New switch added to existing STP

jscimeca715

When a new switch is powered on in a network that has an existing version of STP, does it broadcast a Hello BPDU? If so, STP reacts to a lower bridge ID and changes the root if that works correct? I'm just clearing up my understanding of Cisco BPDU Root Guard and was assuming this would be the standard procedure.

Xenz

You have two situations listed:

A new switch is added to the network. It will participate in the spanning-tree and if it has a lower BID it will attempt to become root. With per-vlan STP this applies to all VLAN's the new switch knows about.


If you do the same thing, but you have root guard on, the existing switch will notice the BPDU with a superior BPDU and change to a state called root-inconsistent which effectively changes the port to blocking. When the BPDU's are no longer received, the port will transition back through the states until it reaches the forwarding state.