Network Access // Lesson 06

Spanning Tree.

Interpret Rapid PVST+ root elections, port roles, states and protection features that keep Layer 2 topologies loop-free.

Rapid PVST+Root bridgePort rolesPortFastGuards
01 // Mental model

Start with the big picture.

The switch with the lowest bridge ID becomes the root; every active port on it is designated.

CCNA focus: Choose the root intentionally, identify one root port per non-root switch, then use the correct guard for the threat. PortFast belongs on true edge ports.
02 // Building blocks

Know what each part does.

01

Root bridge

The switch with the lowest bridge ID becomes the root; every active port on it is designated.

02

Root port

Each non-root switch chooses one best path toward the root bridge.

03

Alternate port

A loop-free backup path remains discarding until it is needed.

04

Protection

PortFast, BPDU Guard, Root Guard and Loop Guard protect different failure boundaries.

03 // Compare and recognise

Read the clues.

ItemWhat to remember
Bridge IDPriority plus extended system ID (VLAN) and MAC address; lowest value wins.
RSTP statesDiscarding, learning and forwarding.
PortFastMoves an edge port quickly to forwarding; it does not disable spanning tree.
BPDU GuardErr-disables a protected edge port if a BPDU arrives.
04 // Configuration and verification

Configuration and verification.

SW1(config)# spanning-tree vlan 10 root primary
SW1(config)# interface gigabitEthernet 0/10
SW1(config-if)# spanning-tree portfast
SW1(config-if)# spanning-tree bpduguard enable
SW1# show spanning-tree vlan 10

Read the example from top to bottom, then verify the resulting state. Configuration is only complete when the output matches the intended design.

05 // Exam and troubleshooting

Turn facts into a method.

  • Priority plus extended system ID (VLAN) and MAC address; lowest value wins.
  • Discarding, learning and forwarding.
  • Moves an edge port quickly to forwarding; it does not disable spanning tree.
  • Err-disables a protected edge port if a BPDU arrives.
Exam checkpoint: Choose the root intentionally, identify one root port per non-root switch, then use the correct guard for the threat. PortFast belongs on true edge ports.
06 // Check yourself

Spanning Tree quiz.

1. Which switch becomes the root bridge?

2. How many root ports does a non-root switch select per spanning-tree instance?

3. Which Rapid STP state combines the older blocking and listening behaviour?

4. What should BPDU Guard do when an edge port receives a BPDU?

5. Does PortFast disable spanning tree on the interface?

Score: 0 / 5