Verifying PPP

Verifying PPP

Objectives:

  • Configure and verify a basic WAN serial connection
  • Configure and verify a PPP connection between Cisco routers

To ensure your PPP connection came up successfully, you can always use the ol’ faithful show ip interface brief command.

In this case, all is well with the Serial 0 PPP connection between the routers because the Protocol is stated as up. Remember, the Status column generally dictates the Physical layer connectivity, whereas the Protocol column focuses on the Data Link connectivity. If you want to get a little more in depth with the PPP negotiation on the interface, issue the show interface <interface> command as follows:

As you can see from the output above, the router has negotiated Link Control Protocol (LCP) options, which is indicated by the LCP Open state. If the LCP negotiations had failed (most likely because of an authentication problem), the LCP state would rotate between Listen, ACKSent, or TERMSent. This is the Cisco router trying to go through the negotiation of the LCP options.

In the line below LCP Open, you can verify all the Network layer communication occurring across the PPP link. In this case, you can see IPCP (indicates the TCP/IP protocol), CCP (Indicates compression is in effect—compressed control protocol [CCP]), and CDPCP (indicates the Cisco Discovery Protocol [CDP]). Technically, CDP is a Data Link protocol; however, Cisco adopted it to connect into the PPP options as a Layer 3 protocol.

EXAM ALERT
Be able to interpret the show interface command output as it relates to the PPP options. Finally, if you would like to see how your PPP compression is working out, you can type the command show compress. This gives you the compression statistics for the line:

In this case, you have no compressed bytes because all the traffic sent over the PPP link was generated by the routers themselves (this traffic is exempt from compression).

Troubleshooting PPP

Objectives:

  • Configure and verify a basic WAN serial connection
  • Configure and verify a PPP connection between Cisco routers
  • Troubleshoot WAN implementation issues

Whenever you reach the troubleshooting section for any topic, you can be guaranteed some debug output. In this case, the debug commands for PPP are pretty darn useful: Most of the output is easy to understand. The most useful command that I have found to troubleshoot a PPP link is the debug ppp negotiation command. Check out this output:

Isn’t that some great stuff?!? Okay, it does take a little bit of screening through all the output, but if you look about halfway through, you can see the exchange of the Robin and Pigeon hostnames. This shows the challenge/response action of the CHAP protocol.

Thankfully, you can see a SUCCESS message to finish it off, showing the Pigeon and Robin routers have successfully authenticated each other. Near the bottom of the output, you see the CCP (compression negotiation) negotiate the Predictor algorithm between the two routers. Finally, at the end of the output, you see the link come up.

If your PPP connection is failing, this debug command will definitely show you the cause (as long as the failure is related to PPP). Another popular command is debug ppp authentication, which gives the same output, but slims it down to just the authentication information (because this is where many failures occur).

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Prasanna

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