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 pppin the interface configuration - If you change the encapsulation, it must match on both ends or the interface will go down
- you can manually switch to PPP encapsulation with
- You can tell if a router is DCE or DTE with
show controllers (interface-ID)
HDLC Frame
| Address | Control | Protocol Code | Information | Frame Check Sequence (FCS) | Flag |
|---|---|---|---|---|---|
| 8 bits | 8 bits | 16 bits | Variable; 0 or more bits; multiples of 8 | 16 bits | 8 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)