S4 however receives BPDUs from S3 on two ports advertising equal cost paths to root so it knows it needs to block one and it uses port prio/port id to determine which.

This is from Todd Lamle tho this explanation contradict from that of Jeremy of CBT nugget…

I do not agree with the explanation for why answer F is incorrect. In RSTP, all VLANs in the network share the SAME root bridge.

Further, answer choice A seems incorrect to me – it implies that the final state of the ports on the switch will be either discarding, learning or forwarding. However, in a root bridge the final state of all ports is FORWARDING.

How can one justify that A is correct and F is not. I am worried I will have to remember to give an incorrect answer to this question if it appears on the actual exam.

Please help in correcting my logic if I am wrong. Also, if anyone who says they had this quesiton on the exam, how similar was it? Were the output and answer choices exactly the same? Please let me know
Thank you!!

Thank You

The only true “Gigabit” link is between SwitchB and SwitchD. All the rest are “FastEthernet” links, because the ports coming off of SwitchA and SwitchC are all FastEthernet.

Therefore, in your illustration, SwitchD will actually advertise it’s root cost as 19 (not 4, as you specify).

SwitchB will compare those two advertisements (they are equal), and then add another 19 to the cost from SwitchA (since that is a FastEthernet link), while adding another 4 to the cost from SwitchD (since that is a true Gigabit link) – giving it a choice of either using Gi0/2 as it’s root port (with a cost of 38) or Gi0/1 as it’s root port (with a cost of 23).

Then, as you said, SwitchB will choose Gi0/1 as it’s root. (but with a cost of 23, not 8 as you specified)

Finally, SwitchA will set it’s port Fa0/2 to Designated/Forwarding (since it has the lower root cost) and SwitchB will set it’s port Gi0/2 to Alt/Discarding on that same link.

I hope this helps to clarify things for those who were asking.

Im really confused about this one… If SWITCH C is the Root Bridge, wouldn’t the ports on Switch C be Designated Ports? If so, How can answer E be correct?

about the Question 9 you explained that –(In this question, we only care about the Access Layer switches (Switch3 & 4). Switch 3 has a lower bridge ID than Switch 4 (because the MAC of Switch3 is smaller than that of Switch4) so both ports of Switch3 will be in forwarding state).– ,it actually don’t matter Because we don’t Verify this parameter first , in fact it is ,the lower path cost to the Root, Imagine that Switch 4 will always need Switch 3 to get to the Root Bridge anyway, meaning Let’s Assume you are right ,and it is just happened that The bridge ID of Switch 4 is lower than Switch 3,that means both ports of Switch4 will be in forwarding state,and one port of Switch 3 Will be blocked,that means if we decided to put a hub in between this link,and connect a computer to this hub,And this computer want to get to some device connected to Switch 1,the Computer frame will be forwarded to Switch 4 and Then Switch port send it back To switch 3,So whats is the point.

A lot of people confuse between electing the root bridge procedure and choosing the best path to get to it.

So the answer i think will be

A lower path cost to the Root
. A lower Sending Bridge ID
A Lower Sending port priority
A lower Sending port number.

“and because Fa0/12 is inferior to Fa0/1″, maybe must says
“and because Fa0/12 is superior to Fa0/1″ ?

Thks.

The exam was 51 questions, only one drag and drop about cable types, all questions are very easy just similar to 9tut.

The lab simulations were Eigrp , ACL1 (similar to 9tut exactly) and ACL2 (modification 3 but with host B to access the finance server)

Good luck for everyone, just be relaxed its very very easy :)

questions 2,3 were in it

The MAC of Switch A has nothing to do with its Fa0/2 port being designated. The fact that Switch A’s Fa0/2 port has a lower cost to the root bridge (Switch C) is the reason it is designated for the A — B segment.

All the best

Regarding Question 8.
Ilya,

-you confirm that the costs on the example are wrong (basically, we need to take into account both speed ends of the link and get the minimum (Bridge C-F0/2=100Mbps and BridgeD-G0/2=1Gbps Cost = 19 ) Correct?

-You confirm that when deciding Designated port for a Segment first thing to check is Path Cost from Sending Bridge and not the Sending ID Bridge. (Basically, same thing as when )
and that is why SwithAF0/2 is Designated. Correct?

Thanks a lot.

Q9: does not exist – A lower Root Bridge ID (Root ID only one, and maybe it repeat: 2. A lower Sending Bridge ID). Must be:

1. A lower path cost to the Root
2. A lower Sending Bridge ID
3. A lower Sending Port ID

Thanks,

In my opinion, explanation is wrong.

Thanks; you are absolutely correct. I did overlook the labels, and assumed everything was 100Mb.

Thanks again.

Gi port Gi port = gigabit link = cost 4
Fa port Fa port = 100Mbps link = cost 19
Gi port Fa port = 100Mbps link = cost 19

Therefore, switch A and switch D will add 19 to the cost they receive. Switch B will add 4 to the route from D and 19 to the route from A.

Hope that makes sense.

The original question doesn’t mention port cost, so I assumed that port cost is the same in either direction from Switch B. If all links are 100Mb, the cost should be the same in each direction, because Switch A and Switch D would only add 4 to the cost towards B.

Please someone help me understand.

Thanks

SwitchB’s shortest path to the root will be B>D>C but the cost of that route is 23 (not 8), compared to the B>A>C which would be 38 (not 23). Cost is calculated on the negotiated port speed, not maximum.

Therefore SwitchD’s cost to the root is 19, not 4.

Just set this up in a lab so I’m pretty sure I’m right :)

In Todd Lammle’s book it says: “Even though the root bridge (SC) has a Gigabit Ethernet port, it’s running at 100 Mbps because SA’s port is a 100-Mbps port, giving it a cost of 19. If the paths between SA and SC were both Gigabit Ethernet, their costs would only be 4, but because they’re running 100 Mbps links instead, the cost jumps to a whopping 19!”

Then why would path cost for SwitchB Gi0/1 in Q8 be 8? It should be 23 because link SwitchC-SwitchD runs only 100Mbps with cost 19, isn’t it?

the question is WRITE wrong! DISCARDING IS NOT A ROLE but A STATE! anyway the answer is OK

It is not true, the switch is running with RSTP and not MSTP so it is root for all vlan!!

now comes to back up port, this one saves as the back up for designeted port on the network segment, as we know each network segment need to have one designeted port to connect to root. so when this designeted port fails on that network segment themn back up port takes over as designeted port

Q3 and Q6 was on test

thanx 9tut

But according to the explanations given by the author of the tuto, switch D advertises a cost of 4 to switch B…

So what is the cost advertised by switch D to switch B?

Does it advertise a cost of 4, witch is the cost of it’s Gig interface connected to switch C ? Or a cost of 19 corresponding to the current speed between switches C and D ?

Thanks to all