What is spanning tree?
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Spanning tree protocol, also known by the acronym STP, is a Local Area
Network protocol which is used to prevent bridging loops in local segments of
the network. This is done through an
election process of a root bridge on the LAN segment. The root bridge is the
logical center of the segment and can see all traffic that is traversing its
network.
What is a Bridge Protocol Data Unit?
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Configuration BPDU is used by Spanning tree protocol to provide and
exchange information between all switches on the local segment. The information in the frame will consist of
the physical MAC address of the source port (originating switch port) and the multicast
address destination of 01:80:C2:00:00:00.
The BPDU allow the switches to elect a root bridge for the segment and designate
port states that will create a single functioning path back to the root bridge.
The information contained in the Bridge Protocol Data Unit consists of
the BPDU Type, Root ID and the Root Path Cost.
The type communications if the BPDU is a configuration BPDU or a
topology change BPDU. The Root ID
communicates to the other switches what the priority is for the source switch
and what the physical address (MAC Address) are for the root bridge on the
segment. The root cost field is used to
calculate the lowest cost back to the root bridge.
Port election states
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Physical links between the switches are then designated either a forwarding
port or a blocking port. The goal of STP
is to create a single patch back to the root bridge, therefore eliminating
multiple paths to the root bridge.
Blocking –
A blocked port is a port that would cause a bridging loop if it was in a forwarding
state. No user data is sent or received
over a blocking port, but it may go into forwarding mode if the other links in
use fail and the spanning tree algorithm determines the port may transition to
the forwarding state. BPDU data is still received in blocking state.
Listening -
The port is processing BPDUs and awaiting possible new information that would
cause it to return to the blocking state. It does not populate the MAC address
table and it does not forward frames.
Learning –
A port which is not yet forward frames. It
does learn source addresses from frames received and adds them to the filtering
database (switching database). It populates the MAC Address table, but does not
forward frames.
Forwarding -
A forwarding port receives and sends data under normal operation.
Disabled –
A disabled port can be manually disabled by the network administrator.
