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adding second trunk to switch - spanning tree question

Hi all

I have 2 core switches, 1 is the stp root, the other the backup root for stp. If I have an access switch with a single link to the core which is the stp  root, if I add a second link on the access switch to the backup root, will this cause an outage on the access switch? Im assuming it wont from practice!



Re: adding second trunk to switch - spanning tree question

You should not have a disruptive event because your access switch will see the backup bridge has a higher priority. It will keep the bridge with the lower priority as root and mark the new trunk port as a backup root.

Cisco Employee

Re: adding second trunk to switch - spanning tree question

Hi Carl,

Jason is correct. Adding the new link will not cause a disruption in STP. When the ports on both ends of the link come up, they will start in the Listening state and exchange BPDUs. From these BDPUs it will be clear that:

  1. The second core switch is "better" in terms of BID so its port will ultimately become Forwarding
  2. The port on the access switch is both "worse" than the port on the core switch, and it does not receive better BPDUs than the current root port of the access switch - so this port will revert to the Blocking state

Jason, one comment though: You have said that the port on the access switch will become a backup port. This terminology has to be used carefully because in terms of Cisco PVST+ and 802.1w RSTP, that port would be an Alternate port, not a Backup port (as the Backup port has a different role).

Best regards,


Re: adding second trunk to switch - spanning tree question

Yes, Alternate Port is the correct term. I wasn't referring to the port role by name, but I suppose using the phrase 'backup root' could cause some confusion--though that is essentially the definition of an alternate port.  A Backup Port is a port which is blocking because it is a redundant path to an upstream bridge. An alternate port is actually an alternate path to the root bridge and can be used to rapidly recover if BPDUs are not seen on the active root port and therefor in RSTP or STP with UplinkFast, serves as a sort of hot standby root port in case of a bridge failure affecting the current root port.

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