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600 Chapter 8: WAN Protocols and Design
Table 8-43 outlines the three Frame Relay LMI types, their origin, and the keyword used in the
Cisco frame-relay lmi-type interface subcommand:
Figure 8-34 shows several examples of ISDN BRI topology, with function groups and reference
points.
Data link connection identifier
(DLCI)
A DLCI is a Frame Relay address and is used in Frame Relay
headers to identify the virtual circuit.
Forward explicit congestion
notification (FECN)
The FECN is the bit in the Frame Relay header that signals to
anyone receiving the frame (switches and DTEs) that congestion is
occurring in the same direction as the frame. Switches and DTEs
can react by slowing the rate by which data is sent in that direction.
Backward explicit congestion
notification (BECN)
The BECN is the bit in the Frame Relay header that signals to
anyone receiving the frame (switches and DTEs) that congestion is
occurring in the opposite (backward) direction as the frame.
Switches and DTEs can react by slowing the rate by which data is
sent in that direction.
Discard eligibility (DE)
The DE is the bit in the Frame Relay header that signals to a switch
that, if frames must be discarded, please choose this frame to
discard instead of another frame without the DE bit set.
Nonbroadcast multiaccess
(NBMA)
NBMA refers to a network in which broadcasts are not supported
but more than two devices can be connected.
Local Management Interface
(LMI)
LMI is the protocol used between a DCE and a DTE to manage the
connection. Signaling messages for SVCs, PVC status messages,
and keepalives are all LMI messages.
Link access procedure--frame
mode bearer services (LAPF)
LAPF is the basic Frame Relay header and trailer, which includes
DLCI, FECN, BECN, and DE bits.
Table 8-43
Frame Relay LMI Types
Name
Document
IOS LMI-Type Parameter
Cisco
Proprietary
cisco
ANSI
T1.617 Annex D
ansi
ITU
Q.933 Annex A
q933a
Table 8-42
Frame Relay Terms and Concepts (Continued)
ch08.fm Page 600 Monday, March 20, 2000 5:17 PM