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Topic: Frame Relay Multipoint Link on a Subinterface Using Inverse ARP

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Justin G. Mitchell
jgmitchell@gmail.com
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This type of frame-relay configuration has some of the same benefits as physical multipoint interfaces, but the actual configuration is more involved.

Our basic configuration involves the following steps:
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01. Enable frame-relay on the physical interface
02. Create the subinterface
03. Assign IP address to the subinterface
04. Assign desired DLCIs to the subinterface



Basic configuration and verification of frame-relay multipoint subinterface:
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Our very basic interface configuration

R3#sh run int s0/0.1
Building configuration...

Current configuration : 176 bytes
!
interface Serial0/0.1 multipoint
 ip address 172.172.123.3 255.255.255.0
 frame-relay interface-dlci 301
 frame-relay interface-dlci 302
 frame-relay interface-dlci 304
end

!
! Verify the DLCIs are associated with the subinterface
!

R3#sh fram pvc | i STAT
DLCI = 301, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial0/0.1
DLCI = 302, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial0/0.1
DLCI = 304, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial0/0.1

!
! Verify inverse ARP is dynamically mapping the remote IP addresses to the DLCIs and subinterface
!

R3#sh fram map       
Serial0/0.1 (up): ip 172.172.123.1 dlci 301(0x12D,0x48D0), dynamic,
              broadcast,, status defined, active
Serial0/0.1 (up): ip 172.172.123.2 dlci 302(0x12E,0x48E0), dynamic,
              broadcast,, status defined, active
Serial0/0.1 (up): ip 172.172.123.4 dlci 304(0x130,0x4C00), dynamic,
              broadcast,, status defined, active



Why we need this configuration:
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If we forget to assign the DLCIs to the subinterface, the mappings will be incorrect as shown below. By default the DLCIs are assigned to the physical interface regardless of other interface configurations.

R3#sh run int s0/0
Building configuration...

Current configuration : 117 bytes
!
interface Serial0/0
 no ip address
 encapsulation frame-relay
 clock rate 2000000
 no frame-relay inverse-arp
end

!
! The following output is from after creating the subinterface, but before assigning the DLCIs to the subinterface.
!

R3#sh fram pvc | i STAT
DLCI = 301, DLCI USAGE = UNUSED, PVC STATUS = ACTIVE, INTERFACE = Serial0/0
DLCI = 302, DLCI USAGE = UNUSED, PVC STATUS = ACTIVE, INTERFACE = Serial0/0
DLCI = 304, DLCI USAGE = UNUSED, PVC STATUS = ACTIVE, INTERFACE = Serial0/0