HDLC and PPP Written Notes

These are my ‘crib notes’ that I’ve made to serve as a last minute refresher. Please forgive the grammer / spelling as I did not develop these notes with publishing in mind.


*** Theory ***

  • HDLC – Cisco made it properity by adding keepalives to see if the line is up or not.  SLARP (Serial Link Address Resolution Protocol) – assigns itself an IP address for TFTP etc configs. Added STAC compression &increases performance with only 7-12 bytes. Can be seen as the netbui of WAN.
  • STAC Compression – do under interface, you will need to do on both routers / both end of links otherwise the link will be kind of down as one side is compressing and the other side isn’t. STAC also taxes the CPU
  • Predictor Compression – less CPU than STAC, but uses more memory. It tries to predict the next data and uses that compression for it
  • MPPC – used for Microsoft environment / clients
  • Troubleshoot – use debug interface serial to check HDLC messages. If 3 messages are missed the link is declared down
    • myseq – is the seq no. You are sending to the otherside
    • myseen – they are acknowledging
    • yourseen – is the number you have seen

  • PPP is an alternate WAN comms to HDLC.
  • Multilink PPP – You can bundle 2 or more serial PPP links as a logical link and then load balance between them.
    • For each serial connection you must specify the multilink-group [1], encapsulation ppp, ppp multilink
    • Then create interface multilink [1]. Give this the IP address, encapusaltion ppp, ppp multilink and multilink-group [1]
    • Show ppp multilink – will show all interfaces assigned and if they are active etc

*** Commands ***



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