two routers named testking1 and testking2 are connected via their serial interfaces as illustrated, but they are unable to communicate. the testking1 router is known to have the correct configuration. given  the partial configurations, identify the fault on the testking2 router that is causing the lack of  
connectivity.  
a. incomplete ip address  
b. insufficient bandwidth  
c. incorrect subnet mask  
d. incompatible encapsulation  
e. link reliability too low  
f. ipcp closed  

answer: d  

explanation:  
hdlc and ppp configuration  
hdlc and ppp configuration is straightforward. you just need to be sure to configure the same wan data-link  protocol on each end of the serial link. otherwise, the routers will misinterpret the incoming frames, because  each wan data-link protocol uses a different frame format. other than configuring some optional features,  that’s all you need to do.