CBT nuggets, ccnp switch, STP

aocferreira

Hello,

i think there's a mistake on stp videos for CBT nuggets series. According to attached pic, can you please confirm?



From switch E point of view, shouldn't Root Port be the one towards switch C ?
And from switch F point of view, shouldn't root port be the one towards switch C?

Later on the video, he says that root port of E goes towards D and root port of F goes towards D also.

Please confirm.

Thanks in advance.

tomtom1

I think you're correct. The process of electing the root port should be like this.

1) Lowest port cost
2) If equal port cost, lowest BID of the switch
3) If equal port cost and BID, the lowest port number is used

So when we run that logic against the given topology, switch C should be preferred over switch D unless we're both missing something

Vask3n

I made a quick and dirty edit to show the root ports should be, let me know if there are any errors on the diagram.

aocferreira

Vask3n, that was also my conclusion.

tomtom1, thanks for feedback also.

So, it seems that they got this wrong...

theodoxa

Root Path Cost is the same going E-D-A or E-C-A. The other paths would all have a higher Root Path Cost. So, the root port would face either D or C. Lowest Sender BID should win. All the switches seem to have default priority, so lowest MAC Address determines who has the lowest Sender BID. I can't read the full MAC Address for Switch D, but if it follows the apparent pattern, C should have the lowest Sender BID.

Lowest Root Path Cost
Lowest Sender Bridge ID
Lowest Sender Port ID

Danielh22185

theodoxa wrote: »

Root Path Cost is the same going E-D-A or E-C-A. The other paths would all have a higher Root Path Cost. So, the root port would face either D or C. Lowest Sender BID should win. All the switches seem to have default priority, so lowest MAC Address determines who has the lowest Sender BID. I can't read the full MAC Address for Switch D, but if it follows the apparent pattern, C should have the lowest Sender BID.

Lowest Root Path Cost
Lowest Sender Bridge ID
Lowest Sender Port ID

^^ This guy nailed it. Yet also brings up a good point. You cannot see the entire MAC address listing for Switch D. However it does seem to follow a pattern and I would assume it should be 32768.0000.0000.000d. Which would make the ports facing switch C the root port as others have already confirmed.


Lowest Root Path Cost
Lowest Sender Bridge ID (MAC address in this case the priority values match on all the switches)
Lowest Sender Port ID

On that note I am going to re-watch this nugget just to confirm for fun and would be good practice for me.

JeanM

Pretty surprising when these errors are discovered in the training material that gets released isn't it ?

Danielh22185

JeanM wrote: »

Pretty surprising when these errors are discovered in the training material that gets released isn't it ?

Happens all the time!