When is a first-hop redundancy protocol needed in the distribution layer?（）
A. when HSRP is not supported by the design
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 switch
F. when the design implements Layer 3 between the access switch and the distribution switch
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