Selecting the designated bridge and designated port
Here are the steps taken by switches in selecting the designated bridge and designated port
for each LAN segment:
1)
Choose the switch with the lowest root path cost from the LAN segment to the root bridge
as the designated bridge. The port through which the designated bridge is attached to the
LAN segment is the designated port.
2)
If multiple switches have the same root path cost, the one with the smallest bridge ID will
be chosen as the designated bridge. The port through which the designated bridge is
attached to the LAN segment is the designated port.
3)
If it happens that there are more than one port through which the designated bridge is
attached to the LAN segment, the port with the highest priority will be selected as the
designated port.
4)
If the priorities of the ports on the same designated bridge are still the same, the port with
the smallest port number will be selected as the designated port.
Tips
：
In a stable STP 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 alternate port can rapidly transit to the new root port once the old root port failed.
The backup port can rapidly transit to the new designated port once the old designated
port failed.
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-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.
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