Part 1: Frame Relay Basic

Frame Relay is WAN technology that works on layer 2 (Data Link) of the ISO technology. Frame Relay initially propose at Consultative Committee on International Telephone and Telegraph(CCITT) in 1984. In 1990, Cisco, DEC, Nortel and Stratacom formed a consorsium to develop frame relay technology . 

Frame relay devices are consist of:

1. DTE(Data Terminal Equipment), located on the customer devices, such as router, bridges,etc).
2. DCE(Data Circuit-Terminating Equipment), located on Frame Relay  PSN (Packet Switched Network)

Frame Relay create virtual circuit to form end-to-end links and uniquely identified by a data link connection identifier (DLCI). DLCI is a value assigned by Frame Relay service provider and is locally significant, which means its values is unique in the LAN, but not in the WAN. Frame Relay virtual circuit fall into two categoris:

1. SVC (Switched Virtual Circuit), connection between DTE devices will be established if there's data to be transferred. If no data is transferred, the connection will be in idle state for a defined period and then terminated.
2. PVC (Permanent Virtual Circuit), connection between DTE devices will be established permanently.  

DLCI Mapping

Remember, Frame Relay only works on layer 2 of the OSI model and it doesn't understand IP addressing. So we need to map IP address to a DLCI number on Frame Relay switch. Mapping can be done statically or dynamically by administrator on the router.

• Static Mapping, manually map next hop address to a local DLCI. Use command "frame-relay  map" on the cisco router to configure static mapping. Here is the example configuration static mapping on R1:         

 R1# sh running-config int s1/0
 Building configuration...

 Current configuration : 156 bytes
 !
 interface Serial1/0
  ip address 10.1.1.1 255.255.255.0
  encapsulation frame-relay
  frame-relay map ip 10.1.1.2 102 broadcast
 end

  

• Dynamic Mapping, using Inverse ARP to map IP address based on DLCI number. Here is the example configuration dynamic mapping on R2:

 R2# sh running-config int s1/0
 Building configuration...
 
 Current configuration : 145 bytes
 !
 interface Serial1/0
  ip address 10.1.1.2 255.255.255.0
  encapsulation frame-relay
  frame-relay interface-dlci 201
 end

Verify your configuration using command "show frame-relay map" or "show frame-relay pvc":

R1#show frame-relay map
Serial1/0 (up): ip 10.1.1.2 dlci 102(0x66,0x1860), static,
              broadcast,
              CISCO, status defined, active

R1#show frame-relay pvc

PVC Statistics for interface Serial1/0 (Frame Relay DTE)

              Active     Inactive      Deleted       Static
  Local          1            0            0            0
  Switched       0            0            0            0
  Unused         0            0            0            0

DLCI = 102, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial1/0

  input pkts 50            output pkts 53           in bytes 3847
  out bytes 4172           dropped pkts 0           in pkts dropped 0
  out pkts dropped 0                out bytes dropped 0
  in FECN pkts 0           in BECN pkts 0           out FECN pkts 0
  out BECN pkts 0          in DE pkts 0             out DE pkts 0
  out bcast pkts 0         out bcast bytes 0
  5 minute input rate 0 bits/sec, 0 packets/sec
  5 minute output rate 0 bits/sec, 0 packets/sec
  pvc create time 01:13:15, last time pvc status changed 01:12:10

        Test ping from R1 to R2:

R1# ping 10.1.1.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.1.1.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent(5/5), round-trip min/avg/max = 4/36/68 ms