Frame Relay was developed as a digital packet-switching technology, whereas older technologies
such as X.25 were analog-based technologies. The technology used in Frame Relay allows it to
multiplex several different data flows over the same physical media. More information on Frame
Relay is presented in Chapter 39, “Troubleshooting Serial Line and Frame Relay Connectivity.”
Frame Relay also uses permanent and switched virtual circuits between the data terminal
equipment (DTE) (customer connection) and the data communication equipment (DCE) (service
provider’s frame relay switch). These virtual circuits have unique identifiers that allow the
Frame Relay to keep track of each logical data flow. The identifier is known as a DLCI (data
link connection identifier). The DLCI number is used to create a logical circuit within a physical
circuit. Multiple logical circuits can be created within one physical circuit.
Look at the following router configuration excerpt:
interface Serial1/5
description Physical Circuit
no ip address
no ip directed-broadcast
encapsulation frame-relay
!
interface Serial1/5.1 point-to-point
description To Building A
ip address 172.16.1.17 255.255.255.252
no ip directed-broadcast
frame-relay interface-dlci 17 IETF
!
interface Serial1/5.2 point-to-point
description To Building B
ip address 172.16.1.25 255.255.255.252
no ip directed-broadcast
frame-relay interface-dlci 22 IETF
From this configuration, you can see that two logical circuits have been defined to communicate
over one physical circuit. Notice that each subinterface or logical circuit has a unique DLCI. Each
DLCI maps to another DLCI within the Frame Relay cloud. This mapping continues throughout the
Frame Relay cloud until it maps to another DTE on the destination side of the virtual circuit.