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Hands-On Lab: Configuring PPP Encapsulation

This hands-on lab covers replacing the default Cisco HDLC encapsulation with the open-standard PPP protocol on a serial link, verifying the LCP and NCP negotiation phases, and confirming the link functions correctly.

PPP ConfigurationLCP NegotiationSerial WAN Protocol

~2 min read · Updated Sep 12, 2026

Lab Objective

Configure PPP encapsulation on both ends of a serial link, verify the negotiation completes successfully, and confirm IP connectivity across the link.

Lab Purpose

Unlike Cisco HDLC, which is a Cisco-proprietary protocol, PPP is an open standard supported by virtually every vendor, making it essential whenever a serial link connects a Cisco device to non-Cisco equipment. PPP also serves as the foundation for the authentication methods covered in later labs.

Lab Topology

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

R1: 10.9.9.1/30
R2: 10.9.9.2/30

Task 1: Configure Basic Addressing and Clocking

Configure both serial interfaces with the addresses shown, applying clock rate on R2.

Task 2: Configure PPP Encapsulation

Change the encapsulation on both routers' serial interfaces from the default to PPP.

Task 3: Verify PPP Negotiation States

Use a show command to confirm both the LCP (Link Control Protocol) and IPCP (IP Control Protocol, an NCP) have reached the "open" state.

Task 4: Verify Connectivity

Confirm R1 can successfully ping R2.

Solution and Verification

R1(config)# interface serial0/0/0
R1(config-if)# ip address 10.9.9.1 255.255.255.252
R1(config-if)# encapsulation ppp
R1(config-if)# no shutdown

R2(config)# interface serial0/0/0
R2(config-if)# ip address 10.9.9.2 255.255.255.252
R2(config-if)# encapsulation ppp
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 PPP, loopback not set

R1# debug ppp negotiation
-- (optional, for observing the live negotiation
--  process during interface flap)

R1# show ppp all

Se0/0/0    LCP Open
           IPCP: IPv4 Open

R1# ping 10.9.9.2

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

Key Takeaway

PPP negotiation happens in layers: LCP first establishes and tests the basic link itself, and only once LCP reaches "Open" does the appropriate NCP (IPCP for IPv4) negotiate the network-layer parameters — a link showing LCP open but IPCP not open indicates the basic link works but something specific to IP configuration (such as an addressing mismatch) still needs attention.

Written & researched by Dr. Shahin Siami

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