Caption script:

Overview：PPP connection

Goal: Visualize PPP layer 2 (LCP) and layer 3 (NCP) connection setup and termination.
Topology: Two routers are connected by a serial link.
Steps: 1) Confgure serial interfaces, encapsulate PPP
           2) Setup PPP connection in 2 steps: First use LCP for link layer, then use NCP for network layer.
           3) Transmit data over PPP connection.
           4) Shutdown interface and terminate PPP connection in 2 steps: NCP, LCP. 

PPP connection setup flow

PPP connection setup takes 3 steps:
1) Link layer connectivity. PPP uses LCP to negotiate selectoions with the remote node.
2) When negotiation succeeds, it authenticates the remote node. (PPP or CHAP)
3) Then PPP uses NCP to negotiate IP parameters and binds IP to PPP.
The first PPP animation (1. Connection) illustrates step 1 and 3.
The second and third animation (PAP, CHAP) visualizes step 2.
This animation is for PPP connection (step 1, 3).

Configure: encapsulate PPP

At R1, R2's serial interface, do:
- configure IP address
- select PPP for encapsulation
Once this is done, PPP starts to configure the link automatically with LCP.

LCP: setup link connection

PPP connection is established in 2 steps: First,  it use LCP to negotiate Link parameters. Tth steps are:
1) R1 (Sender) transmits a Conigure Request frame to propose its selections.
   (e.g., datagram size, escaped characters, magic numbers) and optional authentication. )
2) R2 (Receiver) checks the proposed selections. If agrees, it responds a Configure Ack. 
Note: If negotiation failss. receiver sends a configure Nak.  And sender needs to select a different option 
         to renegotiate.

NCP: set up network connection

After establishing a link layer connection, R1 uses NCP to negotiate network layer selections with R2.
- R1 sends IPCP Configure Request to R2
- R2 agrees with R1's selections and responds a IPCP Config-Ack.
Now R1 and R2 are connected at network layer. They can exchange data packets
Note: NCP binds network protocols to PPP. For IP, its NCP is called IPCP.
Note: Link connection is authetoication before doing NCP negotitaion. We skipped authtication in this tutorial. 
         See the next two tutorials for PAP and CHAP aauthentication.

Verify interface state

Now run "show interface" to check interface state
It is up and is ready for data transfer/

Test PPP connection: R1 ping R2

R1 can ping R2 over the newly established PPP connection.

Shit interface: End IP connection

- Manually shutdown the serial interrface.
- It triggers R1 to send IPCP Terminate Request to R2.
- R2 responds by ICP Terminate Ack.
Note: When NCP is donw, LCP is still up.

Terminate PPP LCP connection

After receiving R2's IPCP response, R1 continutes its showdown.
R1 sends LCP Terminate Request to R2.
R2 responds by sending LCP Terminate Ack to R1.
Now PPP conection is closed completely.

Check interface state

Run show iinterface again. every thing is down  now.

FAQ