PPP

Cisco Jan 26, 2025
  • Point to Point
  • Multipoint (Frame Relay, X.25, ATM)

HDLC is vendor-specific. Default on serial interfaces.

int s0
	encapsulation ppp
Flag (01111110) Address (11111111) Control (00000011) Protocol (C021) LCP DATA CRC
  • Link Establishment and Configuration Negotiation
  • Link Quality Determination
  • Allow network-layer protocol establishment
  • Link termination

MRU - maximum receive unit

Compression can be done with a Hardware card or Software

Software Compression

int s0
	compress [stac|predictor|mppc]

mppc - Microsoft endpoint (requires IOS> 11.1)

LQR - Link Quality Report

  • packets and bytes received
  • compares to what has been sent, error %

Magic Numbers

  • Detects loops in the line
  • Modem in loopback

Authentication

Optional

Password Auth Protocol (PAP)

  • name and password sent in clear text
  • Matched to Local or AAA database

Challenge handshake auth protocol (CHAP)

  • same shared secret on both devices
  • R1 sends a challenge
  • R2 encrypts with the shared secret
  • R1 encrypts with the shared secret and checks for a match

Tags

Craig Garnham

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