RouterA and RouterB, which connect two locations, are unable to communicate. You run the show running-configuration command on both router interfaces,
RouterA and RouterB. The following is a partial output:
routerA#show running-config
interface Serial0
description Router_A
ip address 192.10.191.2 255.255.255.0
encapsulation ppp
no ip mroute-cache
clockrate 64000
routerB#show running-config
interface Serial1
description Router_B
ip address 192.10.192.1 255.255.255.0
encapsulation ppp
no ip mroute-cache
clockrate 64000
Based on the information given in the output, what are two likely causes of the problem? (Choose two.)

A.
The IP address defined is incorrect.

B.
Both routers cannot have a clock rate defined.

C.
Both routers cannot have an identical clock rate.

D.
The Layer 2 framing is misconfigured.

E.
At least one of the routers must have the ip mroute-cache command enabled.

Explanation:
Two possible causes of the problem are that the IP addresses are incorrect as defined, or that both routers have a defined clock rate. The IP addresses on the

routers are in different subnets. The IP addresses need to be changed to fall in the same subnet.
Both routers cannot have a clock rate configured. Only routers with a DCE cable connected should have a clock rate, which provides synchronization to the router
connected to the DTE cable. In a point-to-point serial connection, the DCE cable connects to the DTE cable, providing a communication path between the two
routers. If both computers have a clock rate configured, the routers will not communicate.
A matching clock rate is not the problem. The clock rates between two routers should match. The router connected to the DCE cable will provide the clock rate to
the router connected to the DTE cable, resulting in matching clock rates.
The Layer 2 encapsulation refers to the Data Link protocol used on the link. In this case, the protocol is Point to Point Protocol (PPP), which is configured correctly
on both ends as indicated by the matching encapsulation ppp statements in the output. The connection would be prevented from working if one of the routers were
missing this setting (which would be indicated by the absence of the encapsulation ppp statement in its output), or if a different Layer 2 encapsulation type were
configured, such as High-Level Data Link Control (HDLC).
The ip mroute-cache command is used to fast-switch multicast packets and would not cause the problem in this scenario.
Objective:
Network Fundamentals
Sub-Objective:
Configure, verify, and troubleshoot IPv4 addressing and subnetting

http://docwiki.cisco.com/wiki/Point-to-Point_Protocol
https://www.cisco.com/c/en/us/td/docs/ios/redirect/eol.html