Multipoint connections using inverse ARP
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Multipoint connections on physical interfaces utilizing inverse ARP for mapping are the most basic frame-relay configuration, requiring the least number of steps. For our basic configuration all we need is to enable frame-relay encapsulation on the interface and assign an IP address.

This is an ideal configuration method in a full mesh situation where all the configured DLCIs are on the same subnet. It provides the easiest means of connectivity with the lowest amount of administrative overhead.

R1(config-if)#do sh run int s0/0
Building configuration...

Current configuration : 114 bytes
!
interface Serial0/0
 ip address 172.31.13.1 255.255.255.240
 encapsulation frame-relay
 clock rate 2000000
end

R1(config-if)#do sh fram map
Serial0/0 (up): ip 172.31.13.3 dlci 103(0x67,0x1870), dynamic,
              broadcast,, status defined, active

R1(config-if)#do sh fram pvc | i STAT
DLCI = 102, DLCI USAGE = UNUSED, PVC STATUS = INACTIVE, 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


R3(config-if)#do sh run int s0/0
Building configuration...

Current configuration : 114 bytes
!
interface Serial0/0
 ip address 172.31.13.3 255.255.255.240
 encapsulation frame-relay
 clock rate 2000000
end

R3(config-if)#do sh fram map
Serial0/0 (up): ip 172.31.13.1 dlci 301(0x12D,0x48D0), dynamic,
              broadcast,, status defined, active

R3(config-if)#do sh fram pvc | i STAT
DLCI = 301, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial0/0
DLCI = 302, DLCI USAGE = UNUSED, PVC STATUS = INACTIVE, INTERFACE = Serial0/0
DLCI = 304, DLCI USAGE = UNUSED, PVC STATUS = INACTIVE, INTERFACE = Serial0/0


The MAJOR issue with relying on inverse ARP on multipoint interfaces is the fact that inverse ARP will discover all attached devices and map their IP addresses to a DLCI whether the remote device is in the same IP network or not. In this example I have added an IP address to the physical interface on R2:

R1#sh run int s0/0
Building configuration...

Current configuration : 114 bytes
!
interface Serial0/0
 ip address 172.31.13.1 255.255.255.240
 encapsulation frame-relay
 clock rate 2000000
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
              
              
R2#sh run in
*Mar  1 00:18:08.307: %SYS-5-CONFIG_I: Configured from console by console
R2#sh run int s0/0
Building configuration...

Current configuration : 114 bytes
!
interface Serial0/0
 ip address 172.31.12.2 255.255.255.240
 encapsulation frame-relay
 clock rate 2000000
end

R2#sh fram map
Serial0/0 (up): ip 172.31.13.1 dlci 201(0xC9,0x3090), dynamic,
              broadcast,, status defined, active
Serial0/0 (up): ip 172.31.13.3 dlci 203(0xCB,0x30B0), dynamic,
              broadcast,, status defined, active


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

Current configuration : 114 bytes
!
interface Serial0/0
 ip address 172.31.13.3 255.255.255.240
 encapsulation frame-relay
 clock rate 2000000
end

R3#sh fram map
Serial0/0 (up): ip 172.31.13.1 dlci 301(0x12D,0x48D0), dynamic,
              broadcast,, status defined, active
Serial0/0 (up): ip 172.31.12.2 dlci 302(0x12E,0x48E0), dynamic,
              broadcast,, status defined, active


So, how can we get around this issue WITHOUT disabling inverse ARP on the interface? We can assign the affected DLCIs to a subinterface. This removes the DLCI from the physical interface and prevents the learning of any real information on that DLCI.

R1#sh run | se Serial
interface Serial0/0
 ip address 172.31.13.1 255.255.255.240
 encapsulation frame-relay
 clock rate 2000000
interface Serial0/0.12 point-to-point
 frame-relay interface-dlci 102   
R1#sh fram map
Serial0/0.12 (up): point-to-point dlci, dlci 102(0x66,0x1860), broadcast
          status defined, active
Serial0/0 (up): ip 172.31.13.3 dlci 103(0x67,0x1870), dynamic,
              broadcast,
              CISCO, status defined, active
Serial0/1 (up): ip 172.31.14.4 dlci 888(0x378,0xDC80), dynamic,
              broadcast,
              CISCO, status defined, active
R1#

Another and more preferred option we have, is disabling inverse ARP on the unused DLCIs themselves. This continues to allow the dynamic mapping of IP addresses and DLCIs, but will prevent the learning of information on the unused DLCIs.

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
R1#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)#int s0/0
R1(config-if)#no frame-relay inverse-arp ip 102 
R1(config-if)#do 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
R1(config-if)#do clear fram inarp
! Wait for the inverse ARP process to repopulate the mappings
R1(config-if)#do sh fram map
Serial0/0 (up): ip 172.31.13.3 dlci 103(0x67,0x1870), 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
R1#sh run int s0/0
Building configuration...

Current configuration : 149 bytes
!
interface Serial0/0
 ip address 172.31.13.1 255.255.255.240
 encapsulation frame-relay
 clock rate 2000000
 no frame-relay inverse-arp IP 102
end

R1#