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Hands-On Lab: Verifying Cisco HDLC Encapsulation

This hands-on lab confirms that Cisco HDLC is the default WAN encapsulation on serial interfaces, demonstrates what happens when the two ends of a link use mismatched encapsulation, and covers the verification commands used to diagnose this common WAN connectivity problem.

Cisco HDLCSerial EncapsulationEncapsulation Mismatch

~3 min read · Updated Sep 12, 2026

Lab Objective

Verify that Cisco HDLC is running by default on a serial link between two routers, deliberately introduce an encapsulation mismatch, observe the resulting failure, and correct it.

Lab Purpose

Encapsulation mismatches are a classic serial link problem: the interface can show as physically up, yet no traffic passes, because the two ends are speaking different Layer 2 protocols. Recognizing this specific failure signature quickly saves significant troubleshooting time in the field.

Lab Topology

R1 ---- Serial0/0/0 (DTE) ------ Serial0/0/0 (DCE) ---- R2

R1: 10.8.8.1/30
R2: 10.8.8.2/30

Task 1: Configure Basic Addressing and Clocking

Configure both interfaces with the addresses shown, and apply an appropriate clock rate on R2 (the DCE side).

Task 2: Verify Default Encapsulation

Without configuring encapsulation explicitly, use a show command to confirm which encapsulation is active by default on both routers' serial interfaces.

Task 3: Confirm Connectivity

Ping between the two routers to confirm the link works correctly with matching default encapsulation.

Task 4: Introduce an Encapsulation Mismatch

Change R2's serial interface encapsulation to PPP while leaving R1 on the default HDLC, then attempt to ping again and observe the failure.

Task 5: Correct the Mismatch

Restore matching encapsulation on both routers and confirm connectivity is restored.

Solution and Verification

R1(config)# interface serial0/0/0
R1(config-if)# ip address 10.8.8.1 255.255.255.252
R1(config-if)# no shutdown

R2(config)# interface serial0/0/0
R2(config-if)# ip address 10.8.8.2 255.255.255.252
R2(config-if)# clock rate 64000
R2(config-if)# no shutdown

R1# show interfaces serial0/0/0

Serial0/0/0 is up, line protocol is up
  Hardware is GT96K Serial
  Encapsulation HDLC, loopback not set

R1# ping 10.8.8.2

!!!!!
Success rate is 100 percent (5/5)

R2(config)# interface serial0/0/0
R2(config-if)# encapsulation ppp

R1# ping 10.8.8.2

.....
Success rate is 0 percent (0/5)

R1# show interfaces serial0/0/0

Serial0/0/0 is up, line protocol is down
-- Physically up, but line protocol down --
-- exactly the same symptom seen with a
-- missing clock rate, which is why checking
-- encapsulation on BOTH ends is a required
-- step whenever this specific symptom appears

R2(config)# interface serial0/0/0
R2(config-if)# encapsulation hdlc

R1# ping 10.8.8.2

!!!!!
Success rate is 100 percent (5/5)

Key Takeaway

An encapsulation mismatch and a missing clock rate produce the identical "up, line protocol down" symptom on a serial interface — since both causes look the same from a single show command, a complete serial troubleshooting checklist must always verify both clocking and matching encapsulation on each end.

Written & researched by Dr. Shahin Siami

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