CCENT Question: LAN Switching Logic

certskills
By certskills June 4, 2014 09:05

Today’s post is a #CCENT question about the basics of LAN switching. It has a similar slant to some of the recent questions here in the blog, but focused on layer 2. I’ll post the answer and explanation in a few days. Ask questions and discuss if you like. Enjoy!

The figure shows a small enterprise network. The switches all have default configuration, unless otherwise noted in the question. The PCs have been configured, as well as the routers, so that it is possible for all hosts to ping each other. That also means that all the cabling shown in the figure works, and all interfaces are up.

 

On the left side of the figure, all switch interfaces default to be in the same VLAN (VLAN 1). Similarly, on the right, all devices sit in the same VLAN (VLAN 2).

Use the information above when answering the following question:

Switch SW1 receives an Ethernet frame from host A. SW1 and have SW2 have all default configuration, and they have not learned any dynamic entries for any tables yet. Which of the following are true statements about interfaces out which this Ethernet frame will be forwarded?

A. SW1 F0/1

B. SW1 F0/2

C. SW1 G0/1

D. SW1 G0/2

E. SW2 F0/9

F. R1 S0/0/0

V1-P1-2-1

Happy 25th to Cisco Live!
Answers to the "LAN Switching Logic" Question
certskills
By certskills June 4, 2014 09:05
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24 Comments

  1. Ali June 4, 09:38

    B. SW1 F0/2

    Reply to this comment
  2. A June 4, 09:39

    Hi!

    Aswer: B, C, D

    Reply to this comment
  3. Carlos Hernandez June 4, 10:51

    B, C, D

    Because the Switch has not learned any dynamic entries so the swtich logic is to flood that Ethernet frame.

    Reply to this comment
  4. CCENTSkills June 4, 11:20

    Keep the comments coming! I’ll post my own opinion and explanation in a few days. Thanks…
    Wendell

    Reply to this comment
  5. Adil Ahmed June 4, 11:29

    The switch will act like Hub initially, as the Switch has not learned any dynamic entries

    Correct answer is B,C,D

    Reply to this comment
  6. rbl June 4, 12:33

    B,C,D and E

    Reply to this comment
  7. Micaelis June 4, 12:44

    My answer would be: b,c,d,e. The reason I added sw2 f0/9 it is in 1 broadcast domain.

    Reply to this comment
  8. Mathieu June 4, 12:45

    B, C, D

    Reply to this comment
  9. Toigonbai June 5, 00:08

    B C D E

    Reply to this comment
  10. Tracy Thormahlen June 5, 03:33

    B, C, D, & E

    Reply to this comment
  11. Peter June 5, 08:13

    B, C, D

    Reply to this comment
  12. Rickosic June 5, 23:38

    B,C,D,E
    The question dont specify any device so sw2 port must be there

    Thanks Wendell for your books and help !!!
    I am a little afraid because sdm will crush my near ccna certification. what do u think about our cisco certifications and job offer Wendell?

    Reply to this comment
  13. CCENTSkills June 6, 07:29

    Hi Rickosic,
    Short answer: SDN won’t make your CCNA studies useless (hooray)!
    SDN, if successful in the market – and I think it will be – will be a major change. I’ve thought a lot about SDN and what skills we all need in an SDN world. I’ve even started a blog about that very topic (www.sdnskills.com). Some short points to support why your CCNA (and CCNP R/S and CCNA/CCNP DC) won’t be wasted:

    To learn SDN beyond the basics, you need fairly good R/S skills, and benefit from Data Center skills. EG, Cisco makes CCNP the pre-req for their SDN specialist certifications. Most of what I read and lab about SDN requires skill beyond CCNA just to figure out what’s going on.

    The transition will take time. If SDN is hugely successful, in N years, when most networks use only SDN, will some of what’s in CCNA today be totally useless? Probably. But N may be 10 years, or more.

    The transition to SDN will require both traditional and new SDN skills – and more complexity during that transition.

    Finally, just a big picture comment. IT is a field with lots of change. Don’t worry if what you study now may change, because it will. Period. You should always, forever as an IT person, have a long-term and short-term skills development plan. SDN is a great candidate for the long-term plan of anyone who’s currently studying CCNA. You can even move it up to the short term plan. But you need a great foundation in routing and switching (and more) to “get” SDN.
    Wendell

    Reply to this comment
    • Rickosic June 8, 20:20

      Thank you very much Wendelll =D
      your expalnation was so helpful and hearten in this changin carreer!!
      thank you for your time and share your knowledge. Blessings!

      Reply to this comment
    • Rickosic June 8, 20:20

      Thank you very much Wendelll =D
      your explanation was so helpful and hearten in this changin carreer!!
      thank you for your time and share your knowledge. Blessings!

      Reply to this comment
    • Mauricio June 19, 03:12

      Hi Wendell. Just got your book. I’m very excited about reading it. Can’t wait. I’ve played with Linux a lot. 🙂 Also got my lab gear ready but not sure if I got the correct one. :-0

      Reply to this comment
  14. Gran t June 11, 17:39

    B, C, D & E
    Since the destination could be an unknown local unicast it would be flooded out all ports on SW1 except port F0/1. SW2 would also flood the packet out port F0/9.

    If the packet were a broadcast packet same answer as above including R1 dropping the broadcast packet.

    If the packet was an ICMP Ping to host D, then I believe host A would still have to issue an ARP to get a MAC, which R1 would answer giving its F0/0 MAC. The actual ping would then go through to host D, but that would have to be a second packet.

    You could add R2 and SW3 if you assume that host A already had a MAQC for host D in its ARP table since the question said only the switches had empty MAC tables.

    If the question said the PCs have been configured so they could ping each other I would assume a static MAC entry and an IP entry in host A’s configuration.

    If host already knows the IP of host b, the ping packet would traverse

    Reply to this comment
  15. JB July 20, 07:57

    Thanks for this, im new in the Cisco world and this info is awesome.

    Reply to this comment
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