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STP troubleshooting

crixis
Level 1
Level 1

Why would switch (A) participating in stp in vlan1 have a port blocking switch (B) that says no spanning tree instance on vlan1?

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Accepted Solutions

Martin L
VIP
VIP

no spanning-tree vlan 1 will disable STP process for vlan 1 on switch where command is applied (swB), but he still will pass BPDUs along the way. So, in ring topology where A has 2 paths to Root, path via B is forwarding (Root port) while the other path is Blocking (Alt).  It is like sw B is taken out of topology for vlan 1. 

Regards, ML

 

 

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5 Replies 5

Gopinath_Pigili
Spotlight
Spotlight

Hi crixis,

I guess stp process on any one of the switch mistakenly disabled for vlan1

In both switches execute the following command

Switch(config)#spanning-tree vlan 1

if this didn't resolve the issue...please provide bit more information..like  share topology or configuration....so others can help you with this issue...

 

Best regards
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any idea why some1 would want to disable STP for a particular vlan?  in case of vlan 1 , traffic engineering since u cannot delete vlan 1? Can't think of any good reason right now.

Regards, ML

I'm troubleshooting a problem where i disconnected a switch that was replaced  by a new switch on a new network, my admin thought we could disconnect the old network equipment one by one after we had the new network equipment stood up and online. When i went to do this, after disconnecting the first switch, we started to see alerts from our network monitoring tool that devices (switches) were offline, They were not down, but we could not continously ping them. imguessing thats the reason the montioring tool reported them as down. Eventually we saw brief interruptions to traffic flow so my admin told me to reconnect the old switch. After doing so i have been going to each switch in the physical network and logging in and documenting the results or the following commands,

show version - to determine that devices base mac

show spanning-tree vlan 1 - to see what ports are involved, status and what each switch sees as the root bridge

show cdp neighbors - to help document connected devices and continue searching for root 

while doing this on say 20 switches so far, i have found 3 different root bridges reported, i have only traced back and logged into 1 of them. The switch i mentioned above that is being blocked seems to be in the same topology as the switch i disconnected . It puzzled me why it seemed to be affected by topology changes when it didnt have a instance of stp for vlan 1 running, and then when i found another switch blocking it, i wondered more about it.

My company doesnt use Cisco DNA and doesnt have a graphical toplogy of the networks that cover two physically seperate campuses, so right now im remotely logging into switches and using those 3 commands to try and paint a picture of the networks, find the roots invloved and hopefully learn how to manage disconnecting the legacy network equipment without bringing down the new network.

I was hoping there was a tool or other commands that could help show stp members for a network, but i also think im going to learn alot researching this way.

Thanks for the help and answers thus far. Much appreaciated.

can you more elaborate. 
MHM

Martin L
VIP
VIP

no spanning-tree vlan 1 will disable STP process for vlan 1 on switch where command is applied (swB), but he still will pass BPDUs along the way. So, in ring topology where A has 2 paths to Root, path via B is forwarding (Root port) while the other path is Blocking (Alt).  It is like sw B is taken out of topology for vlan 1. 

Regards, ML

 

 

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