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STP - Coggle Diagram
STP
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Introduction To STP
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ensures that there is only one logical path between all destinations on the network by intentionally
blocking redundant paths that could cause a loop.
The switches running STP are able to compensate for failures by dynamically unblocking the previously blocked ports and permitting
traffic to traverse the alternate paths.
Redundancy :warning:
Increases the availability of the network topology by protecting the network from a single point of failure.
In a Layer 2 design, loops and duplicate frames can occur, having severe consequences.
STP Operation
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Spanning Tree Algorithm
The Spanning Tree Protocol (STP) creates
one logical path through the switch network (all destinations on the network).
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STP sends bridge protocol data units (BPDUs)
between Layer 2 devices in order to create the
one logical path.
A port on S2 is blocked so traffic can only flow
one way between any two devices. 
When Trunk1 fails, the blocked port on S2 is
unblocked and traffic can flow between S2
and S3.
Port Roles
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Designated port – selected on a per-segment (each
link) basis, based on the cost to get back to root bridge
for either side of the link
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Alternate port – (RSTP only) backup port for the designated port when the other side is not
a root port.
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Root Bridge
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As the switches forward their BPDU frames, switches in the
broadcast domain read the root ID information from the BPDU
frame.
If the root ID from the BPDU received is lower than the root ID on
the receiving switch, the receiving switch updates its root ID
identifying the adjacent switch as the root bridge.
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- Originally BID had two fields: bridge priority and MAC
address
- Bridge priority default is 32,768 (can change)
- • Lowest MAC address (if bridge priority is not changed)
becomes determinant for root bridge.
Root Path Cost
The path information is determined by summing up the
individual port costs along the path from the
destination to the root bridge.
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The default port costs are specified by the IEEE and
defined by the speed at which the port operates.
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Can be modified with the spanning-tree cost
interface command
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STP Operation
Extended System ID
If priorities are all set to the default, lowest MAC
address is the determining factor in lowest BID.
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Root Ports
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Specifically, bridges track the Root Path Cost, the cumulative cost of all links to the Root Bridge.
Designated Ports
A Designated Port functions as the single bridge port that both sends and receives traffic to and from that segment and the Root Bridge.
Each segment in a bridged network has one Designated Port, chosen based on cumulative Root Path Cost to the Root Bridge.
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STP Configuration
PVST+ Config
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PortFast and BPDU Guard
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Bridge Protocol Data Unit (BPDU) Guard disable port that has PortFast configured on it if BPDU is received
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