Introduction To Rstp - HP A3100-8 v2 SI Configuration Manual

If the configuration BPDU received on a designated port has a lower priority than the configuration
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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 a
configuration BPDU 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
The most important timing parameters in STP calculation are forward delay, hello time, and max age.
Forward delay: Specifies the delay time for port state transition. A path failure can cause spanning
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tree re-calculation 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 will likely
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 to
ensure that the new configuration BPDU has propagated throughout the network.
• Hello time: Specifies the time interval at which a device sends hello packets to the surrounding
devices to ensure that the paths are fault-free.
Max age: Determines whether a configuration BPDU held by the device has expired. A
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configuration BPDU beyond the max age is discarded.

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 under certain conditions than STP.
NOTE:
• In RSTP, a newly elected root port can enter the forwarding state rapidly if the old root port on the device
has stopped forwarding data and the upstream designated port has started forwarding data.
In RSTP, a newly elected designated port can enter the forwarding state rapidly if the designated port is
•
an edge port (a port that directly connects to a user terminal rather than to another device or a shared
LAN segment) or a port connected to a point-to-point link. If the designated port is an edge port, it can
enter the forwarding state directly. If the designated port is connected to a point-to-point link, it can
enter the forwarding state immediately after the device undergoes handshake with the downstream
device and gets a response.
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