OSI Layer 2: Data-Link Layer Protocols and Applications
101
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
!
interface Serial1/5.3 point-to-point
description To Building C
ip address 172.16.1.29 255.255.255.252
no ip directed-broadcast
frame-relay interface-dlci 18 IETF
!
interface Serial1/5.4 point-to-point
description To Building D
ip address 172.16.1.33 255.255.255.252
no ip directed-broadcast
frame-relay interface-dlci 21 IETF
!
From this configuration, you can see that four logical circuits have been
defined to communicate over one physical circuit. Notice that each sub-
interface 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.
Frame Structure
The frame used in Frame Relay is similar to the frame used in X.25, except for
two major differences. The first difference is that Frame Relay leaves flow-
and-error control to the upper-layer protocols. Frame Relay does provide con-
gestion detection and can notify the upper layers of possible problems, but it
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