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Topic: Hybrid or Back-to-Back Frame Relay configuration

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Justin G. Mitchell
jgmitchell@gmail.com
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Hybrid Frame-relay connections are formed when there is no frame-relay switch between two routers using frame-relay encapsulation. They are connected using a direct serial connection.

R1 <-- serial cable --> R4 (hybrid)
vs.
R1 <-- serial --> FRS <-- serial --> R4 (traditional)

When we configure hybrid frame-relay we have to be careful to examine our network diagrams and the requirements of the question/scenario because we have two options when configuring hybrid FR. The decision is ALWAYS based on the requirements.

Option 1 - Configure Frame-switching on 1 side
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Using this option, there should be some indication of which side should be the DCE or have frame-relay switching enabled either in the diagram or written out in the requirements.

Example: Configure the interface connected to R4 as the DCE using frame-relay encapsulation.

Or in the diagram, one side should be labeled as the DCE.


Each side of the link should be configured with the same interface-dlci command since there is no actual FRS to switch the traffic.


Sample configurations and verification commands:
! frame-relay switching is enabled on R1
R1#sh run | i switching
frame-relay switching
R1#sh run int s0/1
Building configuration...

Current configuration : 173 bytes
!
interface Serial0/1
 ip address 172.31.14.1 255.255.255.240
 encapsulation frame-relay
 clock rate 2000000
 frame-relay interface-dlci 888
 frame-relay intf-type dce
end

R1#sh fram map
Serial0/0 (up): ip 172.31.12.2 dlci 102(0x66,0x1860), dynamic,
              broadcast,, status defined, active
Serial0/0 (up): ip 172.31.13.3 dlci 103(0x67,0x1870), dynamic,
              broadcast,, status defined, active
Serial0/1 (up): ip 172.31.14.4 dlci 888(0x378,0xDC80), dynamic,
              broadcast,, status defined, active
R1#sh fram pvc | i STAT
DLCI = 102, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial0/0
DLCI = 103, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial0/0
DLCI = 104, DLCI USAGE = UNUSED, PVC STATUS = INACTIVE, INTERFACE = Serial0/0
DLCI = 888, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial0/1


R4#sh run | i switching
R4#sh run int s0/1
Building configuration...

Current configuration : 146 bytes
!
interface Serial0/1
 ip address 172.31.14.4 255.255.255.240
 encapsulation frame-relay
 clock rate 2000000
 frame-relay interface-dlci 888
end

R4#sh fram map
Serial0/1 (up): ip 172.31.14.1 dlci 888(0x378,0xDC80), dynamic,
              broadcast,, status defined, active
R4#sh fram pvc | i STAT
DLCI = 888, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial0/1


Option 2 - Configure frame-relay without enabling frame-relay switching
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This option should be used when the requirements restrict the use of frame-relay switching and explicitly tell you to not enable it on either side.

Example: Configure frame-relay encapsulation between R1 and R4 without enabling frame-relay switching on either device.


When configuring this option, we configure each side with the same frame-relay interface-dlci and then disable keepalives on the interface with the no keepalive command. If we forget the no keepalive command, each interface will continue to go up and down, never forming a good connection.


Sample configurations and verification commands:
! prove frame-relay switching hasn't been enabled
R1#sh run | in switching
R1#sh run int s0/1
Building configuration...

Current configuration : 192 bytes
!
interface Serial0/1
 description *** Link to R4 ***
 ip address 172.31.14.1 255.255.255.240
 encapsulation frame-relay
 no keepalive
 clock rate 2000000
 frame-relay interface-dlci 888
end

R1#sh fram map
Serial0/0 (up): ip 172.31.12.2 dlci 102(0x66,0x1860), dynamic,
              broadcast,, status defined, active
Serial0/0 (up): ip 172.31.13.3 dlci 103(0x67,0x1870), dynamic,
              broadcast,, status defined, active
Serial0/1 (up): ip 172.31.14.4 dlci 888(0x378,0xDC80), dynamic,
              broadcast,
R1#


R4#sh run | i switching
R4#sh run int s0/1
Building configuration...

Current configuration : 192 bytes
!
interface Serial0/1
 description *** Link to R1 ***
 ip address 172.31.14.4 255.255.255.240
 encapsulation frame-relay
 no keepalive
 clock rate 2000000
 frame-relay interface-dlci 888
end

R4#sh fram map
Serial0/1 (up): ip 172.31.14.1 dlci 888(0x378,0xDC80), dynamic,
              broadcast,
R4#