Ccna routing and Switching Complete Study Guide

sh controllers s0/0/0

Interface Serial0/0/0

Hardware is PowerQUICC MPC860

DCE V.35, clock rate 2000000

The preceding output shows a DCE interface that has the clock rate set to 

2000000

, 

which is the default for ISR routers. This next output shows a DTE connector, so you don’t 

need enter the 

clock rate

 command on this interface:

SF#

sh controllers s0/0/0

Interface Serial0/0/0

Hardware is PowerQUICC MPC860

DTE V.35 TX and RX clocks detected

Terms such as EIA/TIA-232, V.35, X.21, and HSSI (High-Speed Serial Inter-

face) describe the Physical layer between the DTE (router) and DCE device 

(CSU/DSU).

High-Level Data-Link Control (HDLC) 

Protocol

The High-Level Data-Link Control (HDLC) protocol is a popular ISO-standard, bit-

oriented, Data Link layer protocol. It specifies an encapsulation method for data on syn-

chronous serial data links using frame characters and checksums. HDLC is a point-to-point 

protocol used on leased lines. No authentication is provided by HDLC.

In byte-oriented protocols, control information is encoded using entire bytes. On the other 

hand, bit-oriented protocols use single bits to represent the control information. Some com-

mon bit-oriented protocols are SDLC and HDLC; TCP and IP are byte-oriented protocols.

HDLC is the default encapsulation used by Cisco routers over synchronous serial links. 

And Cisco’s HDLC is proprietary, meaning it won’t communicate with any other vendor’s 

---

High-Level Data-Link Control (HDLC) Protocol 

897

HDLC implementation. But don’t give Cisco grief for it—everyone’s HDLC implementa-

tion is proprietary. Figure 21.10 shows the Cisco HDLC format.

f i g u r e   21.10     Cisco’s HDLC frame format: Each vendor’s HDLC has a proprietary data 

field to support multiprotocol environments.

Cisco HDLC

Flag

Address

Control

Proprietary

Data

FCS

Flag

Supports only single-protocol environments

HDLC

Flag

Address

Control

Data

FCS

Flag

The reason every vendor has a proprietary HDLC encapsulation method is that each 

vendor has a different way for the HDLC protocol to encapsulate multiple Network layer 

protocols. If the vendors didn’t have a way for HDLC to communicate the different layer 3 

protocols, then HDLC would be able to operate in only a single layer 3 protocol environ-

ment. This proprietary header is placed in the data field of the HDLC encapsulation.

It’s pretty simple to configure a serial interface if you’re just going to connect two Cisco 

routers across a T1, for example. Figure 21.11 shows a point-to-point connection between 

two cities.

f i g u r e   21.11     Configuring Cisco’s HDLC proprietary WAN encapsulation