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 block