adding switch to network STP

Unanswered Question
Dec 23rd, 2008
User Badges:

Hi all, when adding a switch to a network, do we have to be careful not to make the network reconverge, incase the new switch has the lowest priority or mac address? if it has, would the whole network reconverge and find the best way to my new switch, and would this only ne true if there were redundsnt links ?

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
Giuseppe Larosa Tue, 12/23/2008 - 09:57
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,

    Founding Member

hello Carl,

when you add a new switch STP recalculation happens in any case.

If the new switch has the lowest bridge id it will become the new root bridge (it can preeempt) and the new spanning tree will be drawn starting from it.

If the new switch has an higher bridge id then the current root bridge it will simply be accepted in a branch of current topology but recalculation has to occur even if it has a single uplink to the current infrastructure

Hope to help


letsgomets Tue, 12/23/2008 - 11:36
User Badges:

Bridge Priority is what you need to be careful of.

Your current root bridge should be configured with a non default bridge priority for the respective VLANs.

If you are putting in a switch straight out of the box there is no concern because there are no VLANs on the box and the bridge priority is default. Now I caveat my last statement that VLAN 1 will be affected. If you have hosts in VLAN 1 there will be an STP topology change for VLAN 1.

If you make redundant connections, spanning tree should stop the loop provided you do not have spanning tree port fast enabled. Make sure whatever port you connect this switch to does not have portfast enabled.

carl_townshend Wed, 12/24/2008 - 09:34
User Badges:

Hi there

so when we plug in a new switch and the stp is recalculated, will this affect the running of the other switches, ie take any links down, or is it just a tcn bpdu ?

gagamboy15 Mon, 12/29/2008 - 07:39
User Badges:

Hello guys,

What are the steps to be considered when putting a new switch in your network to avoid problems?

Thanks in advance.


Jon Marshall Mon, 12/29/2008 - 07:50
User Badges:
  • Super Blue, 32500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN


From an STP point of view the key consideration is to always do it out of hours rather than in core production hours. Adding a switch will create a reconvergence of STP and will affect forwarding for any vlans that are on the new switch.

Also make sure that you have a designated root and secondary so this switch does not become root (unless you want it to of course)

From a non-STP point VTP is the main one. Always make sure the configuration revision number is set to 0 when installing into an existing infrastructure. Easiest way to do this is set the switch to VTP transparent first then back to VTP client (assuming that is what mode you want).

If the switch will have multiple interconnections to other switches for redundancy ensure that portfast has not been enabled on any of the switch interconnect ports.


carl_townshend Tue, 12/30/2008 - 13:52
User Badges:

hi there, when I add a switch, will this cause a tcn to be sent? and what do the other switches do with the tcn, do they forward it to the root ?

viyuan700 Tue, 12/30/2008 - 14:08
User Badges:
  • Silver, 250 points or more

when I add a switch, will this cause a tcn to be sent? and what do the other switches do with the tcn, do they forward it to the root ?

Yes you r right, TCN will be sent when there is topology change and other switch will forward it to root

check the following link if haven't read yet


This Discussion