STP Topology Change Notifications

Q1. What is the purpose of spanning tree portfast?

A. Well, the noticeable part is from the client's view. Portfast means the port does not move through the listening and learning states (30 seconds) which can mean delayed, slow, and even missed startup scripts.

The (arguably) more important point is the affect on your network. Every time a port comes out of blocking and moves to listening, learning, and forwarding, a topology change has occured. The switch sends a Topology change notification to the root switch and EVERY switch in your STP topology will shorten their cam table aging time to the forward delay time (default 15 seconds). This means every device in your STP network that isn't very chatty will have their mac flushed from the switch and need to be relearned. So moral of the story, use portfast on every client port to prevent a whole lot of spanning tree traffic and cam table flushes.


Q2. So describe this Topology Change Notification Process

A. Ok, a port on a switch changes state (we are assuming it is not in portfast mode), so a link comes up or a forwarding port goes down. The switch immediately sends a Topology Change Notice (TCN) out it's root port. (unless the root port was the link that just went down, then the switch will sit and wait to hear from the root or attempt to make itself root) All switches that receive the TCN forward it toward the root.

Once the root receives the TCN, first it sends an acknowledment to the originating switch, then it creates a packet with the Topology Change Bit Set, to notify all switches that there has been a topology change. This is flooded to all switches in the topology. When each switch sees the packet with the topology change bit set, it will lower it's bridge table aging time to the forward delay timer and pass on the topology change packet down the tree. The cam table flush is because the changing topology can mean mac addresses will be appearing on new interfaces. The switches don't yet know how the topology has changed, just that they need to flush any unused mac addresses to prepare for a change.

Ports will go through their regular STP states to create a new topology if one is required.


Q3. When does a port come out of blocking state?

A. If no BPDU (bridge protocol data units, how switches communicate STP) arrives within the max age time, OR if a port receives a BPDU with a better root "cost".

Remember, If a port is not a root port or a designated port, it will be in blocking.


Q4. How often are BPDUs sent

A. By default, 2 seconds