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Frame Relay interface types - Part 1 (Physical Interface)

There are several interface types on frame-relay, according on how you configure your serial interface and on what is the purpose you will achieve.
  1. physical interface
  2. subinterface multipoint
  3. subinterface point-to-point
Lets talk about each of them.


Physical Interface 
- are treated as multi point by default.
- all DLCI's updated from LMI are dynamically assigned to physical interface.
- all destination DTE's are expected to be in same subnet.
- use's dynamic mapping (L3 to L2) by default. 
- can be configured with static mapping using "frame-relay map" command.
- Split horizon is disabled by default.

Subinterface Multipoint 
- all destination DTE's are expected to be in same subnet.
- DLCI's must be manually assigned to the subinterface if inv-arp is used with "frame-relay interface-dlci" command.
- L3/L2 mapping should be manually configured if inv-arp is disabled.
- Split horizon is enabled by default.

Subinterface Point-to-Point
- only a single DLCI can be assigned to point-to-point subinterface using "frame-relay interface-dlci" command.
- the interface does not send out inv-arp queries, but responds to inbound in-arp query.
- L3/L2 static mapping cannot be configured manually as the interface assumes to be in single subnet and the inbound inv-arp querries will be mapped to local DLCI defined.
- Split horizon is enabled by default.

Let us evaluate a frame relay technology using all the interface types. Referring the below fig1.3 we have considered R1 as hub and R2, R3, R4 as spokes. 
 
 
We have three different subnets configured between hub and different spokes. Let us start with 100.100.100.0/29 subnet configured between R1, R2, R3 & R4. We shall configure this on physical interface S1/0 on all four routers and verify the connectivity between each other. 

 On R1
Interface Ser1/0
ip address 100.100.100.1 255.255.255.248
encapsulation frame-relay
 frame-relay map ip 100.100.100.2 102 broadcast
 frame-relay map ip 100.100.100.3 103 broadcast
 frame-relay map ip 100.100.100.4 104 broadcast
 no frame-relay inverse-arp

 On R2
Interface Ser1/0
ip address 100.100.100.2 255.255.255.248
encapsulation frame-relay
 frame-relay map ip 100.100.100.1 201 broadcast
 frame-relay map ip 100.100.100.3 201
 frame-relay map ip 100.100.100.4 201
 no frame-relay inverse-arp

 On R3
Interface Ser1/0
ip address 100.100.100.3 255.255.255.248
encapsulation frame-relay
 frame-relay map ip 100.100.100.1 301 broadcast
 frame-relay map ip 100.100.100.2 301
 frame-relay map ip 100.100.100.4 301
 no frame-relay inverse-arp

 On R4
Interface Ser1/0
ip address 100.100.100.4 255.255.255.248
encapsulation frame-relay
 frame-relay map ip 100.100.100.1 401 broadcast
 frame-relay map ip 100.100.100.2 401
 frame-relay map ip 100.100.100.3 401
 no frame-relay inverse-arp

Note: The inv-arp is disabled on physical interface as the default nature of physical interface will dynamically map all the incoming DLCI's.

 Let us verify the static mappings and connectivity between R1, R2, R3 & R4.
"sh frame-relay pvc" command will display statistics about PVC's for frame relay interfaces. Adding "| i \ ACT" will filter only PVC's which are ACTIVE.

R1#sh frame-relay pvc | i \ ACT
DLCI = 102, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial1/0
DLCI = 103, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial1/0 
DLCI = 104, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial1/0  

"sh frame-relay map" will display the current map entries. Note the interface type, IP address, DLCI's mapped, static/dynamic mapping and the support of broadcast.

R1#sh frame-relay map
Serial1/0 (up): ip 100.100.100.2 dlci 102(0x66,0x1860), static,
              broadcast,
              CISCO, status defined, active
Serial1/0 (up): ip 100.100.100.3 dlci 103(0x67,0x1870), static,
              broadcast,
              CISCO, status defined, active
Serial1/0 (up): ip 100.100.100.4 dlci 104(0x68,0x1880), static,
              broadcast,
              CISCO, status defined, active

We see only static mappings and no dynamic mappings.

R1#ping 100.100.100.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 100.100.100.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 20/28/44 ms
 
R1#ping 100.100.100.3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 100.100.100.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 12/33/72 ms
 
R1#ping 100.100.100.4
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 100.100.100.4, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 16/32/84 ms

Verification on R2

R2#sh frame-relay pvc | i \ ACT
DLCI = 201, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial1/0

R2#sh frame-relay map
Serial1/0 (up): ip 100.100.100.1 dlci 201(0xC9,0x3090), static,
              broadcast,
              CISCO, status defined, active
Serial1/0 (up): ip 100.100.100.3 dlci 201(0xC9,0x3090), static,
              CISCO, status defined, active
Serial1/0 (up): ip 100.100.100.4 dlci 201(0xC9,0x3090), static,
              CISCO, status defined, active
Connectivity from R2 to R1, R3, R4 is verified.

Verification on R3

R3#sh frame-relay pvc | i \ ACT
DLCI = 301, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial1/0

R3#sh frame-relay map
Serial1/0 (up): ip 100.100.100.1 dlci 301(0x12D,0x48D0), static,
              broadcast,
              CISCO, status defined, active
Serial1/0 (up): ip 100.100.100.2 dlci 301(0x12D,0x48D0), static,
              CISCO, status defined, active
Serial1/0 (up): ip 100.100.100.4 dlci 301(0x12D,0x48D0), static,
              CISCO, status defined, active 
Connectivity from R3 to R1, R2, R4 is verified.

Verification on R4

R4#sh frame-relay pvc | i \ ACT
DLCI = 401, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial1/0

R4#sh frame-relay map
Serial1/0 (up): ip 100.100.100.1 dlci 401(0x191,0x6410), static,
              broadcast,
              CISCO, status defined, active
Serial1/0 (up): ip 100.100.100.2 dlci 401(0x191,0x6410), static,
              CISCO, status defined, active
Serial1/0 (up): ip 100.100.100.3 dlci 401(0x191,0x6410), static,
              CISCO, status defined, active
Connectivity from R4 to R1, R2, R3 is verified.

In part1 we have configured the physical interfaces with frame relay encapsulation, disabled the Inv-arp with "no frame-relay inverse-arp" command and statically mapped the DLCI's for 100.100.100.0/29 subnet and verified the PVC's, mappings and connectivity between each router.

Points to note on frame-relay physical interface.
  • Split horizon is disabled by default.
  • Inv-arp is enabled by default.
  • All DLCI's are dynamically mapped using Inv-arp by default.
  • If Inv-arp is disabled, then DLCI's should be statically mapped with "frame-relay map ip" command.
  • All DTE's will be part of same subnet.
 
End of part 1     part2 (Frame Relay Sub interface Multipoint)      part3 (Frame Relay Sub interface Point-to-Point)
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