CCNA Exam 200-125: Why a Switch Doesn't Learn Broadcast Addresses

Why Switches Don't Learn Broadcast Addresses

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Why will a switch never learn a broadcast address?



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A. B. C. D. E.


The correct answer is B. A broadcast frame is never forwarded by a switch.

A switch is a network device that operates at the data link layer of the OSI model, which means it uses MAC addresses to forward frames between connected devices. When a switch receives a frame, it learns the source MAC address of the frame and associates it with the interface on which the frame was received in its switching table. This way, the switch can forward frames to the correct destination without flooding the entire network.

A broadcast frame is a type of Ethernet frame that is sent to every device on a network. It has a destination MAC address of all Fs (FF:FF:FF:FF:FF:FF) in the Ethernet header. Broadcast frames are used for various purposes, such as DHCP requests, ARP requests, and some network protocols.

However, switches are designed to forward frames based on their destination MAC address, not their source or broadcast address. When a switch receives a broadcast frame, it floods it out all ports except the one on which it was received. This behavior ensures that all devices on the network receive the broadcast frame, including those that are not directly connected to the sender.

Because broadcast frames are never forwarded based on their destination MAC address, switches do not learn them in their switching table. Therefore, a switch will never associate the broadcast address (FF:FF:FF:FF:FF:FF) with any particular interface.

In summary, a switch never learns a broadcast address because broadcast frames are never forwarded based on their destination MAC address.