T1 PPP and HDLC Configuration Cisco

By | 11/05/2017

Real World Application & Core Knowledge

As a network engineer you will encounter T1 links very often as T1’s are the de facto standard of nearly all small branch offices. T1’s operate at 1.544Mbps (1544Kbps) and now days commonly drop into a facilities point of presence (pop) or commonly (BPOP) as the POP is most commonly in the basement.

T1’s can also be fractional T1’s where the bandwidth is limited to 512Kbps, 768Kbps, or even 1Mbps. Commonly with fractional T1’s you’ll have the ability to “burst” which gives the router the ability to send more then its committed information rate (CIR) at specific times of the clocking rate. However, those concepts are beyond the scope of the CCNA blueprint.

Most Newer T1’s installations are copper twisted pair T1 which connects to a WIC-1DSU-T1, WIC-1DSU-T1-V2 or other integrated CSU/DSU T1 controllers. However you may encounter older T1 installations where the CSU/DSU is external and you’ll see a v.35 cable ran to a WIC-1T or WIC-2T.

WIC-1T’s are the most common lab serial interfaces and when configuring such interfaces you’ll be required to set the clock rate on the DCE (Data Communications Equipment) which is done by the ISP whereas the DTE (Data Termination Equipment) is done at the consumer end.

In this lab you’ll familiarize yourself with the following commands;

Command Description
show controller serial #/# Shows controller information about the specified serial interface including the clock rate and cable termination type (DTE or DCE)
clock rate # This command is executed in serial interface configuration mode to set the clock rate of a DCE termination point of a serial link.
encapsulation [ HDLC | PPP ] This command when executed in Serial interface configuration mode configures the interface encapsulation to Cisco HDLC (High-Level Data Link Control protocol) or the Industry Standard PPP (Point to Point Protocol)
show interface serial #/# This command when executed in privileged mode will display Serial interface information such as encapsulaton, MTU, up time, current utilization and more.

Lab Prerequisites

  • If you are using GNS3 than load the Free CCNA Workbook GNS3 topology than start devices; R1 and R2.
  • Establish a console session with devices R1 and R2 than configure the devices respected hostname(s).
  • Configure R1’s Serial0/1 interface with the IP address 172.18.21.1/30 and R2’s Serial0/1 interface with the IP address 172.18.21.2/30

Lab Objectives

  • Configure both R1 and R2’s Serial0/1 interface to encapsulate traffic using the HDLC encapsulation.Verify your configuration by using a show command to display the interface encapsulation and by pinging R2 from R1.
  • Configure both R1 and R2’s Serial0/1 interface to encapsulate traffic using the PPP encapsulation. Verify your configuration by using a show command to display the interface encapsulation and by pinging R2 from R1.

Lab Instruction

Objective 1. – Configure both R1 and R2’s Serial0/1 interface to encapsulate traffic using the HDLC encapsulation. Verify your configuration by using a show command to display the interface encapsulation and by pinging R2 from R1.

The configuration part of this objective is a trick question however the commands shown below will demonstrate how to configure HDLC. Keep in mind that HDLC is the default serial interface encapsulation on Cisco routers. You can verify the serial interface encapsulation by using the show interface serial #/# command in privileged mode.

Interface Encapsulation verification shown below;

Ping verification from R1 to R2 shown below;

Objective 2. – Configure both R1 and R2’s Serial0/1 interface to encapsulate traffic using the PPP encapsulation. Verify your configuration by using a show command to display the interface encapsulation and by pinging R2 from R1.

You’ll notice when changing the encapsulation from HDLC to PPP on R1 that the line protocol will go down, this is due to an encapsulation mis-match. Once R2’s Serial0/1 interface is configured with the matching encapsulation the line protocol will go back up.

Encapsulation and ping verification shown below;

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