When is a first-hop redundancy protocol needed in the distribution layer？（）

• 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