[单项选择] During testing of network redundancy in a fully meshed enterprise campus， a problem has been reported. Both distribution multilayer switches are fully configured for redundancy； clients operate normally when the primary multilayer distribution switch is operating. All clients drop connections outside of their respective access layer group when the primary distribution switch is shut down and the secondary distribution switch is still operational. Additional information indicates that the secondary  distribution switch is able to ping major corporate and Internet locations.  Which item would be the best point of troubleshooting for this connectivity issue？ （）
A. client network configuration
B. VLAN mapping
C. inter-switch trunk configuration
D. UDLD configuration
E. first-hop redundancy protocol