21. Spanning Tree Protocol Part 2

STP States

STP uses several states to manage network topology and prevent loops:

STP States

Blocking State (Stable)
- Purpose: Prevent loops by disabling ports.
- Characteristics:
  - Traffic: Does not send or receive regular network traffic.
  - BPDUs: Does not forward.
  - MAC Address Learning: Not performed.
- Ports: Non-designated ports.
Listening State (Transitional)
- Purpose: Prepare for moving to the Forwarding state.
- Characteristics:
  - Duration: 15 seconds (default, controlled by the Forward Delay timer).
  - Traffic: Does not send or receive regular network traffic.
  - BPDUs: Forwarded and received.
  - MAC Address Learning: Not performed.
- Ports: Only designated or root ports enter this state.
Learning State (Transitional)
- Purpose: Begin learning MAC addresses before forwarding traffic.
- Characteristics:
  - Duration: 15 seconds (same Forward Delay timer as Listening state).
  - Traffic: Does not send or receive regular network traffic.
  - BPDUs: Forwarded and received.
  - MAC Address Learning: Performed.
- Ports: Designated or root ports.
Forwarding State (Stable)
- Purpose: Operate normally.
- Characteristics:
  - Traffic: Sends and receives regular network traffic.
  - BPDUs: Forwarded and received.
  - MAC Address Learning: Performed.
- Ports: Root and designated ports.

Hello Timer: Frequency of BPDU transmission. Default is 2 seconds.
Max Age Timer: Determines how long BPDUs are considered valid before re-evaluating the network topology. Default is 20 seconds.
Forward Delay Timer: Time spent in Listening and Learning states. Default is 15 seconds.

Key Points:

BPDUs are only forwarded out of designated ports.
A Forwarding port can transition directly to Blocking state.
A Blocking port must pass through Listening and Learning states before transitioning to Forwarding.

BPDU Header

The root bridge’s STP timers control all STP timers across the network.

PortFast
- Purpose: Allows a port to bypass Listening and Learning states and immediately transition to Forwarding.
- Usage: Should be enabled only on ports connected to end hosts to avoid loops.
- Command:
```
SW1(config)# spanning-tree portfast default
```
- Effect: Enables PortFast on all access ports (not trunk ports).
BPDU Guard
- Purpose: Shuts down a port that receives a BPDU to prevent potential loops.
- Command:
```
SW1(config)# spanning-tree portfast bpduguard default
```
- Effect: Enables BPDU Guard on all PortFast-enabled interfaces.
Root Guard / Loop Guard
- These features help prevent topology changes and loops but are less critical for basic CCNA understanding.

Set Spanning-Tree Mode
- Modern Cisco switches use rapid-pvst by default.
Configure the Primary Root Bridge
- Command:
```
SW1(config)# spanning-tree vlan <vlan-number> root primary
```
- Effect: Sets STP priority to 24576. If another switch with a lower priority exists, this switch’s priority is set to 4096 less than the other switch’s priority.
Configure the Secondary Root Bridge
- Command:
```
SW1(config)# spanning-tree vlan <vlan-number> root secondary
```
- Effect: Sets STP priority to 28672 (4096 higher than the primary).

Configure STP Port Settings

Command:

SW1(config)# interface <interface-id>
SW1(config-if)# spanning-tree cost <cost-value>
SW1(config-if)# spanning-tree port-priority <priority-value>

Effect:
- Cost: Sets the cost to reach the root bridge.
- Port Priority: Sets the port’s priority in STP decisions.

VLAN 1 Topology:
- Originally, SW1 was the primary root bridge.
- Configured SW3 as the primary root and SW2 as the secondary root.
VLAN 2 Topology:
- Changes to VLAN 1 do not affect VLAN 2. VLAN 2 maintains the old topology unless reconfigured.

VLAN 1 Topology VLAN 2 Topology

Understanding these concepts will help you manage STP effectively and ensure a loop-free network environment.