STP

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802.1d Bridge ID: Priority(2) + MAC(6)
System ID Expansion: Priority(4bits) + System ID(12bits) + MAC(6bytes)

• PVST and 802.1s MST

Root Ports - hello's cost + local port cost
Originating Hello from root bridge have cost of 0

IEEE Costs

• 10Mbps - 100
• 100Mbps - 19
• 1Gbps - 4
• 10Gbps - 2

Designated Port Tiebreaker

• Lowest bridge id
• Lowest port priority
• Lowest port number

Topology Change Notification (TCN) BPDU

• Sends out root port until acknowledgment
• Receives Topology Change Ack (TCA) BPDU
• Root eventually receives TCN; TC flag set on nest Hello's from root

802.1d States

• Blocking - stable
• Listening - transitory
• Learning - learn source mac; transitory
• Forwarding - forward data frames; learn source mac; stable
• Disabled - stable

Optimizing

PortFast

Access ports
Immediately forwarding states
Ignores listening and learning state

config# spanning-tree portfast default
config-if# spanning-tree portfast

UplinkFast

Access layer switches
Multiple uplinks to core switches
Replaces root-ports really fast when one goes down

config# spanning-tree uplinkfast [max-update-rate <rate>]

BackboneFast

Detect indirect link failures in network core
Queries switch when root-port stops receiving hellos
Root Link Query (RLQ) BPDU

config# spanning-tree backbonefast

Protecting

Root Guard

Ignored superior BPDUs, puts port into err-disabled state

BPDU Guard

Disabled upon any BPDUs, puts port into err-disabled state

UniDirectional Link Detection (UDLD)

When one of the two directions fails (usually fiber) a loop can occur.
Will auto come back up if Hellos received.
By default uses layer 2 messaging. When fails it falls into err-disabled state.

Aggressive Mode - Tries 8 times, if no response then both sides go into err-disabled

Loop Guard

When one of the two directions fails (usually fiber) a loop can occur.
Falls into loop-inconsistent state