Device Management
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is chosen as the designated port and the BPDU of this port is replaced with the
resulting BPDU. The port regularly sends out the resulting BPDU;
If the BPDU of this port takes the precedence over the resulting BPDU, the
BPDU of this port is not replaced and the port is blocked. The port only can
receive BPDUs.
Tip
In an STP with stable topology, only the root port and designated port can forward data, and
the other ports are blocked. The blocked ports can only receive BPDUs.
STP Timer
1. Hello Time
It specifies the interval to send BPDU packets. It is used to test the links. Hello Time ranges
from 1 to 10 seconds.
2. Max Age
If the BPDU packet is not received after the Max Age, the switch will send the BPDU packet
to all the other switches, and recalculate the spanning tree. Max Age ranges from 6 to 40
seconds.
3. Forward Delay
It means the delay time that the removing of the switch port status. Max Age ranges from 4
to 30 seconds.
When the STP regeneration caused by network malfunction occurs, the STP structure will
get some corresponding change. However, the recomputed new BPDU cannot be
immediately spread over the entire network. Allowing a newly selected root port and
designated port to start forwarding data immediately might give rise to transient loop.
Therefore, STP adopts a kind of state transition mechanism, such that the newly selected
root port and designated port cannot enter into their forwarding status before a 2-times
forwarding delay expires; the forwarding delay can ensure that the new BPDU is already
spread over the entire network.
RSTP(rapid spanning tree protocol)
RSTP (Rapid Spanning Tree Protocol) made improvements on the STP, and achieved a
rapid convergence of network topologies. Its “rapidness” reflects in such a fact that when a
port is selected to serve as root port and designated port, the delay time before it enters into
forwarding status is greatly reduced under a certain condition, so as to reduce the time for
the network to achieve its final topology stabilization (conventional STP needs
