Qtech QSW-2900-24T4-AC [148/209] Disadvantages of stp and rstp
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Qtech QSW-2900-24T4-AC [148/209] Disadvantages of stp and rstp

Qtech QSW-2900-24T-AC [148/209] Disadvantages of stp and rstp
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Figure 3 The final computed spanning tree
& Note:
To facilitate description, the spanning tree computing process in this example is simplified, while the
actual process is more complicated.
2) 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 interval of hello time.
· If it is the root port that received the configuration BPDU and the received configuration BPDU is superior
to the configuration BPDU of the port, the device will increase message age carried in the configuration
BPDU by a certain rule and start a timer to time the configuration BPDU while it sends out this
configuration BPDU through the designated port.
· If the configuration BPDU received on the designated port has a lower priority than the configuration BPDU
of the local port, the port will immediately sends out its better configuration BPDU in response.
· If a path becomes faulty, the root port on this path will no longer receive new configuration BPDUs and the
old configuration BPDUs will be discarded due to timeout. In this case, the device will generate a
configuration BPDU with itself as the root and sends out the BPDU. This triggers a new spanning tree
computing process so that a new path is established to restore the network connectivity.
However, the newly computed configuration BPDU will not be propagated throughout the network immediately,
so the old root ports and designated ports that have not detected the topology change continue forwarding data
through the old path. If the new root port and designated port begin to forward data as soon as they are elected, a
temporary loop may occur. For this reason, STP uses a state transition mechanism. Namely, a newly elected root port
or designated port requires twice the forward delay time before transitioning to the forwarding state, when the new
configuration BPDU has been propagated throughout the network.
10.1.2 Introduction to MSTP
a) V. Why MSTP
1) Disadvantages 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 transitioning to the forwarding state, even if it is a port on a point-to-point link
or it is an edge port, which directly connects to a user terminal rather than to another device or a shared LAN
segment.

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