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Monday, January 31, 2011

Will there be a outage of the network cause of recalculating the Spanning Tree?

We have a simple question about Spanning Tree behavior (We use in our LAN the standard STP 802.1d).We want to add a new switch (STP enabled) to the LAN. This switch has a lower priority as the actual Root Bridge and will never become the new Root Bridge.

Question: If we add the new switch, will there be a outage of the network cause of recalculating the Spanning Tree, or will this only happen if the new Switch will become the Root Bridge?


  • What do you mean that your switch has lower priority? It should be higher than the root bridge for example if root bridge has a priority configured like 16384 and your new switch has default which is 32767, you are all set. You need to cross check this on both the switches. If you add a new switch and it has a lower priority, taking the above example say lower than 16384, it will cause a STP recalculation over the entire network as this will become the new root bridge. If you have already placed the root bridge carefully on your network and this new switch has a higher priority, it will not cause the STP recalculation. The careful placement and configuration of the root bridge is the key in an STP enabled network.

  • The switch with the lowest bridge ID (BID) is elected as the root bridge. Bridge ID combination of bridge priority and mac address so both should be lower than all other switches to elect as a root switch. So your new switch should be higher than the existing root switch to not become as a root switch. If your new switch have same priority as existing root switch you can change it.And yes if the root bridge election process started to elect new root switch then there will be outage.
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