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3
The switch compares the resulting BPDU with the BPDU of the desired port
whose role you want to determine.
If the resulting B
PDU takes the precedence over the BPDU of the port, the
port 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.
Table 7-2 Selecting root port and designated port
Tips
：
In a STP with stable topology, only the root port and designated port can forward data, and the
other ports are blocked. The blocked ports only can receive BPDUs.
RSTP (Rapid Spanning Tree Protocol), evolved from the 802.1D STP standard, enable Ethernet
ports to transit their states rapidly. The premises for the port in the RSTP to transit its state
rapidly are as follows.
The condition for the root port to transit its port state rapidly: The old root port of the
switch stops forwarding data and the designated port of the upstream switch begins to
forward data.
The condition for the designated port to transit its port state rapidly: The designated
port is an edge port or connecting to a point-to-point link. If the designated port is an
edge port, it can directly transit to forwarding state; if the designated port is connecting
to a point-to-point link, it can transit to forwarding state after getting response from the
downstream switch through handshake.
RSTP Elements
Edge Port: Indicates the port connected directly to terminals.
P2P Link: Indicates the link between two switches directly connected.
MSTP (Multiple Spanning Tree Protocol), compatible with both STP and RSTP and subject to
IEEE 802.1s standard, not only enables spanning trees to converge rapidly, but also enables
packets of different VLANs to be forwarded along their respective paths so as to provide
redundant links with a better load-balancing mechanism.
Features of MSTP:
MSTP combines VLANs and spanning tree together via VLAN-to-instance mapping table.
It binds several VLANs to an instance to save communication cost and network
resources.
MSTP divides a spanning tree network into several regions. Each region has several
internal spanning trees, which are independent of each other.
MSTP provides a load-balancing mechanism for the packets transmission in the VLAN.
MSTP is compatible with both STP and RSTP.
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