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markjosepascua
Beginner

Cisco Lab: Course ware regarding STP

How come the resulting STP topology is this:

Screenshot 2021-03-29 10.07.22 AM.png

 

when the output of the show spanning-tree command for SW1, SW2, SW3, and SW4 are these:

Screenshot 2021-03-29 10.09.48 AM.png

Screenshot 2021-03-29 10.14.52 AM.png

Screenshot 2021-03-29 10.15.10 AM.png

 

Screenshot 2021-03-29 10.15.18 AM.png

Please enlighten me. Thank you so much!

2 ACCEPTED SOLUTIONS

Accepted Solutions

Hello,

 

the answer is wrong, apparently. If SW3 would be the root for Vlan1, it would, as stated, only have designated ports. Since it has a root port, it cannot be the root switch.

View solution in original post

Hello
SW1 is the bridge root for vlan 1 so al it ports are forward designated.

All the other switch in this instance looks to have calculated via lowest path cost and lowest port priority.

Sw3 will choose sw1( lowest path cost) and then eth0/2 lowest port priority to Sw1
Sw4 will choose sw1( lowest path cost) and eth0/0 lowest port priority to Sw1

 

So the stp topology looks correct.


Please rate and mark as an accepted solution if you have found any of the information provided useful.
This then could assist others on these forums to find a valuable answer and broadens the community’s global network.

Kind Regards
Paul

View solution in original post

7 REPLIES 7
Georg Pauwen
VIP Master

Hello,

 

what is your specific question/problem regarding this topology ? What STP behaviour is expected ? SW1 is the root, redundant links are blocked...

Hi Georg,

 

Based on the output of the show spanning-tree command: Root bridge is SW1. However the answer in the course (STP Topology: 1st Image) shows that SW3 is the Root Bridge. I wanted to know how is SW3 the root bridge in STP Topology when the output shows in CLI that SW1 should be the Root Bridge.

 

Thanks.

Hello,

 

-->  However the answer in the course (STP Topology: 1st Image) shows that SW3 is the Root Bridge.

 

If you look at the output from SW3, you see that port Et0/2 is the root port. A root switch does not have any root ports, just designated ports. The root port is the port pointing TOWARDS the root switch.

 

That said, you can have different root switches for different Vlans. SW3 might very well be the root for any of the other Vlans.

Hi,

 

this means that the answer in the Course ware is wrong on the STP Topology of VLAN 1? 

markjosepascua_0-1617002350779.png

 

This image confused me because the port that should have been blocked is E0/3 of SW3 and the E0/1 of SW1 should be Forwarding. Also the E0/1 of SW4 should be blocked.

Hello,

 

the answer is wrong, apparently. If SW3 would be the root for Vlan1, it would, as stated, only have designated ports. Since it has a root port, it cannot be the root switch.

Hello
SW1 is the bridge root for vlan 1 so al it ports are forward designated.

All the other switch in this instance looks to have calculated via lowest path cost and lowest port priority.

Sw3 will choose sw1( lowest path cost) and then eth0/2 lowest port priority to Sw1
Sw4 will choose sw1( lowest path cost) and eth0/0 lowest port priority to Sw1

 

So the stp topology looks correct.


Please rate and mark as an accepted solution if you have found any of the information provided useful.
This then could assist others on these forums to find a valuable answer and broadens the community’s global network.

Kind Regards
Paul
paul driver
VIP Expert

Hello

On a side not I dont think the toplogy is wrong, it clearly states  "STP topology for VLAN 1"


Please rate and mark as an accepted solution if you have found any of the information provided useful.
This then could assist others on these forums to find a valuable answer and broadens the community’s global network.

Kind Regards
Paul