Understand Frame Relay and its history. Frame Relay is a streamlined version of X.25 without
the windowing and retransmission capabilities. Frame Relay is a layer 2 protocol that was defined
as a network service for ISDN by the CCITT (now ITU-T). The Group of Four extended Frame
Relay in 1990 to allow for a Local Management Interface (LMI) to assist in PVC management.
Understand the two types of virtual circuits (VCs). Know what a switched virtual circuit
(SVC) is used for and what a permanent virtual circuit (PVC) is used for. Understand why you
would use one type over another.
Know what a DLCI is and how it is mapped to Network layer protocols. The data link
connection identifier (DLCI) is used to identify a PVC in a Frame Relay network. The DLCIto-
Network-layer mapping can be statically configured by an administrator using the framerelay
map command or can be dynamically set by using Inverse ARP (IARP).
Understand the Local Management Interface (LMI). LMI was an extension to Frame Relay
to manage the virtual circuits on a connection. LMI virtual circuit status messages provide communication
and DLCI synchronization between Frame Relay DTE and DCE devices.
Know how to configure Frame Relay and what it uses for congestion control. The
encapsulation frame-relay command is used to configure an interface for Frame Relay
operation. The frame-relay intf-type dce command is used to configure an interface
for DCE operation, but by default, the Frame Relay interface type is DTE. Frame Relay uses
backward explicit congestion notification (BECN) and forward explicit congestion notification
(FECN) messages to control congestion on a Frame Relay switch.
Understand the options for traffic shaping on a Frame Relay interface. There are many
options for traffic shaping to enable a Frame Relay network to operate more efficiently. You
can have the router slow traffic on a VC in response to BECNs received. You can set up queuing
on a per-VC basis and limit traffic going out of a VC. You need to know what the committed
information rate (CIR) and excess information rate (EIR) are.
Know what Cisco IOS commands are used to verify and troubleshoot a Frame Relay connection.
The commands show interface, show frame-relay pvc, show frame-relay map, show
frame-relay lmi, and debug frame-relay lmi are all used to see and verify the operation of
Frame Relay. The command clear frame-relay-inarp is used to delete the dynamic PVC-to-
Network-Layer addressing entries.