Frame Relay Network
The FR network consists of the FR backbone (consisting of FR switches provided
by the FR carrier) providing the FR service. The router functions as the FR
connection device. The router encapsulates FR frames and routes them through the
network based on a Data Link Connection Identifier (DLCI). The DLCI is the
medium access control (MAC) address that identifies the PVC or SVCbetween the
router and the FR destination device. For example, in Figure 21, a packet destined
to go from router B to router D would have a DLCI of 19 to reach router D;
however, a packet destined to go from router D to router B would have a DLCI of
16.
A DLCI can have either local or global significance. Local DLCIs are significant at
the point of entry to the network, but global DLCIs are significant throughout the
network. Tothe user, however, the DLCI that the router uses to route a packet is
the DLCI that the user associates with the frame’s global or local destination. DLCIs
are configured through the FR configuration process or learned through FR
management.
FR PVCs are predefined connections used to route data through a FR network. The
bandwidth allocated for a PVC within the network is a subscription option and must
be allotted to the PVC whether or not the PVC uses it.
A Frame Relay network has the following characteristics:
vTransports frames transparently.The network can modify only the DLCI,
congestion bits, and frame check sequence. High-Level Data Link Control
(HDLC) flags and zero bit insertion provide frame delimiting, alignment, and
transparency.
vDetects transmission, format, and operational errors (frames with an unknown
DLCI)
DLCI 19
Router
B
Frame
Relay
Backbone Router
D
Router
A
DLCI 18
DLCI 17
DLCI 16
physical connection
logical connection
Figure 21. DLCIs in Frame Relay Network
Using Frame Relay
388 MRS V3.2 Software User’s Guide
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