A: LAN Broadcasts

By Wendell September 20, 2018 09:05

The term LAN broadcast may have a specific meaning to you, with the contents of the Ethernet header leaping from your memory. But maybe not. What about the source address? Type field?  Check out the question first, and then come back here to check out the answers.

The Answers:



Background: What does “LAN Broadcast” Mean?

This question revolves around the term LAN Broadcast. Generally, the term refers to an Ethernet frame that, by intent, should be delivered to every device in the same LAN (that is, in the same VLAN). Sometimes, new CCNA learners get that general idea (a frame sent to all devices in a LAN) but do not understand the mechanisms.

More specifically, the term refers to an Ethernet frame with a destination MAC address of FFFF.FFFF.FFFF (known as the broadcast address). All switches in the LAN then know to flood the frame out all ports in that VLAN, with some restrictions on outgoing ports to prevent the frame copies from repeatedly looping around the LAN.


Figure 2: Data Link Addresses PC1 Towards PC2


Why the Right Answer is Right

A LAN broadcast should use the broadcast address as the destination address. Additionally, the source MAC address field should never list the broadcast address (FFFF.FFFF.FFFF) – it makes no sense for the source of the frame to be “all devices.” So, the answer that states the frame uses a broadcast destination MAC, but not a broadcast source MAC, is the correct answer.


Why the Wrong Answers are Wrong

Two wrong answers would require the use of a source MAC address of FFFF.FFFF.FFFF. By convention, the source MAC should never use the broadcast MAC address; the sender (the source) should always list the unicast MAC address of the device that created the frame. Those facts rule out two of the answers.

Finally, one answer mentions the concept of a Broadcast Type field value. As shown in Figure 2, the Type field (also called the Protocol Type field) lists specific values to identify the specific type of protocol header that follows the Ethernet header. The Type field identifies the specific type of header, and does not have a concept or value that means “broadcast.”

Q: LAN Broadcasts
Q: WAN Basics
By Wendell September 20, 2018 09:05
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