A: LAN Broadcasts

 In 200-301 V1 Ch02: Intro to Ethernet, CCENT-OLD, Q&A

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
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Hi Mr. Wendell.
I’m glad to answer to all like this questions after education chapters. It’s very interesting and educating.
But why only two question? I mean where I can find more questions like yours on that themes which I learned in book and want to practice in consaludated learning material.
Thanks a lot!


You’re welcome!
Well, I count 100+ on the two blog sites (blog.certskillscom/ccent and http://www.certskills.com/ccna). But I agree, it’d be nice to have more. I’ve been a bit lax in creating new content – making a plan! Right now, catching up on comments!!
My books come with… appx. 300-350 questions in each book with the PTP QA tool, plus copies of all the “Do I Know This Already” (DIKTA) questions. So when you buy the books, you get somewhere more than 400 questions in each book.
Then if you buy the Premium Edition eBook for another $10 each, that comes with two more practice exams, about 90 questions each. (Make sure and look for the 70% off coupon in the DVD sleeve of the book.) That gets you another 180 or so questions if you buy the ICND1 Premium Edition, and likewise if you buy the ICND2 Premium Edition. You can read more about those here (and if you go there, I just saw that I have some broken figure links there…) https://www.certskills.com/ccna/ann2016-10/

Finally, note that if all you want are the questions, and you haven’t bought the books yet, just buy the Premium Editions. That gives you the books as eBook (many formats), PLUS all the questions that come with the book, plus the two extra exam banks. That is, buy both the ICND1 OCG Premium Edition and the ICND2 Official Cert Guide Premium Edition and you’ll get…1100? 1200? about that many questions.

Hope this helps!

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