CCNA Complete Guide 2nd Edition.pdf - Cisco Learning Home

Point-to-Point Serial Links (Leased Lines)
- The 2 most popular data link (L2) protocols that are being used over point-to-point serial links:
HDLC High-Level Data Link Control. It was derived from Synchronous Data Link Control
(SDLC) created by IBM. The HDLC specification was not intended to encapsulate
(or support) multiple network layer protocols across the same link. It does not include
a Protocol Type field and hence unable to identify the encapsulated network layer
packets. Vendors that support HDLC must define their own proprietary HDLC
headers that contain the Protocol Type field to identify the network layer protocols.
Due to this reason, HDLC implementations are proprietary – Cisco routers will not
work in connections to other vendor routers. It is the default encapsulation type used
on Cisco synchronous serial interfaces. HDLC does not support error recovery.
PPP Point-to-Point Protocol. It is an industry-standard protocol that provides an open
standard for creating point-to-point links between different vendor equipments.
It uses a standardized Protocol Type field – Network Control Protocol (NCP) field in
the header to identify the encapsulated network layer packets. It supports
authentication, multilink connections, and can operates over synchronous and
asynchronous links. PPP uses HDLC to encapsulate and transmit packets over pointto-point
links. PPP supports error recovery but is disabled by default.
Note: HDLC was developed by the International Organization for Standardization (ISO).
- HDLC does not need the arbitration feature as in Ethernet. Because on point-to-point serial links,
routers can send over the 4 wires (2 twisted-pair wires) circuit at any time, and the signal
(eventually data) will arrived at the other end of the link.
- Both HDLC and PPP frames include an address field, but it is not being used and not really
needed over point-to-point links.
- Do not confuse error detection with error recovery! Almost all data link layer protocols including
HDLC and PPP perform error detection with the FCS field in their trailers. Frames that fail the
FCS check will be noticed and discarded (error detection). Error recovery is normally being
performed by higher-layer protocols, eg: TCP.
Point-to-Point Protocol (PPP)
- When PPP was being developed, it includes many additional features that never been seen in
other WAN data link protocols up to that time.
- The PPP specification includes the following 2 subprotocols or sublayers:
i) Link Control Protocol (LCP). Responsible for establishing, configuring, testing,
maintaining, and terminating PPP connections. PPP was designed to work with several
network layer protocols, hence each PPP link has a LCP and a NCP for the corresponding
L3 protocol encapsulated and transmitted over the PPP link.
ii) Network Control Protocols (NCPs). Responsible for encapsulating network layer
protocols and assigning network layer addresses to the remote end over PPP links after
the link establishment and authentication phases. The negotiated network layer protocols
can then be transmitted across the PPP connection.
Ex: IP Control Protocol (IPCP) for IP, IPX Control Protocol (IPXCP) for IPX, and
AppleTalk Control Protocol (ATCP) for AppleTalk. A router configured for PPP
encapsulation will try to identify the network layer protocol and use the appropriate
network control protocol for the corresponding L3 protocol transmitted over the PPP link.
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