单项选择题
A. when the design implements Layer 2 between the access arid distribution blocks
B. when multiple vendor devices need to be supported
C. when preempt tuning of the default gateway is needed
D. when a robust method of backing up the default gateway is needed
E. when the design implements Layer 2 between the access switch and the distribution blocks
F. when the design implements Layer 2 between the access and distribution blocks