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