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Cisco Frame Relay Configurations

Chapter Description

Jonathan Chin dicussess basic Frame Relay operations on Cisco routers in a router-based Frame Relay network. He also explains how to configure a basic frame relay network involving Cisco equipment and how to perform basic monitoring and troubleshooting using relevant Cisco IOS show and debug commands.

Using Frame Relay Point-to-Point Subinterfaces

On Frame Relay networks, a single VC is always provisioned for a point-to-point connection. The same VC originates at a local end and then terminates at the remote end. A subnet address is usually assigned to each point-to-point connection. Therefore, only one DLCI can be configured per point-to-point subinterface. In this example, the local referenced DLCI of the VC at hub router R3 and spoke router R4 are 304 and 403, respectively. The subnet address 192.168.1.0/30 is allocated to this point-to-point network. Typically, a 30-bit subnet mask is used for point-to-point connection to preserve address space.

On point-to-point subinterfaces, the destination is identified and configured with the frame-relay interface-dlci command beginning in interface configuration mode. When configured on a point-to-point subinterface, the command associates the selected point-to-point subinterface with a DLCI. The command also allows users to select the type of Frame Relay encapsulation to be used on the specific VC. The command can be executed without specifying the Frame Relay encapsulation type to be used. By default, the Cisco Frame Relay encapsulation type will be used. Example 4-23 shows the configuration command and the corresponding output of the show frame-relay map.

Example 4-23 Example of frame-relay interface-dlci Command and the Output of show frame-relay map

R4(config)#interface s1/2.403 point-to-point
R4(config-subif)#frame-relay interface-dlci ?
 <16-1007> Define a switched or locally terminated DLCI

R4(config-subif)#frame-relay interface-dlci 403 ?
 cisco   Use CISCO Encapsulation
 ietf   Use RFC1490/RFC2427 Encapsulation
 ppp    Use RFC1973 Encapsulation to support PPP over FR
 protocol Optional protocol information for remote end
<cr>
R4#show frame-relay map
Serial1/2.403 (up): point-to-point dlci, dlci 403(0xC9,0x3090), broadcast
     status defined, active
R4#

As shown in the show frame-relay map output in Example 4-23, the DLCI configured on point-to-point subinterface serial1/0.403 is 403. Note that the created subinterface number mirrors the DLCI of the referenced VC. Generally, when creating a Frame Relay subinterface, it is good practice to assign a Frame Relay subinterface number that mirrors the DLCI value of the Frame Relay PVC assigned to that subinterface.

On point-to-point subinterfaces, you do not need to use the frame-relay map command to perform static address mapping, because it is always assumed that the end point of the point-to-point connection automatically resides on the same subnet as the start point. It is also not required to enable or disable Inverse ARP, because there is only a single remote destination on a point-to-point PVC and discovery is not necessary.

Using Frame Relay Multipoint Subinterfaces

On a Cisco router, by default, physical interfaces are multipoint interfaces. Frame Relay multipoint subinterfaces created on Cisco routers behave very much like the physical interfaces.

When a multipoint subinterface is created under a physical interface, it is necessary to specifically assign DLCIs to the multipoint subinterface. By default, the Cisco IOS software allocates all unassigned DLCIs advertised by the Frame Relay switch to the physical interface on the router. On a multipoint subinterface, the frame-relay interface-dlci dlci command or the frame-relay map protocol protocol-address dlci [broadcast] command in the subinterface configuration mode can be used to associate the multipoint subinterface with specific DLCIs. The frame-relay interface-dlci dlci command performs dynamic address mapping using Inverse ARP to map the next-hop protocol address to the local DLCI on the router. For instance, in the configuration examples in Example 4-22, DLCIs 301, 302, and 304 are automatically associated with the physical interface serial 3/1 on the hub router R3. The frame-relay interface-dlci 301 and frame-relay interface-dlci 302 commands are configured on multipoint subinterface s3/1.301 to specifically assign both DLCIs to the multipoint subinterface. The same command is used to associate DLCI 304 with point-to-point subinterface s3/1.304. Unlike point-to-point subinterfaces, the frame-relay interface-dlci command can be configured multiple times to associate more than one DLCI to a multipoint subinterface.

Similarly, the frame-relay map protocol protocol-address dlci [broadcast] command can be used to specifically assign DLCIs to the multipoint subinterfaces. The optional broadcast keyword in the frame-relay map command is required if broadcast and multicast traffic are to be sent over the specified dlci. Without the broadcast option, dynamic routing protocols such as EIGRP, OSPF and RIPv2 would not be able to advertise multicast route updates over the specified dlci. In comparison with the frame-relay interface-dlci command, the frame-relay map command performs static addressing mapping and it disables Inverse ARP on the specified dlci. In addition, the frame-relay map command is supported on the physical interface and the frame-relay map command should be used when the far end Frame Relay device does not support Inverse ARP.

NOTE

When a multipoint subinterface is created on a physical interface, the DLCIs of virtual circuits are always assigned to the physical interface until they are specifically assigned to the subinterfaces using the frame-relay interface-dlci dlci command or the frame-relay map protocol protocol-address dlci [broadcast] command.

On multipoint subinterfaces, either dynamic or static mapping can be used, depending on the network configurations.

In the hub-and-spoke network exemplified in Figure 4-3, the hub router R3 uses dynamic mapping via inverse ARP to map the next hop protocol addresses 172.16.1.1/29 and 172.16.1.2/29 of routers R1 and R2 to DLCI 301 and 302, respectively. On the spoke router R1, dynamic Inverse ARP mapping is used to map the next hop protocol address 172.16.1.3/29 at router R3 to local DLCI 301. However, because the spoke routers R1 and R2 do not have direct connectivity with each other on the partially meshed network, static mapping must be used between them. For each additional spoke router added to the 172.16.1.0/29 subnet on the hub-and-spoke network in Figure 4-3, an additional frame-relay map command must be supplied on each spoke router to statically map the next hop protocol address to the local DLCI.

The following example verifies the configurations of the routers in the hub-and-spoke network depicted in Figure 4-3. Example 4-24 shows the output of the show frame-relay map command on the hub router R3 and the results of several connectivity checks via the ping command.

Example 4-24 Verifying the Network in Figure 4-3

R3#show frame-relay map
Serial3/1.301 (up): ip 172.16.1.1 dlci 301(0x67,0x1870), dynamic,
       broadcast,, status defined, active
Serial3/1.302 (up): ip 172.16.1.2 dlci 302(0x68,0x1880), dynamic,
       broadcast,, status defined, active
Serial3/1.304 (up): point-to-point dlci, dlci 304(0x66,0x1860), broadcast
     status defined, active
R3#
R3#ping 172.16.1.1 

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 56/57/60 ms
R3#ping 172.16.1.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.1.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 56/56/60 ms
R3#ping 192.168.1.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 56/57/60 ms
R3#

R1#show frame-relay map
Serial4/2 (up): ip 172.16.1.2 dlci 103(0x191,0x6410), static,
       broadcast,
       CISCO, status defined, active
Serial4/2 (up): ip 172.16.1.3 dlci 103(0x191,0x6410), dynamic,
broadcast,, status defined, active
R1#ping 172.16.1.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.1.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 112/115/120 ms
R1#ping 172.16.1.3

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.1.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 56/59/64 ms

R2#show frame-relay map
Serial3/0 (up): ip 172.16.1.1 dlci 203(0x12D,0x48D0), static,
       broadcast,
       CISCO, status defined, active
Serial3/0 (up): ip 172.16.1.3 dlci 203(0x12D,0x48D0), dynamic,
broadcast,, status defined, active
R2#ping 172.16.1.1 

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 112/115/124 ms
R2#ping 172.16.1.3

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.1.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 56/57/60 ms
R2#

R4#show frame-relay map
Serial1/2.403 (up): point-to-point dlci, dlci 403(0xC9,0x3090), broadcast
     status defined, active
R4#ping 192.168.1.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 56/57/60 ms
R4#
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