The disruption in IS-IS connectivity between R1 and R2, despite the interfaces being up, suggests an issue with packet handling or MTU configuration. Action A, disabling padding for hello packets under the serial interface on R2, is necessary because excessive padding can lead to issues in processing hello packets effectively. Action C, changing the MTU to 1500 bytes on R2, addresses potential MTU mismatch issues. MTU mismatches are common problems that can interrupt connectivity in networks using routing protocols like IS-IS; ensuring both routers operate at the same MTU can resolve this issue.

References := The explanation is derived from the principles outlined in Cisco’s Implementing and Operating Cisco Service Provider Network Core Technologies, which emphasizes the importance of proper packet handling and MTU configuration for maintaining IS-IS connectivity.