HDLC
HDLC is the default encapsulation used by Cisco routers over synchronous
serial links. HDLC is a bit-oriented ISO standard Data Link layer
protocol. It specifies a method to encapsulate data over synchronous
serial links using frame characters and checksums. HDLC is a
point-to-point protocol used on leased lines between Cisco devices. If you
need to establish a link between a Cisco device and a non Cisco device, you must
use PPP encapsulation instead of HDLC. No authentication can be used with
HDLC. The reason each vendor has a proprietary encapsulation of HDLC is
that they each have a different way for the HDLC protocol to communicate with
the Network layer protocols, and the ISO standard doesn't allow for multiple
protocols on a single link. They needed a way for HDLC to communicate with
layer-3 protocols so HDLC can carry more than one type of protocol.
Example of Configuring HDLC Encapsulation
Although HDLC is the default encapsulation for serial interfaces, you might
have to change it back to HDLC from another encapsulation method; to do that use
the following commands:
Router3#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Router3(config)#int s0
Router3(config-if)#encapsulation ?
atm-dxi ATM-DXI encapsulation
frame-relay Frame Relay networks
hdlc Serial HDLC synchronous
lapb LAPB (X.25 Level 2)
ppp Point-to-Point protocol
smds Switched Megabit Data Service (SMDS)
x25 X.25
Router3(config-if)#encapsulation hdlc
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