Spanning Tree Topology Change Notification (TCN)

[lagapidis (Lazarus Agapidis)]

Hello Pradeep

@pradeepmp11366

What would be the first state of new switch port and how it works?(I know it starts from blocking state, I want to know the series of events that happens between those two ports - old and the new one)

For the following, we assume that we are using simple STP (802.1D). If you have a stable topology where STP has converged, and you add a new switch to one of the switchports of the switches already in the topology, then the following would happen:

1. The new switch considers itself the root bridge. The port on which it is connected to the other switch will begin briefly in the blocked state as you mentioned.
2. Next, that port will transition to the Listening state. During the listening state, BPDUs are exchanged. Note that both ports on both ends of the link will be transitioning from state to state at the same time assuming the timers are the same, so both ends will be in the Listening state as they exchange BPDUs. During the BPDU exchange, the new switch will learn if it is indeed the root bridge or if the current root bridge will remain, depending on the priority values and the MAC address.

• If the new switch is not the root bridge, its port will assume a role of root port while the interface at the other end of the link will assume a role of designated. (assuming no L2 loop is created with the new switch).
• if the new switch is the root bridge, the port becomes designated and superior BPDUs are sent on the link which will cause the rest of the switches to re-converge their STP orientation.
  The ports continue to transition until the topology converges.

@pradeepmp11366

I am pretty sure that TCN message won’t be sent to root by adjacent switch because nothing to be erased from MAC Addr table(whole purpose of TCN message). Please confirm my understanding on this!

The conditions under which a TCN is sent are very specific and they are stated in detail by Cisco. Cisco states:

A bridge has detected a topology change:

• When a port that was forwarding is going down (blocking for instance).
• When a port transitions to forwarding and the bridge has a designated port. (This means that the bridge is not standalone.)

Once a topology change has been confirmed, a TCN will be sent.

The process to send a notification to all bridges in the network involves two steps:

• The bridge notifies the root bridge of the spanning tree.
• The root bridge “broadcasts” the information into the whole network.

It is only under those circumstances that a TCN is sent. It does not specify that a TCN is sent only if there are actual MAC addresses in the MAC address table of the new switch that is being connected. It doesn’t take this into account. A TCN will be sent and the aging time will be reduced regardless of whether or not there are actually new MAC addresses to be learned.

@pradeepmp11366

3. will there be a RB election process again?

Yes there will be. As stated at the beginning, BPDUs will be exchanged and if the priority and MAC address of the new switch are indeed superior, then yes, an election and a subsequent STP reconvergence will take place.

I hope this has been helpful!

Laz