In Ch. 13 of the CCNA Routing and Switching ICND2 200-105 Official Cert Guide, Table 13-5 describes likely general causes for a ping failure based on the line/protocol status.

The one show command that lists PPP details is the show interfaces command, with an example from R1 listed in Example 13-5. The output looks just like it does for HDLC up until the first highlighted line in the example. The two highlighted lines confirm the configuration (“Encapsulation PPP”). These lines also confirm that LCP has completed its work successfully, as noted with the “LCP Open” phrase. Finally, the output lists the fact that two CPs, CDPCP and IPCP, have also successfully been enabled—all good indications that PPP is working properly.

Example 13-5 Finding PPP, LCP, and NCP Status with show interfaces

R1# show interfaces serial 0/0/0
Serial0/0/0 is up, line protocol is up
  Hardware is WIC MBRD Serial
  Description: link to R2
  Internet address is 192.168.2.1/24
  MTU 1500 bytes, BW 1544 Kbit/sec, DLY 20000 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation PPP, LCP Open  Open: IPCP, CDPCP, loopback not set
! Lines omitted for brevity

Create a 2- to 3-page troubleshooting guide based on this table of ping failures. For each state combination, list the steps you would take to further understand any problems at each appropriate layer and verify that your configuration is correct. Make sure your steps are logical, clear, and appropriate for WAN protocol.