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Frame Relay is a high-performance WAN protocol that was standardized by the ITU-T and
is widely used in the United States. This section describes Frame Relay operation,
configuration, and troubleshooting.
Understanding Frame Relay
Frame Relay is a connection-oriented data-link technology that is streamlined to provide
high performance and efficiency. For error protection, it relies on upper-layer protocols and
dependable fiber and digital networks.
Frame Relay defines the interconnection process between the router and the local access
switching equipment of the service provider. It does not define how the data is transmitted
within the Frame Relay service provider cloud. Figure 8-21 shows that Frame Relay
operates between the router and the frame relay switch.
Figure 8-21 Frame Relay
Devices attached to a Frame Relay WAN fall into the following two categories:
■ Data terminal equipment (DTE): Generally considered to be the terminating
equipment for a specific network. DTE devices are typically located on the customer
premises and can be owned by the customer. Examples of DTE devices are Frame
Relay Access Devices (FRAD), routers, and bridges.
■ Data communications equipment (DCE): Carrier-owned internetworking devices.
The purpose of DCE devices is to provide clocking and switching services in a network
and to transmit data through the WAN. In most cases, the switches in a WAN are Frame
Relay switches.
DCE or Frame
Relay Switch
Frame Relay works here.
326 Chapter 8: Extending the Network into the WAN
Frame Relay provides a means for statistically multiplexing many logical data
conversations, referred to as virtual circuits (VC), over a single physical transmission link
by assigning connection identifiers to each pair of DTE devices. The service provider
switching equipment constructs a switching table that maps the connection identifier 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 prior to the transmission of the first frame. Figure 8-22 illustrates
a Frame Relay connection and identifies the many components within Frame Relay.
Figure 8-22 Frame Relay Components
The following terms are used frequently in Frame Relay discussions and can be the same
or slightly different from the terms your Frame Relay service provider uses:
■ Local access rate: Clock speed (port speed) of the connection (local loop) to the
Frame Relay cloud. The local access rate is the rate at which data travels into or out of
the network, regardless of other settings.
■ Virtual circuit (VC): Logical circuit, uniquely identified by a data-link connection
identifier (DLCI), that is created to ensure bidirectional communication from one DTE
device to another. A number of VCs can be multiplexed into a single physical circuit
for transmission across the network. This capability can often reduce the complexity
PVC
PVC
Router B
Router A
Router C
DLCI: 200
DLCI: 100
DLCI: 500
DLCI: 400
Local Access
Local Access Loop = 64 Kbps
Loop = T1
Local Access
Loop = 64 Kbps
LMI
100 = Active
400 = Active
Establishing a WAN Connection with Frame Relay 327
of the equipment and network that are required to connect multiple DTE devices. A VC
can pass through any number of intermediate DCE devices (Frame Relay switches). A
VC can be either a permanent virtual circuit (PVC) or a switched virtual circuit (SVC).
■ Permanent virtual circuit (PVC): Provides 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 call teardown that is used with an SVC.
■ Switched virtual circuit (SVC): Provides temporary connections that are used in
situations that require only sporadic data transfer between DTE devices across the
Frame Relay network. SVCs are dynamically established on demand and are torn
down when transmission is complete.
■ Data-link connection identifier (DLCI): Contains a 10-bit number in the address
field of the Frame Relay frame header that identifies the VC. DLCIs have local
significance because the identifier references the point between the local router and the
local Frame Relay switch to which the DLCI is connected. Therefore, devices at
opposite ends of a connection can use different DLCI values to refer to the same virtual
connection.
■ Committed information rate (CIR): Specifies the maximum average data rate that
the network undertakes to deliver under normal conditions. When subscribing to a
Frame Relay service, you specify the local access rate, for example, 56 kbps or T1.
Typically, you are also asked to specify a CIR for each DLCI. If you send information
faster than the CIR on a given DLCI, the network flags some frames with a discard
eligible (DE) bit. The network does its best to deliver all packets but discards any DE
packets first if congestion occurs. Many inexpensive Frame Relay services are based
on a CIR of 0. A CIR of 0 means that every frame is a DE frame, and the network
throws any frame away when it needs to. The DE bit is within the address field of the
Frame Relay frame header.
■ Inverse Address Resolution Protocol (ARP): A method of dynamically associating
the network layer address of the remote router with a local DLCI. Inverse ARP allows
a router to automatically discover the network address of the remote DTE device that
is associated with a VC.
■ Local Management Interface (LMI): A signaling standard between the router (DTE
device) and the local Frame Relay switch (DCE device) that is responsible for managing
the connection and maintaining status between the router and the Frame Relay switch.
NOTE With ANSI T1.617 and ITU-T Q.933 (Layer 3) and Q.922 (Layer 2), Frame
Relay now supports SVCs. Cisco IOS Release 11.2 or later supports Frame Relay
SVCs. This book does not cover information on configuring Frame Relay SVCs.
328 Chapter 8: Extending the Network into the WAN
■ Forward explicit congestion notification (FECN): A bit in the address field of 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 (Frame
Relay switches) set the FECN bit value of the frames to 1. When these frames reach the
destination DTE device, the address field with the FECN bit set indicates that these
frames 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 might be initiated or the indication might be ignored.
■ Backward explicit congestion notification (BECN): A bit in the address field of the
Frame Relay frame header. DCE devices set the value of the BECN bit to 1 in frames
that travel in the opposite direction of frames that have their FECN bit set. Setting
BECN bits to 1 informs the receiving DTE device that a particular path through the
network is congested. The DTE device can then relay this information to a higher-layer
protocol for processing. Depending on the implementation, flow control might be
initiated or the indication might be ignored.
Example: Frame Relay Terminology—DLCI
As shown in Figure 8-22, Router A has two virtual circuits that are configured on one
physical interface. A DLCI of 100 identifies the VC that connects to Router B. A DLCI of
400 identifies the VC that connects to Router C. At the other end, a different DLCI number
can be used to identify the VC.
Frame Relay allows you to interconnect your remote sites in a variety of topologies.
Figure 8-23 illustrates these topologies.
Figure 8-23 Frame Relay Topologies
Full-Mesh
Partial-Mesh
Star (Hub-and-Spoke)
Establishing a WAN Connection with Frame Relay 329
Each topology is further described as follows:
■ Partial-mesh topology: Not all sites have direct access to all other sites. Depending
on the traffic patterns in your network, you might want to have additional PVCs
connect to remote sites that have large data traffic requirements.
■ Full-mesh topology: All routers have VCs to all other destinations. Full-mesh
topology, although costly, provides direct connections from each site to all other sites
and allows redundancy. When one link goes down, a router can reroute traffic through
another site. As the number of nodes in this topology increases, a full-mesh topology
can become very expensive. Use the n (n – 1) / 2 formula to calculate the total number
of links that are required to implement a full-mesh topology, where n is the number of
nodes. For example, to fully mesh a network of 10 nodes, 45 links are required—10
(10 – 1) / 2.
■ Star topology: Remote sites are connected to a central site that generally provides a
service or an application. The star topology, also known as a hub-and-spoke
configuration, is the most popular Frame Relay network topology. This is the least
expensive topology because it requires the least number of PVCs. In the figure, the
central router provides a multipoint connection because it typically uses a single
interface to interconnect multiple PVCs.
By default, a Frame Relay network provides nonbroadcast multiaccess (NBMA)
connectivity between remote sites. An NBMA environment is treated like other broadcast
media environments, such as Ethernet, where all the routers are on the same subnet.
However, to reduce cost, NBMA clouds are usually built in a hub-and-spoke topology. With
a hub-and-spoke topology, the physical topology does not provide the multiaccess
capabilities that Ethernet does, so each router might not have separate PVCs to reach the
other remote routers on the same subnet. Split horizon is one of the main issues you
encounter when Frame Relay is running multiple PVCs over a single interface.
In any Frame Relay topology, when a single interface must be used to interconnect multiple
sites, you can have reachability issues because of the NBMA nature of Frame Relay. The
Frame Relay NBMA topology can cause the following two problems:
■ Routing update reachability: Split horizon updates reduce routing loops by
preventing a routing update that is received on an interface from being forwarded out
the same interface. In a scenario using a hub-and-spoke Frame Relay topology, a
remote router (a spoke router) sends an update to the headquarters router (the hub
router) that is connecting multiple PVCs over a single physical interface. The
headquarters router then receives the broadcast on its physical interface but cannot
330 Chapter 8: Extending the Network into the WAN
forward that routing update through the same interface to other remote (spoke) routers.
Split horizon is not a problem if a single PVC exists on a physical interface because
this type of connection would be more of a point-to-point connection type.
■ Broadcast replication: With routers that support multipoint connections over a single
interface that terminate many PVCs, the router must replicate broadcast packets,
such as routing update broadcasts, on each PVC to the remote routers. These replicated
broadcast packets consume bandwidth and cause significant latency variations in
user traffic.
The following methods exist to solve the routing update reachability issue:
■ To solve the reachability issues brought on by split horizon, turn off split horizon.
However, two problems exist with this solution. First, although most network layer
protocols, such as IP, do allow you to disable split horizon, not all network layer
protocols allow you to do this. Second, disabling split horizon increases the chances of
routing loops in your network.
■ Use a fully meshed topology; however, this topology increases the cost.
■ Use subinterfaces. To enable the forwarding of broadcast routing updates in a hub-andspoke
Frame Relay topology, you can configure the hub router with logically assigned
interfaces called subinterfaces, which are logical subdivisions of a physical interface.
In split horizon routing environments, routing updates that are received on one
subinterface can be sent out another subinterface. In subinterface configuration, each
VC can be configured as a point-to-point connection, which allows each subinterface
to act like a leased line. When you use a Frame Relay point-to-point subinterface, each
subinterface is on its own subnet.
Figure 8-24 shows how to resolve the issues using subinterfaces.
Figure 8-24 Using Subinterfaces with Frame Relay
S0.1 10.1.1.1/24
S0
Logical Interface Physical
Interface
Subnet A
10.1.1.2/24
10.2.2.2/24
10.3.3.4/24
Subnet B
S0.2 10.2.2.1/24
S0.3 10.3.3.1/24
Establishing a WAN Connection with Frame Relay 331
A Frame Relay connection requires that, on a VC, the local DLCI be mapped to a
destination network layer address, such as an IP address. Routers can automatically
discover their local DLCI from the local Frame Relay switch using the LMI
protocol.
On Cisco routers, the local DLCI can be dynamically mapped to the remote router network
layer addresses with Inverse ARP. Inverse ARP associates a given DLCI to the next-hop
protocol address for a specific connection. Inverse ARP is described in RFC 1293.
Example: Frame Relay Address Mapping
As shown in Figure 8-25, using Inverse ARP, the router on the left can automatically
discover the remote router IP address and then map it to the local DLCI. In this case, the
local DLCI of 500 is mapped to the 10.1.1.1 IP address. Therefore, when the router must
send data to 10.1.1.1, it uses DLCI 500.
Figure 8-25 Frame Relay Address Mapping
Instead of using Inverse ARP to automatically map the local DLCIs to the remote router
network layer addresses, you can manually configure a static Frame Relay map in the
map table.
Frame Relay signaling is required between the router and the Frame Relay switch.
Figure 8-26 shows how the signaling is used to get information about the different DLCIs.
IP (10.1.1.1)
PVC
10.1.1.1
Inverse ARP or
Frame Relay Map
Frame Relay DLCI (500)
CSU/DSU
DLCI: 500
332 Chapter 8: Extending the Network into the WAN
Figure 8-26 Frame Relay Signaling
The LMI is a signaling standard between the router and the Frame Relay switch. The LMI
is responsible for managing the connection and maintaining the status between the devices.
Although the LMI is configurable, beginning in Cisco IOS Release 11.2, the Cisco router
tries to autosense which LMI type the Frame Relay switch is using. The router sends one
or more full LMI status requests to the Frame Relay switch. The Frame Relay switch
responds with one or more LMI types, and the router configures itself with the last LMI
type received. Cisco routers support the following three LMI types:
■ Cisco: LMI type defined jointly by Cisco, StrataCom, Northern Telecom (Nortel), and
Digital Equipment Corporation
■ ANSI: ANSI T1.617 Annex D
■ Q.933A: ITU-T Q.933 Annex A
You can also manually configure the appropriate LMI type from the three supported types
to ensure proper Frame Relay operation.
When the router receives LMI information, it updates its VC status to one of the following
three states:
■ Active: Indicates that the VC connection is active and that routers can exchange data
over the Frame Relay network.
■ Inactive: Indicates that the local connection to the Frame Relay switch is working, but
the remote router connection to the remote Frame Relay switch is not working.
■ Deleted: Indicates that either no LMI is being received from the Frame Relay switch
or that no service exists between the router and local Frame Relay switch.
PVC
CSU/DSU
DLCI: 500
DLCI: 400
10.1.1.1
PVC
Keepalive
LMI
500 = Active
400 = Inactive
Establishing a WAN Connection with Frame Relay 333
The following is a summary of how Inverse ARP and LMI signaling work with a Frame
Relay connection:
1. Each router connects to the Frame Relay switch through a channel service unit/data
service unit (CSU/DSU).
2. When Frame Relay is configured on an interface, the router sends an LMI status
inquiry message to the Frame Relay switch. The message notifies the switch of the
router status and asks the switch for the connection status of the router VCs.
3. When the Frame Relay switch receives the request, it responds with an LMI status
message that includes the local DLCIs of the PVCs to the remote routers to which the
local router can send data.
4. For each active DLCI, each router sends an Inverse ARP packet to introduce itself.
Figure 8-27 illustrates the first four steps of this process.
Figure 8-27 Stages of Inverse ARP and LMI Operation
5. When a router receives an Inverse ARP message, it creates a map entry in its Frame Relay
map table that includes the local DLCI and the remote router network layer address.
Note that the router DLCI is the local DLCI, not the DLCI that the remote router is using.
Any of the three connection states can appear in the Frame Relay map table.
6. Every 60 seconds, routers send Inverse ARP messages on all active DLCIs. Every 10
seconds, the router exchanges LMI information with the switch (keepalives).
NOTE If Inverse ARP is not working or the remote router does not support Inverse
ARP, you must manually configure static Frame Relay maps, which map the local
DLCIs to the remote network layer addresses.
1
DLCI = 100
Frame Relay
Cloud
172.168.5.5
DLCI = 400
172.168.5.7
Status Inquiry
Local DLCI 100 = Active
Status Inquiry
Local DLCI 400 = Active
Hello, I am 172.168.5.5.
2
4
3
2
3
334 Chapter 8: Extending the Network into the WAN
7. The router changes the status of each DLCI to active, inactive, or deleted, based on the
LMI response from the Frame Relay switch.
Figure 8-28 illustrates Steps 5–7 of this process.
Figure 8-28 Stages of Inverse ARP and LMI Operation Continued
Configuring Frame Relay
A basic Frame Relay configuration assumes that you want to configure Frame Relay on one
or more physical interfaces and that the routers support LMI and Inverse ARP.
The following steps are used to configure basic Frame Relay:
Step 1 Select the interface needed for Frame Relay. Use the interface configuration mode.
RouterX(config)# interface serial1
After the interface configuration is entered, the command-line interface (CLI)
prompt changes from (config)# to (config-if)#.
Step 2 Configure a network layer address, for example, an IP address.
RouterX(config-if)# ip address 10.16.0.1 255.255.255.0
Step 3 Select the Frame Relay encapsulation type that is used to encapsulate
end-to-end data traffic. Use the encapsulation frame-relay interface
configuration command.
RouterX(config-if)# encapsulation frame-relay [cisco | ietf]
DLCI = 100
Frame Relay
Cloud
172.168.5.5
DLCI = 400
172.168.5.7
Hello, I am 172.168.5.5.
Keepalives
Hello, I am 172.168.5.7.
6
4
5
7
Frame Relay Map
172.168.5.5 DLCI 400 Active
5
Frame Relay Map
172.168.5.7 DLCI 100 Active
Keepalives
7
Establishing a WAN Connection with Frame Relay 335
The option cisco means that Cisco encapsulation is being used. Use this option if
you are connecting to another Cisco router. You do not need to enter the keyword
cisco because it is the default encapsulation. The option ietf sets the encapsulation
method to comply with the Internet Engineering Task Force (IETF) standard (RFC
2427). Select this option if you are connecting to a router from another vendor.
Step 4 Establish LMI connection using the frame-relay lmi-type interface
configuration command.
RouterX(config-if)# frame-relay lmi-type {ansi | cisco | q933a}
This command is needed only if you are using Cisco IOS Software Release 11.1
or earlier. With Cisco IOS Software Release 11.2 or later, the LMI type is
autosensed and no configuration is needed. The option cisco is the default. The
LMI type is set on a per-interface basis and is shown in the output of the show
interfaces EXEC command.
Step 5 Configure the bandwidth for the link using the bandwidth [kilobits]
interface configuration command. For example:
RouterX(config-if)# bandwidth 64
This command affects routing operations performed by protocols such as
Enhanced Interior Gateway Routing Protocol (EIGRP) and Open Shortest Path
First (OSPF), as well as other calculations.
Step 6 Enable Inverse ARP if it was disabled on the router. Use the frame-relay
inverse-arp [protocol] [dlci] interface configuration command.
The protocol parameter indicates the protocol in use. Supported protocols include
IP, Internetwork Packet Exchange (IPX), AppleTalk, DECnet, Banyan Virtual
Integrated Network Service (VINES), and Xerox Network Services (XNS). The
dlci parameter indicates the DLCI on the local interface with which you want to
exchange Inverse ARP messages. Inverse ARP is on by default and does not
appear in the configuration output.
Consider the following configuration, where IP is the protocol, and the DLCI is 16:
RouterX(config-if)# frame-relay inverse-arp ip 16
When the remote router does not support Inverse ARP, the Frame Relay peers have different
Frame Relay encapsulation types. Or when you want to control broadcast and multicast
traffic over the PVC, you must statically map the local DLCI to the remote router network
layer address. These static Frame Relay map entries are referred to as static maps.
336 Chapter 8: Extending the Network into the WAN
Use the following command to statically map the remote network layer address to the local
DLCI:
RouterX(config-if)# frame-relay map protocol protocol-address dlci [broadcast]
[ietf | cisco | payload-compress packet-by-packet]
Table 8-2 details the parameters for this command.
You can configure subinterfaces in one of the following two modes:
■ Point-to-point: A single point-to-point subinterface is used to establish one PVC
connection to another physical interface or subinterface on a remote router. In this case,
each pair of the point-to-point routers is on its own subnet, and each point-to-point
subinterface has a single DLCI. In a point-to-point environment, because each
subinterface acts like a point-to-point interface, update traffic is not subject to the split
horizon rule.
■ Multipoint: A single multipoint subinterface is used to establish multiple PVC
connections to multiple physical interfaces or subinterfaces on remote routers. In this
case, all the participating interfaces are in the same subnet. In this environment,
because the subinterface acts like a regular NBMA Frame Relay interface, update
traffic is subject to the split horizon rule.
Example: Configuring Frame Relay Point-to-Point Subinterfaces
In Figure 8-29, Router A has two point-to-point subinterfaces. The s0.110 subinterface
connects to Router B, and the s0.120 subinterface connects to Router C. Each subinterface
is on a different subnet.
Table 8-2 frame-relay map Command Parameters
Parameter Description
protocol Defines the supported protocol, bridging, or logical link control. The
choices include AppleTalk, DECnet, Data-Link Switching (DLSW),
IP, IPX, Logical Link Control, type 2 (LLC2), remote source-route
bridging (RSRB), Banyan VINES, and XNS.
protocol-address Defines the network layer address of the destination router interface.
dlci Defines the local DLCI that is used to connect to the remote protocol
address.
broadcast (Optional) Allows broadcasts and multicasts over the VC. This
permits the use of dynamic routing protocols over the VC.
ietf | cisco Enables IETF or Cisco encapsulations.
payload-compress
packet-by-packet
(Optional) Enables packet-by-packet payload compression, using the
Stacker method. This is a Cisco-proprietary compression method.
Establishing a WAN Connection with Frame Relay 337
Figure 8-29 Point-to-Point Subinterfaces
Follow these steps to configure subinterfaces on a physical interface:
Step 1 Select the interface upon which you want to create subinterfaces and enter
interface configuration mode.
Step 2 Remove any network layer address that is assigned to the physical
interface and assign the network layer address to the subinterface.
Step 3 Configure Frame Relay encapsulation.
Step 4 Use the following command to select the subinterface you want to
configure and to designate it as a point-to-point subinterface:
RouterX(config-if)# interface serial number.subinterface-number pointto-
point}
Table 8-3 describes the options for this command.
You are required to enter either the multipoint or point-to-point parameter; no default is available.
Table 8-3 interface serial Command Parameters
Parameter Description
.subinterface-number Subinterface number in the range 1–4,294,967,293. The
interface number that precedes the period (.) must match the
physical interface number to which this subinterface belongs.
point-to-point Select this option if you want each pair of point-to-point
routers to have its own subnet.
A
10.17.0.1
S0.110
S0.120
10.18.0.1
DLCI = 110
DLCI = 120
10.17.0.2
10.18.0.2
B
C
interface Serial0
no ip address
encapsulation frame-relay
!
interface Serial0.110 point-to-point
ip address 10.17.0.1 255.255.255.0
bandwidth 64
frame-relay interface-dlci 110
!
interface Serial0.120 point-to-point
ip address 10.18.0.1 255.255.255.0
bandwidth 64
frame-relay interface-dlci 120
!
338 Chapter 8: Extending the Network into the WAN
Step 5 If you configured the subinterface as the point-to-point subinterface, you
must configure the local DLCI for the subinterface to distinguish it from
the physical interface. The command to configure the local DLCI on the
subinterface follows:
RouterX(config-subif)# frame-relay interface-dlci dlci-number
The dlci-number parameter defines the local DLCI number that is being linked to
the subinterface. No other methods exist to link an LMI-derived DLCI to a
subinterface because the LMI does not know about subinterfaces.
Do not use the frame-relay interface-dlci command on physical interfaces.
Example: Configuring Frame Relay Multipoint Subinterfaces
In Figure 8-30, all the routers are on the 10.17.0.0/24 subnet. Router A is configured with
a multipoint subinterface with three PVCs. The PVC with DLCI 120 is used to connect to
Router B, the PVC with DLCI 130 is used to connect to Router C, and the PVC with DLCI
140 is used to connect to Router D.
Figure 8-30 Frame Relay Multipoint Subinterface
Split horizon is disabled by default on Frame Relay multipoint main interfaces and enabled
by default on Frame Relay multipoint subinterfaces. In the figure, which uses a multipoint
NOTE If you defined a subinterface for point-to-point communication, you cannot
reassign the same subinterface number to use for multipoint communication without first
rebooting the router. Instead, use a different subinterface number.
S2.1 = 10.17.0.2/24
DLCI = 130
DLCI = 140
DLCI = 120
S2.1 = 10.17.0.3/24
S2.1 = 10.17.0.4/24
B
C
D
S2.2 = 10.17.0.1/24
A
interface Serial2
no ip address
encapsulation frame-relay
!
interface Serial2.2 multipoint
ip address 10.17.0.1 255.255.255.0
bandwidth 64
frame-relay map ip 10.17.0.2 120 broadcast
frame-relay map ip 10.17.0.3 130 broadcast
frame-relay map ip 10.17.0.4 140 broadcast
no ip split-horizon
Establishing a WAN Connection with Frame Relay 339
subinterface, split horizon must be manually disabled at Router A to overcome the split
horizon issue.
Follow these steps to configure subinterfaces on a physical interface:
Step 1 Select the interface upon which you want to create subinterfaces and enter
interface configuration mode.
Step 2 Remove any network layer address, like the IP address, assigned to the
physical interface and assign the network layer address to the
subinterface.
Step 3 Configure Frame Relay encapsulation.
Step 4 Use the following command to select the subinterface you want to
configure and to designate it as a multipoint subinterface:
RouterX(config-if)# interface serial number.subinterface-number
multipoint
Table 8-4 describes the options for this command.
You are required to enter either the multipoint or point-to-point parameter; no default is available.
Step 5 If you have configured the subinterface as multipoint and Inverse ARP is
enabled, you must configure the local DLCI for the subinterface to
distinguish it from the physical interface. This configuration is not
required for multipoint subinterfaces that are configured with static route
maps. The command to configure the local DLCI on the subinterface
follows:
RouterX(config-subif)# frame-relay interface-dlci dlci-number
The dlci-number parameter defines the local DLCI number that is being linked to
the subinterface. No other methods exist to link an LMI-derived DLCI to a
subinterface because the LMI does not know about subinterfaces.
Do not use the frame-relay interface-dlci command on physical interfaces.
Table 8-4 interface serial Command Parameters
Parameter Description
.subinterface-number Subinterface number in the range 1–4,294,967,293. The
interface number that precedes the period (.) must match
the physical interface number to which this subinterface
belongs.
multipoint Select this option if you want all routers in the same
subnet.
340 Chapter 8: Extending the Network into the WAN
Verifying Frame Relay
The show interfaces command displays information regarding the encapsulation and
Layer 1 and Layer 2 status. Verify that the encapsulation is set to Frame Relay.
The command also displays information about the LMI type and the LMI DLCI. The LMI
DLCI is not the DLCI that identifies the PVC across which data is passed. That DLCI is
shown in the show frame-relay pvc command.
The output also displays the Frame Relay DTE or DCE type. Normally, the router will be
the DTE. However, a Cisco router can be configured as the Frame Relay switch; in this case,
the type will be DCE. Example 8-3 shows the output from this command.
Use the show frame-relay lmi command to display LMI traffic statistics. For example, this
command shows the number of status messages exchanged between the local router and the
local Frame Relay switch. Example 8-4 shows the output of this command.
NOTE If you defined a subinterface for point-to-point communication, you cannot
reassign the same subinterface number to use for multipoint communication without first
rebooting the router. Instead, use a different subinterface number.
Example 8-3 Verify Frame Relay Information with the show interfaces Command
RouterX# show interfaces s0
Serial0 is up, line protocol is up
Hardware is HD64570
Internet address is 10.140.1.2/24
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 1/255
Encapsulation FRAME-RELAY, loopback not set, keepalive set (10 sec)
LMI enq sent 19, LMI stat recvd 20, LMI upd recvd 0, DTE LMI up
LMI enq recvd 0, LMI stat sent 0, LMI upd sent 0
LMI DLCI 1023 LMI type is CISCO frame relay DTE
FR SVC disabled, LAPF state down
Broadcast queue 0/64, broadcasts sent/dropped 8/0, interface broadcasts 5
Last input 00:00:02, output 00:00:02, output hang never
Last clearing of “show interface” counters never
Queueing strategy: fifo
Output queue 0/40, 0 drops; input queue 0/75, 0 drops
<Output omitted>
Example 8-4 Displaying LMI Traffic Statistics with the show frame-relay lmi
Command
RouterX# show frame-relay lmi
LMI Statistics for interface Serial0 (Frame Relay DTE) LMI TYPE = CISCO
Invalid Unnumbered info 0 Invalid Prot Disc 0
Establishing a WAN Connection with Frame Relay 341
Table 8-5 describes a few of the fields in the show frame-relay lmi display.
Use the debug frame-relay lmi command to determine whether the router and the Frame
Relay switch are sending and receiving LMI packets properly. Example 8-5 shows the
output associated with this command.
Invalid dummy Call Ref 0 Invalid Msg Type 0
Invalid Status Message 0 Invalid Lock Shift 0
Invalid Information ID 0 Invalid Report IE Len 0
Invalid Report Request 0 Invalid Keep IE Len 0
Num Status Enq. Sent 113100 Num Status msgs Rcvd
Table 8-5 show frame-relay lmi Fields
Field Description
LMI Type Signaling or LMI specification; options are Cisco, ANSI, or ITU-T
Num Status Enq. Sent Number of LMI status inquiry messages sent
Num Status Msgs
Rcvd
Number of LMI status messages received
Example 8-5 Confirming LMI Packet Traffic Delivery/Receipt with the debug frame-relay
lmi Command
RouterX# debug frame-relay lmi
Frame Relay LMI debugging is on
Displaying all Frame Relay LMI data
RouterX#
1w2d: Serial0(out): StEnq, myseq 140, yourseen 139, DTE up
1w2d: datagramstart = 0xE008EC, datagramsize = 13
1w2d: FR encap = 0xFCF10309
1w2d: 00 75 01 01 01 03 02 8C 8B
1w2d:
1w2d: Serial0(in): Status, myseq 140
1w2d: RT IE 1, length 1, type 1
1w2d: KA IE 3, length 2, yourseq 140, myseq 140
1w2d: Serial0(out): StEnq, myseq 141, yourseen 140, DTE up
1w2d: datagramstart = 0xE008EC, datagramsize = 13
1w2d: FR encap = 0xFCF10309
1w2d: 00 75 01 01 01 03 02 8D 8C
1w2d:
1w2d: Serial0(in): Status, myseq 142
1w2d: RT IE 1, length 1, type 0
1w2d: KA IE 3, length 2, yourseq 142, myseq 142
1w2d: PVC IE 0x7 , length 0x6 , dlci 100, status 0x2 , bw 0
Example 8-4 Displaying LMI Traffic Statistics with the show frame-relay lmi
Command (Continued)
342 Chapter 8: Extending the Network into the WAN
The first four lines describe an LMI exchange. The first line describes the LMI request that
the router has sent to the Frame Relay switch. The second line describes the LMI reply
that the router has received from the Frame Relay switch. The third and fourth lines
describe the response to this request from the switch. This LMI exchange is followed by
two similar LMI exchanges. The last six lines consist of a full LMI status message that
includes a description of the two PVCs of the router.
Table 8-6 describes the significant fields shown in Example 8-5.
Table 8-6 debug frame-relay lmi Output Fields
Field Description
Serial0(out) Indicates that the LMI request was sent out on interface Serial 0
StEnq The command mode of message, which can be one of the following:
StEnq: Status inquiry
Status: Status reply
myseq 140 Myseq counter, which maps to the CURRENT SEQ counter of the router
yourseen 139 Yourseen counter, which maps to the LAST RCVD SEQ counter of the
switch
DTE up State of the line protocol (up or down) for the DTE (user) port
RT IE 1 Value of the report type (RT) information element (IE)
length 1 Length of the report type information element in bytes
type 1 Report type is RT IE
KA IE 3 Value of the keepalive information element
length 2 Length of the keepalive information element in bytes
yourseq 142 Yourseq counter, which maps to the CURRENT SEQ counter of the switch
myseq 142 Myseq counter, which maps to the CURRENT SEQ counter of the router
PVC IE 0x7 Value of the PVC information element type
length 0x6 Length of the PVC IE in bytes
dlci 100 DLCI value, in decimal, for this PVC
Establishing a WAN Connection with Frame Relay 343
The “(out)” output is an LMI status message that is sent by the router. The “(in)” output is
a message that is received from the Frame Relay switch.
The “type 0” output indicates a full LMI status message. The “type 1” output indicates an
LMI exchange.
The “dlci 100, status 0x2” output means that the status of DLCI 100 is active. The common
values of the DLCI status field are as follows:
■ 0x0: “Added” and “inactive” mean that the switch has this DLCI programmed, but for
some reason—for example, the other end of this PVC is down—it is not usable.
■ 0x2: “Added” and “active” mean that the Frame Relay switch has the DLCI, and
everything is operational. You can start sending traffic with this DLCI in the header.
■ 0x4: “Deleted” means that the Frame Relay switch does not have this DLCI
programmed for the router but that it was programmed at some point in the past. This
status could also happen because the DLCIs are reversed on the router or because the
PVC was deleted by the service provider in the Frame Relay cloud.
status 0x2 Status value; possible values are as follows:
0x00: Added/inactive
0x02: Added/active
0x04: Deleted
0x08: New/inactive
0x0a: New/active
bw 0 CIR for the DLCI
Table 8-6 debug frame-relay lmi Output Fields (Continued)
Field Description
344 Chapter 8: Extending the Network into the WAN
Use the show frame-relay pvc [interface interface] [dlci] command to display the status
of each configured PVC as well as traffic statistics. Example 8-6 shows the output of this
command.
Table 8-7 describes the fields of the show frame-relay pvc command display.
Example 8-6 Displaying PVC Status and Traffic Statistics with the show frame-relay pvc
Command
RouterX# show frame-relay pvc 100
PVC Statistics for interface Serial0 (Frame Relay DTE)
DLCI = 100, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial0
input pkts 28 output pkts 10 in bytes 8398
out bytes 1198 dropped pkts 0 in FECN pkts 0
in BECN pkts 0 out FECN pkts 0 out BECN pkts 0
in DE pkts 0 out DE pkts 0
out bcast pkts 10 out bcast bytes 1198
pvc create time 00:03:46, last time pvc status changed 00:03:47
Table 8-7 show frame-relay pvc Output Fields
Field Description
DLCI One of the DLCI numbers for the PVC.
DLCI USAGE Lists “SWITCHED” when the router or access server is used as a switch
or “LOCAL” when the router or access server is used as a DTE device.
PVC STATUS Status of the PVC. The DCE device reports the status, and the DTE
device receives the status. When you disable the LMI mechanism on the
interface by using the no keepalive command, the PVC status is
STATIC. Otherwise, the PVC status is exchanged using the LMI
protocol, as follows:
STATIC: LMI is disabled on the interface.
ACTIVE: The PVC is operational and can transmit packets.
INACTIVE: The PVC is configured but is down.
DELETED: The PVC is not present (DTE device only), which means
that no status is received from the LMI protocol.
Establishing a WAN Connection with Frame Relay 345
If the frame-relay end-to-end keepalive command is used, the end-toend
keepalive (EEK) status is reported in addition to the LMI status. Two
examples follow:
ACTIVE (EEK UP): The PVC is operational according to LMI and
end-to-end keepalives.
ACTIVE (EEK DOWN): The PVC is operational according to LMI, but
end-to-end keepalive has failed.
INTERFACE Specific subinterface that is associated with this DLCI.
LOCAL PVC
STATUS
Status of the PVC that is configured locally on the Network-to-Network
Interface (NNI).
NNI PVC
STATUS
Status of the PVC that is learned over the NNI link.
input pkts Number of packets that are received on this PVC.
output pkts Number of packets that are sent on this PVC.
in bytes Number of bytes that are received on this PVC.
out bytes Number of bytes that are sent on this PVC.
dropped pkts Number of incoming and outgoing packets that are dropped by the router
at the Frame Relay level.
in pkts dropped Number of incoming packets that have been dropped. Incoming packets
can be dropped for a number of reasons:
Inactive PVC
Policing
Packets received above DE discard level
Dropped fragments
Memory allocation failures
Configuration problems
out pkts
dropped
Number of outgoing packets that have been dropped, including shaping
drops and late drops.
out bytes
dropped
Number of outgoing bytes that have been dropped.
continues
Table 8-7 show frame-relay pvc Output Fields (Continued)
Field Description
346 Chapter 8: Extending the Network into the WAN
Use the show frame-relay map command to display the current map entries and
information about the connections. Example 8-7 shows the output of this command.
The following information explains the show frame-relay map output that appears in the
example:
■ The “100” output is the local DLCI number in decimal.
■ The “0x64” output is the hex conversion of the DLCI number (0x64 = 100 decimal).
■ The “0x1840” output is the value as it would appear “on the wire” because of the way
the DLCI bits are spread out in the address field of the Frame Relay frame.
late-dropped
out pkts
Number of outgoing packets that have been dropped because of a quality
of service (QoS) policy, such as VC queuing or Frame Relay traffic
shaping. This field is not displayed when the value is 0.
late-dropped
out bytes
Number of outgoing bytes dropped because of a QoS policy, such as VC
queuing or Frame Relay traffic shaping. This field is not displayed when
the value is 0.
in FECN pkts Number of packets that are received with the FECN bit set.
in BECN pkts Number of packets that are received with the BECN bit set.
out FECN pkts Number of packets that are sent with the FECN bit set.
out BECN pkts Number of packets that are sent with the BECN bit set.
in DE pkts Number of DE packets that have been received.
out DE pkts Number of DE packets that have been sent.
out bcast pkts Number of output broadcast packets.
out bcast bytes Number of output broadcast bytes.
Example 8-7 Displaying Frame Relay Map Entries and Connection Information with the show
frame-relay map Command
RouterX# show frame-relay map
Serial0 (up): ip 10.140.1.1 dlci 100(0x64,0x1840), dynamic,
broadcast,, status defined, active
RouterX# clear frame-relay-inarp
RouterX# show frame map
RouterX#
Table 8-7 show frame-relay pvc Output Fields (Continued)
Field Description
Troubleshooting Frame Relay WANs 347
■ The “10.140.1.1” output is the remote router IP address (a dynamic entry that is learned
through the Inverse ARP process).
■ Broadcast and multicast are enabled on the PVC because broadcast is stated in the third
line.
■ The PVC status is active.
To clear dynamically created Frame Relay maps, which are created using Inverse ARP, use
the clear frame-relay-inarp privileged EXEC command.
Summary of Establishing a WAN Connection with Frame Relay
The following summarizes the key points that were discussed in the previous sections:
■ Frame Relay PVCs are identified with DLCIs, and the status of the PVCs is reported
through the LMI protocol.
■ Frame Relay point-to-point subinterfaces require a separate subnet for each PVC, and
multipoint subinterfaces share a single subnet with Frame Relay peers.
■ To display connectivity with the Frame Relay provider, use the show frame-relay lmi
command. To display connectivity with the Frame Relay peer, use the show framerelay
pvc and show frame-relay map commands.
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