The first step in configuring Frame Relay is to select the interface and then enable the Frame
Relay encapsulation on the serial interface. You do this with the
encapsulation frame-relay
command. As you will notice in the following router configuration commands, there are two
options:
cisco
and
ietf
.
Router#
config t
Router(config)#interface serial 0
Router(config-if)#encapsulation frame-relay [cisco or ietf]
Cisco is the default encapsulation, which means that you have another Cisco router on the
remote end with which your router will communicate. You will use the IETF encapsulation
when communicating with a remote router that is not a Cisco device.
After you configure the encapsulation to the serial interface, you then need to add the Network
layer address, DLCI number, and LMI type. Cisco’s capability to autosense the LMI type has
greatly simplified configuration. The following router configuration shows the process of specifying
the IP address and DLCI number, but not the LMI type because it is automatically detected:
Router(config-if)#ip address 172.16.10.1 255.255.255.0
Router(config-if)#frame-relay interface-dlci 16
Router(config-if)#no shutdown
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Frame Relay Local Management Interface (LMI)
In 1990, the Group of Four developed extensions to the Frame Relay standard to help ease the
management and configuration burden. One of these extensions was the
Local Management
Interface (LMI)
. LMI provides for virtual circuit status messages and multicasting.
Cisco routers support three versions of the LMI standard: Cisco, ANSI, and ITU-T (Q.933a).
LMI autosense, the automatic detection of the LMI type, was introduced in IOS version 11.2. LMI
autosense determines the LMI type by rapidly trying each of them in order: ANSI, ITU-T (Q.933a),
and then Cisco. If it cannot determine the LMI type within 60 seconds, it will terminate the
autosense process and revert to the Cisco LMI type.
After LMI is established between the router and the switch, the next stage is DLCI determination
and IARP. The router will query the switch, asking what the DLCI(s) is/are for this
circuit. The router will configure itself with that DLCI(s) and query the switch to determine
the status of the circuit.
This query is the first stage of discovery. The query that is sent includes the local router’s network
information. The remote router will record the network information and reply in kind.
The local router will map the DLCI it learned from static or dynamic addressing to other network
addresses it discovered from queries.
When an IARP is made, the router updates its map table with one of three possible LMI connection
states:
Active
The connection is active, and the routers can exchange data through the PVC.
Inactive
The local connection to the Frame Relay switch is working, but the remote end of the
PVC is not communicating to the Frame Relay switch.
Deleted
No LMI keepalive information from the switch to the router is being received for this
PVC. This could be because no LMI is actually being exchanged or because the DLCI is not configured
on the ingress switch.
Figure 29.4 shows that the Chicago office PVC to Miami is deleted because the Miami office
is not receiving keepalives from the Frame Relay switch. Neither this inactive state nor this deleted
state affect the other connections (PVCs) that Chicago might have to other locations.
FIGURE 2 9 . 4
LMI connection states
PVC
DLCI = 16
LMI = inactive
Keepalive
PVC
DLCI = 17
LMI = deleted
No keepalive
management and configuration burden. One of these extensions was the
Local Management
Interface (LMI)
. LMI provides for virtual circuit status messages and multicasting.
Cisco routers support three versions of the LMI standard: Cisco, ANSI, and ITU-T (Q.933a).
LMI autosense, the automatic detection of the LMI type, was introduced in IOS version 11.2. LMI
autosense determines the LMI type by rapidly trying each of them in order: ANSI, ITU-T (Q.933a),
and then Cisco. If it cannot determine the LMI type within 60 seconds, it will terminate the
autosense process and revert to the Cisco LMI type.
After LMI is established between the router and the switch, the next stage is DLCI determination
and IARP. The router will query the switch, asking what the DLCI(s) is/are for this
circuit. The router will configure itself with that DLCI(s) and query the switch to determine
the status of the circuit.
This query is the first stage of discovery. The query that is sent includes the local router’s network
information. The remote router will record the network information and reply in kind.
The local router will map the DLCI it learned from static or dynamic addressing to other network
addresses it discovered from queries.
When an IARP is made, the router updates its map table with one of three possible LMI connection
states:
Active
The connection is active, and the routers can exchange data through the PVC.
Inactive
The local connection to the Frame Relay switch is working, but the remote end of the
PVC is not communicating to the Frame Relay switch.
Deleted
No LMI keepalive information from the switch to the router is being received for this
PVC. This could be because no LMI is actually being exchanged or because the DLCI is not configured
on the ingress switch.
Figure 29.4 shows that the Chicago office PVC to Miami is deleted because the Miami office
is not receiving keepalives from the Frame Relay switch. Neither this inactive state nor this deleted
state affect the other connections (PVCs) that Chicago might have to other locations.
FIGURE 2 9 . 4
LMI connection states
PVC
DLCI = 16
LMI = inactive
Keepalive
PVC
DLCI = 17
LMI = deleted
No keepalive
DCLI Mapping
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.
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.
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.
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