Configure serial interface with encapsulation HDLC

Configure serial interface with encapsulation HDLC

- in CCNA

Cisco High-Level Data Link Controller (HDLC) is the Cisco proprietary protocol for sending data over synchronous serial links. Cisco HDLC is the default for data encapsulation at Layer 2 (data link) of the Open System Interconnection (OSI) stack for efficient packet delineation and error control.

To configure HDLC encapsulation is:

Step 1.- Specifies the serial interface and enters interface configuration mode.

Router(config)# interface serial wic/slot/port

Step 2.- Configures synchronous serial encapsulation.

Router(config-if)# encapsulation { hdlc | frame-relay | ppp }

The above command is only required if the encapsulation is not HDLC; remember that HDLC encapsulation is the default.

Verification commands:

show interface serial wic/slot/port
show controllers serial wic/slot/port


Config R1:

R1(config)#interface serial 0/2/0
R1(config-if)#encapsulation hdlc
R1(config-if)#clock rate 64000
R1(config-if)#ip address
R1(config-if)#no shut

Config R2:

R2(config)#interface serial 0/1/1
R2(config-if)#encapsulation hdlc
R2(config-if)#ip address
R2(config-if)#no shut

%LINK-5-CHANGED: Interface Serial0/1/1, changed state to up
%LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/1/1, changed state to up



R1#show interfaces serial 0/2/0
Serial0/2/0 is up, line protocol is up (connected)
Hardware is HD64570
Internet address is
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation HDLC, loopback not set, keepalive set (10 sec)
Last input never, output never, output hang never
Last clearing of "show interface" counters never

R1#show controllers serial 0/2/0
Interface Serial0/2/0
Hardware is PowerQUICC MPC860
DCE V.35, clock rate 64000
idb at 0x81081AC4, driver data structure at 0x81084AC0
SCC Registers:
General [GSMR]=0x2:0x00000000, Protocol-specific [PSMR]=0x8
Events [SCCE]=0x0000, Mask [SCCM]=0x0000, Status [SCCS]=0x00

If there is some issue with HDLC implementation, protocol status will be down. There are three possible reasons:

  • Remote side router in Non-Cisco Router.
  • Remote side is using other protocol such as PPP.
  • DCE device is not providing clock rate to DTE device.


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