CCENT Question: LAN Switching Logic

 In 200-301 V1 Ch05: Ethernet Switching, 200-301 V1 Part 2: Ethernet, CCENT-OLD, Q&A

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

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Answers to the "LAN Switching Logic" Question
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[…] posted a question about LAN Switching a few days back. Here are the answers! Post with any related clarifications or […]

[…] posted a question about LAN Switching a few days back. Here are the answers! Post with any related clarifications or […]

Ali

B. SW1 F0/2

Ahmad

Answer is B,C,D and E.

A

Hi!

Aswer: B, C, D

omar

B,C,D

Carlos Hernandez

B, C, D

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

lyjo

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

Adil Ahmed

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

Correct answer is B,C,D

rbl

B,C,D and E

Micaelis

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

Mathieu

B, C, D

Toigonbai

B C D E

Tracy Thormahlen

B, C, D, & E

Peter

B, C, D

Rickosic

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?

Rickosic

sorry I mean SDN (software defined networking) trend

lyjo

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

Rickosic

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!

Rickosic

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!

Mauricio

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

[…] posted a question about LAN Switching a few days back. Here are the answers! Post with any related clarifications or […]

Gran t

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

JB

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

Punya Athma

B, C, D, E

As this frame from PC A is an unknown uni cast frame for SW1 the switch floods out this frame through all the other interfaces except F0/1 from which the frame entered the Switch. Then the frame reaches SW2 and for SW2 also it is an unknown unicast frame. At the same time there is only one port F0/9 connected to PC, C. SW2 send the frame throughF0/9.

Punya Atma

SW1 floods out the Ethernet frame it received from host A, as this Switch is in its default config, i.e, not yet learned any mac-addresses and so the mac-address table is empty. So this frame is unknown uni cast for SW1.
The correct answer choices are:- B, C, and, D.
Yet this frame leaves SW1’s G0/1 port and reaches SW2’s G0/2 port.
SW2 also in its default config, and its mac-table too is empty. Additionally in the topology figure SW2 is shown with its only one FastEthernet Port F0/9.
Thereby the frame leaves out of SW2’s F0/9 port.
So the answer choices are:- B, C, D, and, E.

Still more to be analysed about this question, because the question simply and blankly starts, “Switch SW1 receives an Ethernet frame from host A”. But nothing elaborately inform about the frame:-
There are two possibilities or guesses that the destination IP address of this frame could be either within the local VLAN 1, or in the other VLAN 2.
So, if the destination IP address is within the other VLAN 2, Router R1 forwards this frame out of its S 0/0/0 interface.
Therefore the answer choice, ‘F’, also to be added.

imikcisco

B, C, D and E because it deviates in the same transmission domain, except for the interface receives the frame

Antonio Pena

There are two different scenarios depending on which subnet is going the frame sending by host A.

Scenario one: the frame keeps in the subnet (10.1.1.254/24)
For this scenario B, C and E are true.

Scenario two: the frame goes to the subnet (10.1.3.254/24)
For this scenario D and F are true.

Wendell Odom

Hi Antonio,
On your scenario two, imagine host A sends a packet to a host in subnet 10.1.3.0/24 as you suggest. The frame that encapsulates the packet sent by host A arrives at router R1. R1 de-encapsulates the packet, so the Ethernet frame ends there. So, for this question, the router does not forward an Ethernet frame. It forwards the IP packet. In effect, the frame does not leave the local LAN.
So, instead of two scenarios, it’s one: a frame that travels in the LAN to another device in the LAN, whether that’s a host or a router.
Hope this helps,
Wendell

Antonio Pena

Hi! Yes, I understood. Now I see it clarity. I have to take more care analyzing the questions sceneries. Thank you so much!

Valentí

Answer: B, C, D, E

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