An older interface, largely displaced by Ethernet

  • One side functions as DCE (Data Communications Equipment)
  • The other side functions as DTE (Data Terminal Equipment)
  • The DCE side specifies the clock rate (speed) of the connection
  • Default encapsulation on a serial interface is HDLC (actually cHDLC (Cisco HDLC) on Cisco devices)
    • you can manually switch to PPP encapsulation with encapsulation ppp in the interface configuration
    • If you change the encapsulation, it must match on both ends or the interface will go down
  • You can tell if a router is DCE or DTE with show controllers (interface-ID)

HDLC Frame

AddressControlProtocol CodeInformationFrame Check
Sequence (FCS)
8 bits8 bits16 bitsVariable; 0 or more bits; multiples of 816 bits8 bits
  • Control field is always set to zero (0x00)
  • Protocol Code field denotes the protocol of the encapsulated data (e.g. 0x0800 for Internet Protocol)