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STP reconverge causes a momentary disconnect throughout the entire network. WHY?

Posted on 2010-09-22
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Last Modified: 2012-05-10
Whenever I am at my test bench setting up new switches--mostly Cisco but also HP and Dell-whenever I restart the new switch the SPT reconverges and knocks out network connectivity network wide for a minute or two. I get very unhappy user community and I have never seen this happen before when setting up new switches--they are connected to the network during final setup--and I don't know why this happens. Please help and thank you in advance.
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Question by:dprue
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9 Comments
 
LVL 16

Expert Comment

by:InteraX
ID: 33737855
Is that switch the current root bridge. If this goes down, all your switches will need to elect a new root bridge. If it then comes back, a new root bridge will then be re-elected.
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Author Comment

by:dprue
ID: 33738115
I don't know why it would be a root bridge. It is a spare router i am putting into service. how do I know if it is playing as a root bridge?
 
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LVL 16

Expert Comment

by:InteraX
ID: 33738142
I think it's sh stp
Bandwidth and priority nnumber etc all play a factor.
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Expert Comment

by:InteraX
ID: 33738177
BTW, that's a cisco command
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LVL 16

Expert Comment

by:InteraX
ID: 33738257
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Accepted Solution

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jbrmiller earned 1500 total points
ID: 33757189
depending on the switch interconection to the network , it may be a pathway to other switches to the rootbridge.

If you are still setting up your switch, i recommend to connect it only with a single trunk to the network, so the switch will be a spoke and will not be a relevant part of your spanning tree.
when you really finish setting up the switch, then connect it in the final way, STP will reconverge and you will have a small blackout.

the blackout can be shortened by using rapid-pvst instead pvst. rapid-pvst decreases the learning time of stp to half or less.

sideways... Network implementations and modifications should be maked out of business hours.

Regards.
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Author Closing Comment

by:dprue
ID: 33781859
I was setting up the initial config with a single connection to the network so the switch was really a stub. I know that network interruptions should be done after-hours but configuring a switch just hanging on the network with one port should not be causing a network disconnect when booting up or shutting down. I did not get the answer I was looking for, like maybe how to set the switch so it could not possibly act as the root switch while I configure it and there not cause a reconvergence.
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LVL 16

Expert Comment

by:InteraX
ID: 33786862
Is it a Cisco switch?

To set a cisco switch as the root bridge, use the following command.

spanning-tree vlan <vlan-id> root primary

If you have other cisco switches, issue this on one of those.
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LVL 16

Expert Comment

by:InteraX
ID: 33786868
Further info from the 2950 config guide.

The switch maintains a separate spanning-tree instance for each active VLAN configured on it. A bridge ID, consisting of the switch priority and the switch MAC address, is associated with each instance. For each VLAN, the switch with the lowest bridge ID becomes the root switch for that VLAN.

To configure a switch to become the root, the switch priority can be modified from the default value (32768) to a significantly lower value so that the switch becomes the root switch for the specified VLAN. Use the spanning-tree vlan vlan-id root global configuration command to alter the switch priority. When you enter this command on a switch, it checks the switch priority of the current root switch for each VLAN and sets its own switch priority for the specified VLAN to 8192 if this value causes this switch to become the root for the specified VLAN. If any root switch for the specified VLAN has a switch priority lower than 8192, the switch sets its own priority for the specified VLAN to 1 less than the lowest switch priority.

For example, if all switches in the network have the switch priority for VLAN 100 set to the default value of 32768, entering the spanning-tree vlan 100 root primary global configuration command on a switch sets the switch priority for VLAN 100 to 8192, causing the switch to become the root switch for VLAN 100.



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Note The root switch for each instance of STP should be a backbone or distribution switch. Do not configure an access switch as the spanning-tree primary root.


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Use the diameter keyword to specify the network diameter (that is, the maximum number of switch hops between any two end stations in the network). When you specify the network diameter, the switch automatically sets an optimal hello time, forward-delay time, and maximum-age time for a network of that diameter, which can significantly reduce the convergence time. You can use the hello keyword to override the automatically calculated hello time.



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Note We recommend that you avoid manually configuring the hello time, forward-delay time, and maximum-age time after configuring the switch as the root switch.
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