PPP Wan Conections

WAN encapsulation:

  • Leased Line: PPP, HDLC
  • Packet Switching – Frame Relay, X.25, ATM
  • Circuit Switching – PPP, HDLC
  • Metro Ethernet – Ethernet
  • Broadband – PPPoE, PPPoA, Ethernet

HDLC – Cisco’s default point to point encapsulation for circuit switched and dedicated circuits

PPP –  Router to Router or host to network over synchronous or asynchronous circuits

Frame Relay –  Switched data link layer protocol handles multiple virtual circuits.  Successor to X.25

ATM – International standard for cell relay.  Uses fixed 53 byte cell switched technology.  Allows processing in hardware.

Broadband – PPPoE and PPPoA (256kbps to 24Mbps), provide transmission over the PSTN.  Cable Ethernet, data over cable TV infrastructure, 3-30Mbps.

Metro Ethernet – Point to point and multipoint.  Ethernet over dark fibre, SONET/SDH, RPR.

Overview of PPP

PPP supports 2 types of Physical Interfaces

Asynchronous Serial: POTS dialup

Synchronous Serial: ISDN or leased lines

Link Control Protocol (LLP) is used to negotiate and setup control options on the WAN link.

3 Phases of PPP Session Establishment

  1. Link Establishment: Each PP device sends LCP packets to configure and test the data link.  If a configuration option is not included in an LCP packet the default value is assumed.
  2. Authentication: If used, authentication takes place before the network layer protocol phase.  PAP and CHAP are supported.
  3. Network Layer Protocol: PPP devices send NCP packets to choose and configure the protocol for example IP.

PAP: Passwords are sent i clear text with no protection from playback/replay attacks.

CHAP: Uses 3 way handshake at the stat of the link and periodically thereafter to verify identity of the remote node.

Configuring and Verifying PPP

Enable PPP on an interface

Enable Authentication:

  • Configure router hostname
  • Configure username/password
  • PAP/CHAP

PPP summary

There are two components of PPP;

LCP negotiates the connection

NCP encapsulates the traffic

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