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Spanning Tree Protocol (STP) (Types of Spanning Tree Protocols (Rapid…
Spanning Tree Protocol (STP)
Spanning Tree Algorithm
Port Roles
Root ports
- Switch ports closest to the root bridge in terms of overall cost to the root bridge.
Designated ports
- All non-root ports that are still permitted to forward traffic on the network.
Alternate and backup ports
- Alternate ports and backup ports are in discarding or blocking state to prevent loops.
Disabled ports
- A disabled port is a switch port that is shut down.
Rroot Bridge
The switch with the lowest BID automatically becomes the root bridge.
Bridge ID (BID)
The
extended system ID
is used to specify a VLAN ID or a multiple spanning tree protocol (MSTP) instance ID.
The
MAC address
field initially contains the MAC address of the sending switch.
The
bridge priority
value is automatically assigned, but can be modified.
Root Path Cost
bridge protocol data unit (BPDU) frames
Format
The next four fields are used to identify the root bridge and the root path cost to the root bridge.
The last four fields are all timer fields that determine how frequently BPDU messages are sent and how long the information received through the BPDU process is retained.
The first four fields identify the protocol, version, message type, and status flags.
Types of Spanning Tree Protocols
Rapid Spanning Tree Protocol (RSTP) or IEEE 802.1w
PVST+
Rapid PVST+
STP
Multiple Spanning Tree Protocol (MSTP)
When a switch port is configured with
PortFast
that port transitions from blocking to forwarding state immediately
When
BPDU guard
is enabled, it puts the port in an errdisabled (error-disabled) state on receipt of a BPDU.
A big part of troubleshooting consists of comparing the actual state of the network against the expected state of the network and spotting the differences to gather clues about the troubleshooting problem.