Wide-Area Networks - CCIE Routing and Switching Exam Certification Guide - page 216

LAPD

ISDN uses LAPD as the signaling protocol at Layer 2. LAPD is specified in the ITU-T Q.921
standard. You use LAPD on the D channel to ensure proper flow and transmission of the
signaling and control information.

The LAPD frame format is shown in Figure 5-12. The address field is similar to the address
field in HDLC. The Service Access Point Identifier (SAPI) identifies the LAPD services that
are provided to the upper layer (Layer 3). The C/R bit indicates if the frame is a command or
response. The terminal endpoint identifier (TEI) identifies single or multiple terminals. If the
TEI is all 1s, it indicates a broadcast.

LAPD Frame Format

Figure 5-12

Flag Address Control Data FCS Flag

Address Field:

SAPI C/R EA TEI EA

HDLC

HDLC is a leased-line encapsulation method that you commonly use in point-to-point connec-
tions. The HDLC standard was developed by the ISO (ISO 3309). Cisco modified the standard
by adding a Type field to the frame. Cisco routers use this proprietary version of HDLC. The B
channels in ISDN can use HDLC encapsulation. The frame format for HDLC is the same as in
LAPD except for some differences in the Address field. On a Cisco router, HDLC encapsulation
can only connect with another Cisco router; it is enabled by default.

PPP

PPP is an encapsulation protocol for transporting Layer-3 traffic over point-to-point links. Also,
PPP can be configured over other WAN serial lines, including Frame Relay PVCs.

PPP encapsulation can also establish an end-to-end PPP connection over other networks, such
as Ethernet (PPPoE). This use of PPP is not discussed in this topic.

PPP consists of three major components:
•

Encapsulation of packets over serial links using HDLC frame structure as a basis

•

Establishment, maintenance, and termination of circuits with the Link Control Program
(LCP)

•

Multiplexing of upper-layer protocols with Network Control Programs (NCPs)