Frame Relay Basics

Frame Relay is a popular WAN technology that is cost-effective compared with lease-lines using PPP or HDLC. Frame Relay is a packet-switching network , meaning it doesn't take the same path each time to send data . Frame Relay networks by default are NBMA (non-broadcast multi-access networks) and are made up of hundreds of FR switches. Understanding the FR cloud isn't important because the ISP maintains it, understanding how to tie into the FR cloud is vital. 

Frame Relay uses virtual circuits to send data across the FR cloud. The two types are private virtual circuits (PVC) and switch private circuits (SVC). PVC are more commonly used because they are always up and ready to send data while SVC are up only when data is needed to be sent.

These PVCs are establishment between DTEs while traversing the Frame Relays DCE equipment. Frame Relay uses local management interface (LMI) messages as an heartbeat to keep virtual circuits alive . These keepalives must match between my DTE to the FR's DCE. Cisco routers support three types of LMIs;

• Cisco
• Ansi
• Q933a

Cisco also has an auto-sense feature that sends out all three type of LMIs and configures itself depending on how the FR DCE answers. LMIs are only half of the story.

Data-link Connection Identifiers (DLCIs) are local significant addresses used to identify a PVC to a remote DTE. DLCI addresses are given by the service provider and must be correctly configured to map remote networks.

Configuring Frame Relay on a serial interface requires a couple of steps, first the encapsulation must be changed to frame relay, there are two types of FR encapsulations cisco and ietf, default is Cisco.

Turning frame-relay inverse is a good idea because Frame Relay networks are more stable when manually mapping DLCIs then letting inverse-arp discover the mappings. Mapping frame relay we need the protocol(IP), remote IP address (172.12.123.2 ), local DLCI (112) and broadcast command if broadcasts will be needed across the PVC. 

R1(config)#interface serial 0/0
R1(config-if)#encapsulation frame-relay
R1(config-if)#ip address 172.12.123.1 255.255.255.0
R1(config-if)#frame map ip 172.12.123.2 112 broadcast
R1(config-if)#frame map ip 172.12.123.3 113 broadcast
R1(config-if)#no frame-relay inverse-arp
R1(config-if)#no shutdown

The show frame-relay map command can be used to determine the status of PVCs, LMI type, topology, PVC state;

• Inactive -problem at the remote DTE
• Active - DTE to DTE correctly configured
• Deleted - Local DLCI problem with FR DCE

R1#show frame map
Serial0/0 (up): ip 172.12.123.2 dlci 112(0x70,0x1C00), static,
              broadcast,
              CISCO, status defined, inactive
Serial0/0 (up): ip 172.12.123.3 dlci 113(0x71,0x1C10), static,
              broadcast,
              CISCO, status defined, inactive

R2(config)#interface serial 0/0
R2(config-if)#encapsulation frame-relay
R2(config-if)#no frame inverse-arp
R2(config-if)#ip address 172.12.123.2 255.255.255.0
R2(config-if)#frame map ip 172.12.123.1 221 broadcast
R2(config-if)#frame map ip 172.12.123.3 321 broadcast
R2(config-if)#no shutdown

R2#show frame map

Serial0/0 (up): ip 172.12.123.1 dlci 221(0xDD,0x34D0), static,

              broadcast,

              CISCO, status defined, active

Serial0/0 (up): ip 172.12.123.3 dlci 321(0x141,0x5010), static,

              broadcast,

              CISCO, status defined, inactive

R2#ping 172.12.123.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 172.12.123.1, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 12/34/56 ms

R2#ping 172.12.123.3

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 172.12.123.3, timeout is 2 seconds:

.....

Success rate is 0 percent (0/5)

R3(config)#interface serial 0/0

R3(config-if)#encapsulation frame-relay

R3(config-if)#no frame-relay inverse-arp

R3(config-if)#ip address 172.12.123.3 255.255.255.0

R3(config-if)#frame map ip 172.12.123.1 321 broadcast

R3(config-if)#frame map ip 172.12.123.2 321 broadcast

R3(config-if)#no shutdown

R3#show frame map

Serial0/0 (up): ip 172.12.123.1 dlci 321(0x141,0x5010), static,

              broadcast,

              CISCO, status defined, active

Serial0/0 (up): ip 172.12.123.2 dlci 321(0x141,0x5010), static,

              broadcast,

              CISCO, status defined, active

Both PVCs are configured correctly between DTEs and I can verified by pinging both remote routers

R3#ping 172.12.123.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 172.12.123.1, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 1/28/44 ms

R3#ping 172.12.123.2

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 172.12.123.2, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 24/32/36 ms

Another way to verified Frame Relay is using the show frame lmi command, If sent is higher (increasing )than recd (unchanged) LMIs and if timeouts are increasing, there is a problem with the PVC.

R1#show frame lmi

LMI Statistics for interface Serial0/0 (Frame Relay DTE) LMI TYPE = ANSI

  Invalid Unnumbered info 0             Invalid Prot Disc 0

  Invalid dummy Call Ref 0              Invalid Msg Type 0

  Invalid Status Message 0              Invalid Lock Shift 0

  Invalid Information ID 0              Invalid Report IE Len 0

  Invalid Report Request 0              Invalid Keep IE Len 0

  Num Status Enq. Sent 8                Num Status msgs Rcvd 8

  Num Update Status Rcvd 0              Num Status Timeouts 0

  Last Full Status Req 00:00:30         Last Full Status Rcvd 00:00:30

The command show frame pvc will summarized PVC information including ,PVC states, local DLCIs ,interfaces and FECN/BECN/DE bits. Forward Explicit Congesntion Notification (FECN) sets a notification bit on frames going to the remote router that there in congestion in the FR cloud and it needs to slow down, while Backwards Explicit Congestion Notification (BECN) sets a bit back to the sending router that there is congestion in the FR cloud and to slow down. The Discard Eligibility (DE) set a discard bit on frames (unimportant) so that if frames must be discarded these frames will be discarded first.

R1#show frame pvc

PVC Statistics for interface Serial0/0 (Frame Relay DTE)

              Active     Inactive      Deleted       Static

  Local          2            0            0            0

  Switched       0            0            0            0

  Unused         0            0            0            0

DLCI = 112, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial0/0

  input pkts 0             output pkts 0            in bytes 0

  out bytes 0              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 00:15:52, last time pvc status changed 00:15:52

DLCI = 113, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial0/0

  input pkts 0             output pkts 0            in bytes 0

  out bytes 0              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 00:15:55, last time pvc status changed 00:15:55