Westermo U200 Operator manal - Manual (Page 23)

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received on the root port of the switch, the switch also forwards it with an updated message to
all attached LANs for which it is the designated switch.
If a switch receives a configuration BPDU that contains inferior information to that currently
stored for that port, it discards the BPDU. If the switch is a designated switch for the LAN from
which the inferior BPDU was received, it sends that LAN a BPDU containing the up-to-date
information stored for that port. In this way, inferior information is discarded, and superior
information is propagated on the network.
A BPDU exchange results in these actions:
• One switch in the network is elected as the root switch (the logical centre of the
spanning-tree topology in a switched network).
• For each VLAN, the switch with the highest switch priority (the lowest numerical
priority value) is elected as the root switch. If all switches are configured with the
default priority (0x8000), the switch with the lowest MAC address in the VLAN
becomes the root switch.
• A root port is selected for each switch (except the root switch). This port provides the
best path (lowest cost) when the switch forwards packets to the root switch.
• The shortest distance to the root switch is calculated for each switch based on the
path cost.
• A designated switch for each LAN segment is selected. The designated switch incurs
the lowest path cost when forwarding packets from that LAN to the root switch. The
port through which the designated switch is attached to the LAN is called the
designated port.
• Ports included in the spanning-tree instance are selected. Root ports and designated
ports are put in the forwarding state.
• All paths that are not needed to reach the root switch from anywhere in the switched
network are placed in the spanning-tree blocking mode.
The user can easily set the root of the network by configuring one of the switches in the
network as the RSTP focal point (see Installation manual). This will result in a lower priority
value for this switch than for the other switches in the network.
Propagation delays can occur when protocol information passes through a switched LAN. As
a result, topology changes can take place at different times and at different places in a
switched network. When a port transitions directly from nonparticipation in the spanning-tree
topology to the forwarding state, it can create temporary data loops. Ports must wait for new
topology information to propagate through the switched LAN before starting to forward
frames. They must allow the frame lifetime to expire for forwarded frames that have used the
old topology.
Each port on a switch using spanning tree exists in one of these states:
• Blocking - the port does not participate in frame forwarding.
• Listening - the first transitional state after the blocking state when the spanning tree
determines that the port should participate in frame forwarding.
• Learning - the port prepares to participate in frame forwarding.
• Forwarding – the port forwards frames.
• Disabled – the port is not participating in spanning tree because of a shutdown port,
no link on the port, or no spanning-tree instance running on the port.
A port moves through these states:
• From initialization to blocking
• From blocking to listening or to disabled
• From listening to learning or to disabled
• From learning to forwarding or to disabled
• From forwarding to disabled