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Reset switch takes down whole network for 30 seconds

Posted on 2010-08-12
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I have a network set up with about 8 switches set up in a star topology with a central switch connecting to each of the other switches with fiber. I am trying to determine why when I reset a switch out on one of the ends of the star it seems to take down the whole network for about 30 seconds or so. Any Ideas of where I can look? These are dell switches.
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Question by:snogopher
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by:Matt V
Matt V earned 150 total points
ID: 33424298
Do the switches have spanning tree enabled?  There is a pause while the STP tables are updated when a new switch powers up.
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by:Kvistofta
Kvistofta earned 150 total points
ID: 33424369
30 seconds is a standard value for Spanning Tree. Sounds like you have a topology change with root election happening when you boot that switch. What happens then is that all ports are going from a listening and learning state before forwarding traffic, in order to avoid loops in your network. That takes by standard exactly 30 seconds.

There are newer versions of spanning tree that converges in sub-second. Check if you can change to that in your Dell switches. "Rapid spanning tree" is something you should look for.

/Kvistofta
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Author Comment

by:snogopher
ID: 33424408
It appears they are currently set to classic stp, they have an option for rapid stp or multiple stp also. If I change to rapid stp do all switches on the network need to support this?
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Expert Comment

by:Wolfhere
ID: 33424416
Following on Matt's comment, try configuring the port on the main switch that this particular switch is coming in from - spanning-tree portfast. This would negate any detection of loop backs between the two switches...but if physical security is not an issue, you can leave portfast on (spanning-tree portfast would be the logical opposite of spanning-tree algorithm)
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by:Kvistofta
ID: 33424484
Setting inter-switch-links as portfast is absolutely not recommended.

/Kvistofta
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Valutus earned 200 total points
ID: 33428370
port-fast can definitely lead to issues and should generally be avoided.  As per the other experts setting your spanning tree protocol to RSTP is the best solution.  The convergence time of RSTP is a lot less (standard hello time of 2 seconds) and generally un-noticeable as most applications will retransmit dropped packets.  Keep in mind that you need to set RSTP on all your switches otherwise the switches will revert to classic STP (for the port connected to the device that isn't running RSTP).
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by:snogopher
ID: 33430706
This info was very helpful, thanks all
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