Cisco CCNA Packet Tracer Ultimate labs: PPP & PPP CHAP: Answers Part 1

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The Point-to-Point Protocol (PPP) provides a standard method for transporting multi-protocol datagrams over point-to-point links. PPP is comprised of three main components:

● A method for encapsulating multi-protocol datagrams.
● A Link Control Protocol (LCP) for establishing, configuring, and testing the data-link connection.
● A family of Network Control Protocols (NCPs) for establishing and configuring different network-layer protocols.

The Challenge Handshake Authentication Protocol (CHAP) (defined in RFC 1994) verifies the identity of the peer by means of a three-way handshake. These are the general steps performed in CHAP:

After the LCP (Link Control Protocol) phase is complete, and CHAP is negotiated between both devices, the authenticator sends a challenge message to the peer.

The peer responds with a value calculated through a one-way hash function (Message Digest 5 (MD5)).

The authenticator checks the response against its own calculation of the expected hash value. If the values match, the authentication is successful. Otherwise, the connection is terminated.

This authentication method depends on a “secret” known only to the authenticator and the peer. The secret is not sent over the link. Although the authentication is only one-way, you can negotiate CHAP in both directions, with the help of the same secret set for mutual authentication.

For more information on the advantages and disadvantages of CHAP, refer to RFC 1994


In this lab you need to configure Point-to-Point Protocol or PPP.
Okay, so we’ll start the required tasks first.
The first thing we need to do is configure the link between Customer 1 and ISP1 with PPP.
So in other words, this link here needs to be configured with PPP. I’ll start with ISP 1.

The ISP router has booted up. Go to enable mode, show ip interface brief
We can see in the output that this interface gigabit 0.0.1 is configured with IP address interfaces up up.

This interface serial 0.1.0 the link to the customer is currently administratively shutdown.
show interface serial 0.1.0
shows us that this interface is configured with default encapsulation of HDLC.
The interface is once again administratively shutdown.
So layer 1 and layer 2 are down, notice again that the default encapsulation is HDLC.

So I’ll go on to the interface and configure an IP address because no IP address is currently configured on the interface.
So ip address, the subnet mask it used here is /24 subnet mask.
In the real world, on Point-to-Point Protocol links, you’re probably going to use a /30 mask to conserve IP addresses. But in a lab like this we don’t have to worry too much about that.

Next thing I’ll configure is the encapsulation which I’m going to set to PPP and then I’ll no shut or enable the interface.
So show interface serial 0.1.0
interface is currently up at Layer 1, Layer 2 is down because we haven’t configured the other side of the link. Notice the capsulation is now PPP, LCP or Link Control Protocol, is closed. NCPs or Network Control Protocols such as IPCP and CDPCP are also closed because the link is down.

so show run
that’s the configuration of the ISP side.
Let’s do something similar on the customer side.

So show interface serial 0.1.0
The physical interfaces is up but the line protocol is down that’s because the encapsulation is HDLC on this side but on the ISP side, it’s PPP.
So again, show interface serial 1/0 on the ISP side. Layer 1 is up, Layer 2 is down. Same on the other side, Layer 1 is up, Layer 2 is down.

The routers are using different encapsulations, so we need to configure them to use the same encapsulation.

Before I do that, do show ip interface brief……

  • David Bombal