such as routers,
through a Frame Relay network. A Frame Relay
bearer service
was defined as a network service
within the framework of ISDN. It was designed to be more efficient and faster than X.25. The
major difference between Frame Relay and traditional ISDN is that in Frame Relay, the control
information needed to keep the link synchronized is not in a separate channel as it is in ISDN, but
instead is included with the data. This single stream of data provides for flow control, congestion
control, and frame routing. Frame Relay is a form of packet switching, whereas ISDN is still considered
circuit switching.
NOTE:You should understand that the error and congestion control works only at the
Data Link layer and that Frame Relay also relies on upper layer protocols and
applications for error correction.
Relay is, how it is used, and how it came about.
What Is Frame Relay?
Frame Relay
is a telecommunications service designed for cost-efficient data transmission
across a WAN. Frame Relay puts data in a variable-size unit called a
frame
and leaves any
necessary error correction up to the end points. This provides for a high-speed, low-overhead,
efficient network.
Frame Relay is a layer 2 (Data Link layer) connection-oriented protocol that creates virtual circuits
(VCs)—usually permanent virtual circuits (PVCs)—between two end devices such as routers,
through a Frame Relay network. A Frame Relay
bearer service
was defined as a network service
within the framework of ISDN. It was designed to be more efficient and faster than X.25. The
major difference between Frame Relay and traditional ISDN is that in Frame Relay, the control
information needed to keep the link synchronized is not in a separate channel as it is in ISDN, but
instead is included with the data. This single stream of data provides for flow control, congestion
control, and frame routing. Frame Relay is a form of packet switching, whereas ISDN is still considered
circuit switching.
NOTE:You should understand that the error and congestion control works only at the
Data Link layer and that Frame Relay also relies on upper layer protocols and
applications for error correction.
Frame Relay Cisco
THE CCNP EXAM TOPICS COVERED IN THIS
CHAPTER INCLUDE THE FOLLOWING:
Describe how different WAN technologies can be used to
provide remote access to a network, including asynchronous
dial-in, Frame Relay, ISDN, cable modem, and DSL.
Describe traffic control methods used to manage traffic flow
on WAN links.
Explain the operation of remote network access control
methods.
Identify PPP components, and explain the use of PPP as an
access and encapsulation method.
Configure Frame Relay operation and traffic control on
WAN links.
Design a Cisco Frame Relay infrastructure to provide access
between remote network components.
Troubleshoot nonfunctional remote access systems.
The use of
packet-switching
protocols has become the most
popular method for moving traffic across a wide area network
(WAN). One particular packet-switching protocol—Frame
Relay—has become the dominant player in the packet-switching market. Other methods of
passing data between routers across the WAN include dedicated lines, time division multiplexing
(TDM), ATM, ISDN, DSL, and others.
NOTE:
Because DSL is so much cheaper and faster than Frame Relay, it could eventually
replace Frame Relay and ISDN as the dominant player in the WAN markets.
As network sizes increase, you should pay particular attention to how DSL is
playing a larger role in network deployment. However, DSL presently has too
many distance limitations to completely replace Frame Relay anytime soon.
Understanding the theory and function of Frame Relay is important for numerous reasons.
Not only is it still tested on Cisco’s Remote Access exam, but when you get a Cisco-related job,
you will most likely see quite a few networks that depend on Frame Relay. Mastery of the information
covered in this chapter will enable you to gain an in-depth understanding of how and
why you would implement Frame Relay on your internetwork. This chapter goes over what
Frame Relay is, the components of Frame Relay, Frame Relay configuration, and how to verify
that Frame Relay is running properly.
CHAPTER INCLUDE THE FOLLOWING:
Describe how different WAN technologies can be used to
provide remote access to a network, including asynchronous
dial-in, Frame Relay, ISDN, cable modem, and DSL.
Describe traffic control methods used to manage traffic flow
on WAN links.
Explain the operation of remote network access control
methods.
Identify PPP components, and explain the use of PPP as an
access and encapsulation method.
Configure Frame Relay operation and traffic control on
WAN links.
Design a Cisco Frame Relay infrastructure to provide access
between remote network components.
Troubleshoot nonfunctional remote access systems.
The use of
packet-switching
protocols has become the most
popular method for moving traffic across a wide area network
(WAN). One particular packet-switching protocol—Frame
Relay—has become the dominant player in the packet-switching market. Other methods of
passing data between routers across the WAN include dedicated lines, time division multiplexing
(TDM), ATM, ISDN, DSL, and others.
NOTE:
Because DSL is so much cheaper and faster than Frame Relay, it could eventually
replace Frame Relay and ISDN as the dominant player in the WAN markets.
As network sizes increase, you should pay particular attention to how DSL is
playing a larger role in network deployment. However, DSL presently has too
many distance limitations to completely replace Frame Relay anytime soon.
Understanding the theory and function of Frame Relay is important for numerous reasons.
Not only is it still tested on Cisco’s Remote Access exam, but when you get a Cisco-related job,
you will most likely see quite a few networks that depend on Frame Relay. Mastery of the information
covered in this chapter will enable you to gain an in-depth understanding of how and
why you would implement Frame Relay on your internetwork. This chapter goes over what
Frame Relay is, the components of Frame Relay, Frame Relay configuration, and how to verify
that Frame Relay is running properly.
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