The configuration BPDU forwarding mechanism of STP

The configuration BPDUs of STP are forwarded according to these guidelines:

•Upon network initiation, every device regards itself as the root bridge, generates configuration BPDUs with itself as the root, and sends the configuration BPDUs at a regular hello interval.

•If the root port received a configuration BPDU and the received configuration BPDU is superior to the configuration BPDU of the port, the device increases the message age carried in the configuration BPDU following a certain rule, and it starts a timer to time the configuration BPDU while sending this configuration BPDU through the designated port.

•If the configuration BPDU received on a designated port has a lower priority than the configuration BPDU of the local port, the port immediately sends its own configuration BPDU in response.

•If a path becomes faulty, the root port on this path no longer receives new configuration BPDUs and the old configuration BPDUs will be discarded because of timeout. The device generates configuration BPDUs with itself as the root and sends the BPDUs and TCN BPDUs. This triggers a new spanning tree calculation process to establish a new path to restore the network connectivity.

However, the newly calculated configuration BPDU cannot be propagated throughout the network immediately, so the old root ports and designated ports that have not detected the topology change continue forwarding data along the old path. If the new root ports and designated ports begin to forward data as soon as they are elected, a temporary loop might occur.

STP timers

STP calculation involves the following timers:

•Forward delay—The delay time for device state transition. A path failure can cause spanning tree recalculation to adapt the spanning tree structure to the change. However, the resulting new configuration BPDU cannot propagate throughout the network immediately. If the newly elected root ports and designated ports start to forward data immediately, a temporary loop is likely to occur.

For this reason, as a mechanism for state transition in STP, the newly elected root ports or designated ports require twice the forward delay time before they transit to the forwarding state, which makes sure the new configuration BPDU has propagated throughout the network.

•Hello time—The time interval at which a device sends hello packets to the neighboring devices to make sure the paths are fault-free.

•Max age—A parameter used to determine whether a configuration BPDU held by the device has expired. The device discards the BPDU if the max age is exceeded.

Introduction to RSTP

Developed based on the 802.1w standard of IEEE, RSTP is an optimized version of STP. It achieves rapid network convergence by allowing a newly elected root port or designated port to enter the forwarding state much faster than STP.

If the old root port on the device has stopped forwarding data and the upstream designated port has started forwarding data, a newly elected RSTP root port rapidly enters the forwarding state.

A newly elected RSTP designated port rapidly enters the forwarding state if it is an edge port (a port that directly connects to a user terminal rather than to another network device or a shared LAN segment) or it connects to a point-to-point link. Edge ports directly enter the forwarding state. Connecting to a point-to-point link, a designated port enters the forwarding state immediately after the device receives a handshake response from the directly connected device.

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