I found this question and answer in CISCO PRESS ICND2 CHAPTER 12 REVIEW QUESTIONS!
Two routers have a serial link between them,with the link configured to use PPP and with EIGRP configured correctly for all the interfaces.The engineer can ping the IP ADDRESS on the other end of the link,but not the IP ADDRESS of the other router's LAN interface.Which of the following is the likely cause of this problem?
a. The CSU/DSU connected to the other router is not switched ON
b. The serial IP ADDRESS on the router at the other end of the link is not in the same subnet
c. CHAP authentication failed.
d. The router on the other end of the link has been configured to use HDLC.
Explanation given: With PPP, two routers can use IP address in different subnets on opposite ends of the links and a ping to the other router's serial IP address works.However,this subnet match causes routing protocol to fail when forming neighbor relationships to exchange routes ,so neither router learns EIGRP routes from the other.
But in another one famous CISCO literature,I have found that
"If you have a point-to-point link but the encapsulations aren't the same, the link will never come up.
After that it is mentioned in the TOPIC MISMATCHED IP ADDRESSES,
Mismatched IP Addresses
A tricky problem to spot is if you have HDLC or PPP configured on your serial interface but your IP addresses are wrong. Things seem to be just fine because the interfaces will show that they are up. But it won't work since ip addresses are in different subnets."
I found contrary of these 2 answers.
Can anybody please explain?