Spanning Tree Protocol: STP

STP stabilizes the network into a loop free topology by causing some interfaces to simply not forward or receive traffic.  Ports that are not passing traffic are considered to be in a Blocking State, where ports that ARE passing traffic are considered to be in a Forwarding State.  The ports that are in a Forwarding state provide the loop free path over the network.

STP Protocol messages are called BPDU’s (Bridge Protocol Data Units)

bpdu

Protocol Identifier

This value is set to 0x0000

Protocol Version

This value is set to 0x00

BPDU Type

This field identifies the types of BPDUs: Configuration BPDUs (0x00) ot Topology Change Notification BPDUs (0x80)

Flags

This field uses 2 out of 8 bits to handle topology change acknowledgement and the topology change flag

Root Bridge ID and Root Path Cost

These fields identify the root bridge in the network

Sending Bridge Id and Sending Port Id

Includes the senders own bridge identifier and the port on the sender that actually fowrarded the BPDU

Message Age

This is an estimate of the BPDUs age

Max Age, Hello Time, and Forward Delay

These timers always reflect the timer settings configured on the root switch.

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