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
Example:
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 10.1.1.1 255.255.255.252
R1(config-if)#no shut
R1(config)#
Config R2:
R2(config)#interface serial 0/1/1
R2(config-if)#encapsulation hdlc
R2(config-if)#ip address 10.1.1.2 255.255.255.252
R2(config-if)#no shut
R2(config-if)#exit
%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
R2(config)#
Validation:
R1#show interfaces serial 0/2/0
Serial0/2/0 is up, line protocol is up (connected)
Hardware is HD64570
Internet address is 10.1.1.1/30
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.
Reference:
