Introduction To Rstp; Introduction To Mstp; Why Mstp - HP 5820X series Configuration Manual

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 will be discarded because of timeout. 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 the following timing parameters.
Forward delay: Specifies the delay time for device state transition. A path failure can cause
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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: Specifies the time interval at which a device sends hello packets to the surrounding
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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 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 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.

Introduction to MSTP

Why MSTP

Limitations of STP and RSTP
STP does not support rapid state transition of ports. A newly elected root port or designated port must
wait twice the forward delay time before transiting to the forwarding state, even if it is a port on a point-
to-point link or an edge port.
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