Rstp - HP A6600 Configuration Manual

The BPDU forwarding mechanism in STP
• Upon network initiation, every switch 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 it is the root port that received a configuration BPDU and the received configuration BPDU is
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superior to the configuration BPDU of the port, the device increases the message age carried in the
configuration BPDU following a certain rule and starts a timer to time the configuration BPDU while
sending out this configuration BPDU through the designated port.
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 out its own configuration BPDU in response.
If a path becomes faulty, the root port on this path no longer receives new configuration BPDUs and
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the old configuration BPDUs are discarded due to timeout. In this case, the device generates a
configuration BPDU with itself as the root and sends out 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 may occur.
STP timers
STP calculation involves three important timing parameters: forward delay, hello time, and max age.
• Forward delay is the delay time for device state transition.
A path failure can cause spanning 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 right away, 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 transiting to the forwarding state to ensure that the new
configuration BPDU has propagated throughout the network.
Hello time is the time interval at which a device sends hello packets to the surrounding devices to
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ensure that the paths are fault-free.
Max age is a parameter used to determine whether a configuration BPDU held by the device has
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expired. A configuration BPDU beyond the max age is discarded.

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 quicker under certain conditions than in STP.
In RSTP, a newly elected root port can enter the forwarding state rapidly if this condition is met: 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 this condition is met:
the designated port is an edge port (a port directly connects to a user terminal rather than to another
device or a shared LAN segment) or a port connected with 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 with a
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