
Frame Relay Overview
117376-A Rev. A
2-3
Frame Relay Packets
Figure 2-2 illustrates the structure of a frame relay packet. The packet’s header
field includes the following:
• Data link connection identifier (DLCI)
The DLCI is the permanent virtual circuit (PVC) identification number. The
frame relay network uses the DLCI to direct basic data flow.
• Command/response bit (C/R)
ITU-T (formerly CCITT) standards do not use this bit.
• Forward explicit congestion notification (FECN) and backward explicit
congestion notification (BECN)
The FECN and BECN indicate congestion on the network. See “Congestion
Control,” on page 2-
14 for information about how the frame relay software
uses these bits.
• Discard eligibility (DE)
The DE bit allows the router to mark specific frames as low priority (discard
eligible) before transmitting them to the frame relay network.
• Extended address bit (EA)
The EA bit signals whether the next byte is part of the address. This bit
indicates the last byte of the DLCI.
Figure 2-2. Frame Relay Header, 2-Byte Format
DLCI (high order) C/R
DE
EA
F
E
C
N
DLCI (low order)
8 7 6 5 4 3 2 1
B
E
C
N
Flag
Frame
relay
header
Information
(data)
CRC
Flag
EA
FR0002A
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