Frame Relay

Frame Relay is layer 2 packet-switched network protocol, it creates circuits between end-point devices:

• Switched Virtual Circuits (SVC) - Established dynamically by sending signaling messages to the network (CALL SETUP, DATA TRANSFER, IDLE, CALL TERMINATION).
• Permanent Virtual Circuits (PVCs) - preconfigured by the carrier, and after they are set up, only operate in DATA TRANSFER and IDLE modes. Note that some publications refer to PVCs as private VCs.

DLCI (data link connection identifier)

• DLCI has no significance beyond the single link
• 0 to 15 and 1,008 to 1,023 are reserved, typically the range of 16 to 1,007 is used by customers

Inverse Address Resolution Protocol (Inverse ARP) - resolves IPv4 to DLCI therefore IPv4 addresses must be available before VCs can be used. Frame Relay for IPv6 uses Inverse Neighbor Discovery (IND) to obtain a Layer 3 IPv6 address from a Layer 2 DLCI

Local Management Interface (LMI) - a definition of the messages used between the DTE (R1) and the DCE (the Frame Relay switch owned by the service provider). It is a keepalive mechanism that provides status information, every 10s pulls info from the network. LMI types are: CISCO, ANSI, Q933A

Configuration

R1(config)# interface S0/0/1
R1(config-if)# bandwidth 64                                !for OSPF, EIGRP and others routing protocols
R1(config-if)# ip address 10.1.1.1 255.255.255.0
R1(config-if)# ipv6 address 2001:db8:cafe:1::1/64
R1(config-if)# ipv6 address fe80::1 link-local
R1(config-if)# encapsulation frame-relay
R1(config-if)# no frame-relay inverse-arp                  !disables iarp
R1(config-if)# frame-relay map ip 10.1.1.2 102 broadcast   !broadcast is to help routing protocols to broadcast its updates on NBMA networks
R1(config-if)# frame-relay map ipv6 2001:DB8:CAFE:1::2 102
R1(config-if)# frame-relay map ipv6 FE80::2 102 broadcast

R2(config) interface S0/0/1
R2(config-if)# bandwidth 64
R2(config-if)# ip address 10.1.1.2 255.255.255.0
R2(config-if)# ipv6 address 2001:db8:cafe:1::2/64
R2(config-if)# ipv6 address fe80::2 link-local
R2(config-if)# encapsulation frame-relay
R2(config-if)# no frame-relay inverse-arp 
R2(config-if)# frame-relay map ip 10.1.1.1 201 broadcast
R2(config-if)# frame-relay map ipv6 2001:DB8:CAFE:1::1 201
R2(config-if)# frame-relay map ipv6 FE80::1 201 broadcast

Configuration with sub-interfaces

Frame Relay cloud in fact is a FR switch holding PVC to a port maps. Its configuration is shown below.

R1(config)# interface S0/0/1
R1(config-if)# encapsulation frame-relay
R1(config-if)# no shutdown
R1(config-if)# interface s 0/0/1.102 point-to-point    !ptp is to help resolve split horizon and other issues with NBMA networks
R1(config-subif)# ip address 10.1.1.1 255.255.255.252
R1(config-subif)# bandwidth 64
R1(config-subif)# frame-relay interface-dlci 102
R1(config-fr-dlci)# exit
R1(config-subif)# R1(config-if)# interface s 0/0/1.103 point-to-point
R1(config-subif)# ip address 10.1.1.5 255.255.255.252
R1(config-subif)# bandwidth 64
R1(config-subif)# frame-relay interface-dlci 103

Routing considerations

Point-to-Point Subinterfaces - In hub and spoke topologies:

• Subinterfaces act as leased lines
• Each point-to-point subinterface requires its own subnet

Multipoint Subinterfaces – In partial-mesh and full-mesh topologies:

• Subinterfaces act as NBMA so they do not resolve the split horizon issue
• routers exist in the same subnet

Troubleshooting

Changing configuration requires power cycling an interface

show frame-relay map
show frame-relay lmi
show interfaces serial
show frame-relay pvc
clear counters
clear frame-relay inarp
! ACTIVE - DTE to DCE (router to a Frame Relay switch) circuit
! INACTIVE - connected to DCE (switch) but without DTE detected at the other end
! DELETED - switch doesn't recognize DLCI configured on a router as a valid for its interface
debug frame-relay lmi