Point-to-Point Protocol (PPP)
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F I G U R E 1 0 . 2
Cisco HDLC frame format
As shown in the figure, the reason that every vendor has a proprietary
HDLC encapsulation method is that each vendor has a different way for
the HDLC protocol to communicate with the Network layer protocols. If the
vendors didn't have a way for HDLC to communicate with the different layer-
3 protocols, then HDLC would only be able to carry one protocol. This
proprietary header is placed in the data field of the HDLC encapsulation.
If you had only one Cisco router and you needed to connect to, say, a
Bay router because you had your other Cisco router on order, then you
couldn't use the default HDLC serial encapsulation. You would use some-
thing like PPP, which is an ISO-standard way of identifying the upper-layer
protocols.
Point-to-Point Protocol (PPP)
P
PP (Point-to-Point Protocol) is a data-link protocol that can be used
over either asynchronous serial (dial-up) or synchronous serial (ISDN)
media and that uses the LCP (Link Control Protocol) to build and maintain
data-link connections.
The basic purpose of PPP is to transport layer-3 packets across a Data
Link layer point-to-point link. Figure 10.3 shows the protocol stack com-
pared to the OSI reference model.
Address
Flag
· Each vendor's HDLC has a proprietary data field to support multiprotocol environments.
· Supports only single-protocol environments.
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Address
Control
Data
FCS
Flag
Control
Proprietary
Data
FCS
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Cisco HDLC
HDLC
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