Computer NetworksSpanning Tree Protocol

Spanning Tree Protocol (STP): Loop-Free Bridged Networks

LevelIntermediate

Duration75 mins

TopicSpanning Tree Protocol

2 / 5

STP Operation — The Protocol Mechanics

The Symphony of Silent Bridges

Every two seconds, across millions of enterprise networks worldwide, billions of small messages silently traverse switched networks. These messages carry no user data, generate no visible traffic, and are invisible to applications. Yet without them, modern networking would be impossible.

These are Bridge Protocol Data Units (BPDUs)—the heartbeat of the Spanning Tree Protocol. Through continuous BPDU exchange, bridges across a network collectively and automatically determine which paths should be active and which should be blocked, all without any central controller or human intervention.

STP is a marvel of distributed computing: individual bridges, each with only local information about their directly connected links, communicate exclusively through BPDUs to construct a globally consistent, loop-free topology. Understanding how this works requires diving into the protocol mechanics—the message formats, the decision algorithms, and the timers that govern STP behavior.

What You Will Learn

By the end of this page, you will understand the complete operational mechanics of STP: how Bridge Protocol Data Units (BPDUs) are structured and exchanged, the critical role of Bridge ID and path costs, the Hello/MaxAge/ForwardDelay timers that govern protocol behavior, and how bridges use this information to collectively compute the spanning tree. You'll master the distributed algorithm that makes loop prevention automatic.

Bridge Protocol Data Units (BPDUs)

BPDUs are the fundamental communication mechanism of STP. They are L2 frames that bridges use to discover each other, exchange topology information, and detect changes that require spanning tree recalculation.

BPDU Transmission Rules:

Destination MAC Address: 01:80:C2:00:00:00 (reserved multicast address for spanning tree)
Transmission Interval: Every 2 seconds by default (Hello Time)
Transmission Ports: Sent out all designated and root ports
Reception Behavior: Received on all ports, processed by STP state machine
Not Forwarded: BPDUs are never forwarded to other ports—only locally processed

Why the Special Multicast Address?

The 01:80:C2:00:00:00 address is in the reserved range that switches recognize as "process locally, never forward." This ensures BPDUs remain within each link segment and don't flood through the network like regular multicast frames.

IEEE 802.1D BPDU Frame Format
Field	Size (bytes)	Description
Protocol Identifier	2	0x0000 for IEEE 802.1D STP
Protocol Version	1	0x00 for original STP, 0x02 for RSTP
BPDU Type	1	0x00 = Configuration, 0x80 = TCN
Flags	1	TC (Topology Change), TCA (TC Acknowledgment)
Root Bridge ID	8	Priority (2) + MAC Address (6) of root
Root Path Cost	4	Cumulative cost to reach root bridge
Bridge ID	8	Priority (2) + MAC Address (6) of sending bridge
Port ID	2	Priority (1) + Port Number (1) of sending port
Message Age	2	Time since BPDU originated at root
Max Age	2	Maximum BPDU lifetime (default 20 seconds)
Hello Time	2	BPDU transmission interval (default 2 seconds)
Forward Delay	2	Time in Listening/Learning states (default 15 seconds)

Total BPDU Size: 35 bytes (plus Ethernet header and trailer = ~60 bytes on wire)

BPDU Types:

Configuration BPDU (Type 0x00):

The primary BPDU type, containing complete topology information
Sent by designated bridges on designated ports
Used for root election, path cost calculation, and port role determination
Contains all timer values to synchronize the network

Topology Change Notification BPDU (Type 0x80):

Minimal BPDU used to signal topology changes
Contains only Protocol ID, Version, and Type (4 bytes total)
Sent toward the root when a port transitions state
Triggers rapid MAC address table updates across the spanning tree

BPDU Generation at the Root

Only the root bridge generates original BPDUs. All other bridges receive BPDUs from the root (directly or relayed), modify them to add their own path cost and bridge ID, and forward them out their designated ports. The Message Age field tracks how far the BPDU has traveled from the root.

Bridge Identification and Priority

Every bridge participating in STP must be uniquely identifiable and comparable. The Bridge ID serves this purpose and is the key to root bridge election.

Bridge ID Structure (8 bytes):

┌────────────────────┬────────────────────────────────────────────┐
│  Bridge Priority   │              MAC Address                   │
│    (2 bytes)       │              (6 bytes)                     │
├────────────────────┼────────────────────────────────────────────┤
│  0x8000 (32768)    │        00:1A:2B:3C:4D:5E                   │
│  Default Priority  │        Burned-in or Assigned               │
└────────────────────┴────────────────────────────────────────────┘

Complete Bridge ID: 32768.00:1A:2B:3C:4D:5E

Bridge Priority:

Range: 0 to 65535
Default: 32768 (0x8000)
Lower values = Higher priority
Modern implementations (RSTP/MSTP) use increments of 4096 to accommodate Extended System ID

Extended System ID:

In VLAN-aware environments, the same switch may participate in multiple spanning tree instances (one per VLAN or per VLAN group). The Extended System ID allows the same physical switch to have different Bridge IDs per VLAN:

Original 2-byte Priority:     [Priority 16 bits]

With Extended System ID:      [Priority 4 bits][VLAN ID 12 bits]
                                    │                │
                                    │                └── 0-4095 VLANs
                                    └── Priority: 0, 4096, 8192, ..., 61440

Example for VLAN 100:
  Priority = 32768 = 0x8000 = [1000][000001100100]
                              = Base 32768 + VLAN 100
  Effective Bridge ID = 32868.00:1A:2B:3C:4D:5E

Bridge ID Comparison Rules

•Compare priority first: Lower numeric priority wins
•If priority equal, compare MAC address: Lower MAC address wins
•Result is deterministic: Given any two bridges, one is definitively 'better'
•Lowest Bridge ID becomes root: The bridge with the lowest Bridge ID is elected root

Bridge ID Comparison Examples
Bridge A ID	Bridge B ID	Winner	Reason
4096.00:11:22:33:44:55	32768.00:AA:BB:CC:DD:EE	Bridge A	Lower priority (4096 < 32768)
32768.00:11:22:33:44:55	32768.00:AA:BB:CC:DD:EE	Bridge A	Equal priority, lower MAC
32768.00:AA:BB:CC:DD:EE	32768.00:11:22:33:44:55	Bridge B	Equal priority, B has lower MAC
0.00:FF:FF:FF:FF:FF	4096.00:00:00:00:00:01	Bridge A	Priority 0 beats all

Best Practice: Set Priority Explicitly

Never rely on MAC addresses for root bridge selection. MAC addresses are essentially random and change when hardware is replaced. Always configure explicit priorities: set the preferred root to 4096 or 8192, the backup root to 16384 or 20480, and leave access switches at default 32768.

Path Cost Calculation

Once the root bridge is elected, every other bridge must determine the best path to reach it. Path cost provides the metric for this calculation, with lower costs indicating more desirable paths.

How Path Cost Works:

Each port has an associated port cost (based on link speed)
When a bridge receives a BPDU, it adds its incoming port's cost to the BPDU's Root Path Cost
The Root Path Cost is the cumulative cost from this bridge to the root
The port with the lowest Root Path Cost becomes the Root Port

                        ROOT BRIDGE
                    Cost to root = 0
                            │
                    ────────┴────────
                   100Mbps  │  1Gbps
                   cost=19  │  cost=4
                            │
              ┌─────────────┼─────────────┐
              │             │             │
          SW-A (19)     SW-B (4)      SW-C (19)
      path cost = 19   path cost=4   path cost=19

IEEE 802.1D-1998 Port Costs (Original)
Link Speed	Port Cost	Notes
4 Mbps	250	Token Ring
10 Mbps	100	Original Ethernet
16 Mbps	62	Fast Token Ring
100 Mbps	19	Fast Ethernet
1 Gbps	4	Gigabit Ethernet
10 Gbps	2	10 Gigabit Ethernet

IEEE 802.1D-2004 Port Costs (Revised)
Link Speed	Recommended Cost	Notes
10 Mbps	2,000,000	More granularity for high speeds
100 Mbps	200,000	Proportional to bandwidth
1 Gbps	20,000	10x faster = 10x lower cost
10 Gbps	2,000	Standard for modern data centers
100 Gbps	200	Emerging standard
1 Tbps	20	Future-proofing

Path Cost Calculation Example:

                      ROOT (Priority 4096)
                            │
                     ┌──────┴──────┐
               1Gbps │             │ 100Mbps
               (4)   │             │ (19)
                     │             │
                   SW-A          SW-B
                     │             │
               1Gbps │             │ 1Gbps
               (4)   │             │ (4)
                     │             │
                     └──────┬──────┘
                          SW-C
                     (Access Layer)

SW-C's Path Cost Calculation:

Path through SW-A: 4 (to ROOT) + 4 (to SW-C) = 8
Path through SW-B: 19 (to ROOT) + 4 (to SW-C) = 23
Winner: Path through SW-A (cost 8)
SW-C's Root Port: Port connecting to SW-A

Administrative Cost Override

Administrators can manually configure port costs to influence path selection. This is useful when physical topology doesn't match desired traffic patterns, or when specific paths should be preferred for policy reasons. However, misconfigurations can cause suboptimal routing or unexpected failover behavior.

STP Timers and Their Critical Roles

STP relies on three configurable timers that control protocol behavior. Understanding these timers is essential for both troubleshooting and optimization.

The Three STP Timers:

Hello Time (Default: 2 seconds)

•Purpose: Interval between BPDU transmissions
•Range: 1-10 seconds
•Set by: Root bridge only (propagated in BPDUs)
•Impact of shorter values: Faster failure detection, more CPU/bandwidth usage
•Impact of longer values: Slower failure detection, lower overhead

Max Age (Default: 20 seconds)

•Purpose: Maximum time a bridge stores BPDU information
•Range: 6-40 seconds
•Set by: Root bridge only
•Failure detection: If no BPDU received for Max Age, port assumes link failure
•Determines convergence speed: Longer Max Age = slower reaction to failures

Forward Delay (Default: 15 seconds)

•Purpose: Time spent in Listening and Learning states
•Range: 4-30 seconds
•Set by: Root bridge only
•Port transition: Port waits Forward Delay twice (Listening → Learning → Forwarding)
•Total transition time: 2 × Forward Delay = 30 seconds by default

STP Timer Summary and Convergence Impact
Timer	Default	Range	Affects
Hello Time	2 seconds	1-10	BPDU frequency, overhead
Max Age	20 seconds	6-40	Failure detection time
Forward Delay	15 seconds	4-30	Port transition time
Total Convergence	30-50 seconds	—	Max Age + 2×Forward Delay

Why Such Long Default Timers?

The default timers were designed for networks with significant propagation delays and many bridges:

Network diameter consideration: BPDUs must propagate through the entire network before decisions are made
Message Age tracking: The Message Age field in BPDUs increments as they traverse bridges. Max Age must accommodate the maximum network diameter.
Stability priority: STP prioritizes stability over speed. Rushing port transitions risks temporary loops.

The 5-4-3-2-1 Rule Implication:

Original Ethernet specified maximum 7 bridges in a spanning tree path. With 1 second added per bridge hop (for processing), plus propagation delays, the 20-second Max Age ensures BPDUs can traverse the full diameter before timing out.

Timer Modification Warning

Never modify STP timers without understanding the implications. Reducing timers can cause instability when the network is larger than the timers allow. The formula for valid timers is: 2 × (Forward Delay - 1) ≥ Max Age ≥ 2 × (Hello Time + 1). Most networks should use RSTP instead of modifying original STP timers.

BPDU Processing and Decision Algorithm

When a bridge receives a BPDU, it must decide whether this BPDU contains information "better" than what it currently knows. This comparison determines port roles and ultimately the spanning tree topology.

BPDU Comparison: The Four-Part Tie-Breaker

When comparing two BPDUs (received vs. stored), bridges use this priority order:

BPDU Superiority Rules (in order)

•Lowest Root Bridge ID wins — The BPDU claiming a lower root is always superior
•Lowest Root Path Cost wins — If same root, the path with lower cost is better
•Lowest Sender Bridge ID wins — If same root and cost, the lower sending bridge ID wins
•Lowest Port ID wins — If all else equal, lower port ID (priority + number) wins

bpdu_comparison.pseudo
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// BPDU Comparison Algorithm
// Returns: true if received BPDU is superior to stored BPDU
 
function isReceivedBPDUsuperior(received: BPDU, stored: BPDU): boolean {
    // Step 1: Compare Root Bridge IDs
    if (received.rootBridgeId < stored.rootBridgeId) {
        return true;  // Lower root ID = superior
    }
    if (received.rootBridgeId > stored.rootBridgeId) {
        return false; // Higher root ID = inferior
    }
    
    // Root IDs equal, continue comparison
    
    // Step 2: Compare Root Path Costs
    let receivedCost = received.rootPathCost + thisPort.pathCost;
    if (receivedCost < stored.rootPathCost) {
        return true;  // Lower cost = superior
    }
    if (receivedCost > stored.rootPathCost) {
        return false; // Higher cost = inferior
    }
    
    // Path costs equal, continue comparison
    
    // Step 3: Compare Sender Bridge IDs
    if (received.senderBridgeId < stored.senderBridgeId) {
        return true;  // Lower sender ID = superior
    }
    if (received.senderBridgeId > stored.senderBridgeId) {
        return false; // Higher sender ID = inferior
    }
    
    // Sender IDs equal (same switch, different ports), compare Port IDs
    
    // Step 4: Compare Port IDs
    if (received.portId < stored.portId) {
        return true;  // Lower port ID = superior
    }
    
    return false; // Received BPDU is not superior
}

What Happens When a Superior BPDU Arrives:

Update stored information: The port stores the superior BPDU's data
Recalculate port role: Determine if this port should become Root Port
Propagate changes: If this bridge's topology information changed, update outgoing BPDUs
Trigger topology change: If port states change, initiate TCN if appropriate

What Happens When an Inferior BPDU Arrives:

If the received BPDU is inferior to stored information:

The BPDU is ignored for topology decisions
The port continues in its current role
If this port is Designated, it responds with its own superior BPDU

This mechanism ensures that "better" topology information always wins, and the entire network converges to agree on the same root and spanning tree.

The Beauty of Distributed Convergence

Through this simple comparison algorithm running on every bridge, the entire network converges to the same spanning tree without any central coordinator. Each bridge makes local decisions based on BPDUs, but the result is globally consistent. This is a classic example of distributed consensus in networking.

Port Role Determination

Every port on every bridge in the spanning tree is assigned one of three roles. Understanding these roles is fundamental to understanding STP behavior.

The Three Port Roles in 802.1D STP:

Root Port (RP)

•Definition: The port with the best (lowest cost) path to the root bridge
•Count: Exactly ONE root port per non-root bridge
•Direction: Points TOWARD the root bridge
•State: Always in Forwarding state (when topology stable)
•BPDU behavior: Receives BPDUs from upstream designated bridge

Designated Port (DP)

•Definition: The port responsible for forwarding traffic toward a segment
•Count: Exactly ONE designated port per network segment
•Direction: Points AWAY from the root bridge
•State: Always in Forwarding state (when topology stable)
•BPDU behavior: Sends BPDUs onto the segment

Blocked Port (Non-Designated / Alternate)

•Definition: A port that would create a loop if it forwarded traffic
•Count: All ports that aren't Root or Designated
•Direction: N/A (not actively forwarding)
•State: Blocking state (receives and processes BPDUs only)
•BPDU behavior: Receives but does not send Configuration BPDUs

Port Role Selection Algorithm:

For each bridge:

1. Am I the Root Bridge? (My Bridge ID is lowest in all received BPDUs)
   └── YES: All my ports are DESIGNATED PORTS
   └── NO: Continue...

2. Determine my ROOT PORT:
   - Compare all received BPDUs
   - Port receiving best BPDU = ROOT PORT
   - Best = lowest (Root ID, then Path Cost, then Sender ID, then Port ID)

3. For each remaining port, determine DESIGNATED vs BLOCKED:
   - Am I the best path to the root for this segment?
   - Compare my BPDU (from root port) against received BPDUs on this port
   - If my BPDU is better → This port is DESIGNATED
   - If my BPDU is worse → This port is BLOCKED

4. The Root Bridge's ports are always DESIGNATED (it is the best path by definition)

Port Role Summary
Role	Exists On	State	BPDU Behavior	Traffic Direction
Root	Non-root bridges	Forwarding	Receives	Toward root
Designated	All bridges	Forwarding	Sends	Away from root
Blocked	Non-root bridges	Blocking	Receives only	None (blocked)

Root Bridge Port Roles

The root bridge has NO root port (it is the root, there's nowhere closer to go). ALL ports on the root bridge are designated ports—the root is by definition the best path for every connected segment.

BPDU Processing: Worked Examples

Let's trace through BPDU processing in a concrete example to solidify understanding.

Network Topology:

              SW1                    SW2                    SW3
    (32768.00:00:00:00:00:01)  (16384.00:00:00:00:00:02)  (32768.00:00:00:00:00:03)
              │                        │                        │
         Port 1                   Port 1                   Port 1
              │                        │                        │
              └─────────Link A─────────┴─────────Link B─────────┘
                                  (100 Mbps each, cost = 19)

Step 1: Initial State

When switches boot, each believes it is the root:

SW1 sends: Root = 32768.00:00:00:00:00:01, Cost = 0, Sender = SW1
SW2 sends: Root = 16384.00:00:00:00:00:02, Cost = 0, Sender = SW2
SW3 sends: Root = 32768.00:00:00:00:00:03, Cost = 0, Sender = SW3

Step 2: BPDU Comparison at SW1

SW1's Port 1 receives BPDU from SW2 claiming:

Root = 16384.00:00:00:00:00:02 (SW2)
Root Path Cost = 0
Sender Bridge = SW2

SW1 compares:

SW2's claimed root (16384.xxx) < SW1's claimed root (32768.xxx)
SW2 wins! SW1 now knows SW2 is the root.

SW1 updates:

Root Bridge = 16384.00:00:00:00:00:02
My Root Path Cost = 0 + 19 (Link A cost) = 19
Port 1 becomes ROOT PORT

Step 3: BPDU Comparison at SW3

SW3 receives SW2's BPDU (same as SW1):

SW2's claimed root (16384.xxx) < SW3's claimed root (32768.xxx)
SW2 wins! SW3 now knows SW2 is the root.

SW3 updates:

Root Bridge = 16384.00:00:00:00:00:02
My Root Path Cost = 0 + 19 (Link B cost) = 19
Port 1 becomes ROOT PORT

Step 4: Designated Port Decision on Link Between SW1-SW3

(Assume SW1 and SW3 also have a direct 100Mbps link between them)

Both SW1 and SW3 have:

Same Root: 16384.00:00:00:00:00:02
Same Root Path Cost: 19

Tie-breaker needed! Compare Sender Bridge IDs:

SW1: 32768.00:00:00:00:00:01
SW3: 32768.00:00:00:00:00:03
SW1's ID (00:00:00:00:00:01) < SW3's ID (00:00:00:00:00:03)

SW1 wins!

SW1's port toward SW3 becomes DESIGNATED
SW3's port toward SW1 becomes BLOCKED

Final State:

                    SW2 (ROOT)
                   /         \
              [RP]/           \[RP]
                 /             \
              SW1───[DP]  [BLK]───SW3
                  \              /
                   \            /
                    [DP]  [BLK]

Convergence Complete

The network has converged to a loop-free spanning tree. SW2 is the root (lowest priority). SW1 and SW3 have root ports pointing to SW2. The redundant link between SW1-SW3 has one port blocked. Traffic flows without loops, but the physical redundancy is preserved for failover.

Summary: Mastering STP Operation

We've covered the complete operational mechanics of the Spanning Tree Protocol—the message formats, identifiers, and algorithms that enable distributed loop prevention.

Key Takeaways

•BPDUs are the communication mechanism — Configuration BPDUs carry topology information; TCN BPDUs signal changes.
•Bridge ID determines root election — Priority (configurable) + MAC address; lowest wins.
•Path cost determines best route — Higher bandwidth = lower cost. Cumulative cost to root determines Root Port.
•Three timers govern behavior — Hello (2s), Max Age (20s), Forward Delay (15s); set only at root.
•Four-way tie-breaker for BPDUs — Root ID → Path Cost → Sender Bridge ID → Port ID.
•Three port roles — Root Port (toward root), Designated Port (forwarding to segment), Blocked Port (standby).

What's Next:

Now that we understand how BPDUs work and how port roles are determined, we need to examine the most critical decision in any STP network: Root Bridge Election. The next page provides an in-depth exploration of how root bridges are elected, why root bridge placement matters enormously for traffic patterns, and best practices for root bridge design.

Page Complete

You now understand the operational mechanics of STP—how bridges communicate through BPDUs, how port roles are determined, and how the distributed algorithm converges to a loop-free topology. Next, we'll dive deep into root bridge election and its critical importance to network design.

2 / 5

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Computer NetworksSpanning Tree Protocol

Spanning Tree Protocol (STP): Loop-Free Bridged Networks

LevelIntermediate

Duration75 mins

TopicSpanning Tree Protocol

2 / 5

STP Operation — The Protocol Mechanics

The Symphony of Silent Bridges

What You Will Learn

Bridge Protocol Data Units (BPDUs)

BPDU Transmission Rules:

Destination MAC Address: 01:80:C2:00:00:00 (reserved multicast address for spanning tree)
Transmission Interval: Every 2 seconds by default (Hello Time)
Transmission Ports: Sent out all designated and root ports
Reception Behavior: Received on all ports, processed by STP state machine
Not Forwarded: BPDUs are never forwarded to other ports—only locally processed

Why the Special Multicast Address?

IEEE 802.1D BPDU Frame Format
Field	Size (bytes)	Description
Protocol Identifier	2	0x0000 for IEEE 802.1D STP
Protocol Version	1	0x00 for original STP, 0x02 for RSTP
BPDU Type	1	0x00 = Configuration, 0x80 = TCN
Flags	1	TC (Topology Change), TCA (TC Acknowledgment)
Root Bridge ID	8	Priority (2) + MAC Address (6) of root
Root Path Cost	4	Cumulative cost to reach root bridge
Bridge ID	8	Priority (2) + MAC Address (6) of sending bridge
Port ID	2	Priority (1) + Port Number (1) of sending port
Message Age	2	Time since BPDU originated at root
Max Age	2	Maximum BPDU lifetime (default 20 seconds)
Hello Time	2	BPDU transmission interval (default 2 seconds)
Forward Delay	2	Time in Listening/Learning states (default 15 seconds)

Total BPDU Size: 35 bytes (plus Ethernet header and trailer = ~60 bytes on wire)

BPDU Types:

Configuration BPDU (Type 0x00):

The primary BPDU type, containing complete topology information
Sent by designated bridges on designated ports
Used for root election, path cost calculation, and port role determination
Contains all timer values to synchronize the network

Topology Change Notification BPDU (Type 0x80):

Minimal BPDU used to signal topology changes
Contains only Protocol ID, Version, and Type (4 bytes total)
Sent toward the root when a port transitions state
Triggers rapid MAC address table updates across the spanning tree

BPDU Generation at the Root

Bridge Identification and Priority

Every bridge participating in STP must be uniquely identifiable and comparable. The Bridge ID serves this purpose and is the key to root bridge election.

Bridge ID Structure (8 bytes):

┌────────────────────┬────────────────────────────────────────────┐
│  Bridge Priority   │              MAC Address                   │
│    (2 bytes)       │              (6 bytes)                     │
├────────────────────┼────────────────────────────────────────────┤
│  0x8000 (32768)    │        00:1A:2B:3C:4D:5E                   │
│  Default Priority  │        Burned-in or Assigned               │
└────────────────────┴────────────────────────────────────────────┘

Complete Bridge ID: 32768.00:1A:2B:3C:4D:5E

Bridge Priority:

Range: 0 to 65535
Default: 32768 (0x8000)
Lower values = Higher priority
Modern implementations (RSTP/MSTP) use increments of 4096 to accommodate Extended System ID

Extended System ID:

Original 2-byte Priority:     [Priority 16 bits]

With Extended System ID:      [Priority 4 bits][VLAN ID 12 bits]
                                    │                │
                                    │                └── 0-4095 VLANs
                                    └── Priority: 0, 4096, 8192, ..., 61440

Example for VLAN 100:
  Priority = 32768 = 0x8000 = [1000][000001100100]
                              = Base 32768 + VLAN 100
  Effective Bridge ID = 32868.00:1A:2B:3C:4D:5E

Bridge ID Comparison Rules

•Compare priority first: Lower numeric priority wins
•If priority equal, compare MAC address: Lower MAC address wins
•Result is deterministic: Given any two bridges, one is definitively 'better'
•Lowest Bridge ID becomes root: The bridge with the lowest Bridge ID is elected root

Bridge ID Comparison Examples
Bridge A ID	Bridge B ID	Winner	Reason
4096.00:11:22:33:44:55	32768.00:AA:BB:CC:DD:EE	Bridge A	Lower priority (4096 < 32768)
32768.00:11:22:33:44:55	32768.00:AA:BB:CC:DD:EE	Bridge A	Equal priority, lower MAC
32768.00:AA:BB:CC:DD:EE	32768.00:11:22:33:44:55	Bridge B	Equal priority, B has lower MAC
0.00:FF:FF:FF:FF:FF	4096.00:00:00:00:00:01	Bridge A	Priority 0 beats all

Best Practice: Set Priority Explicitly

Path Cost Calculation

Once the root bridge is elected, every other bridge must determine the best path to reach it. Path cost provides the metric for this calculation, with lower costs indicating more desirable paths.

How Path Cost Works:

Each port has an associated port cost (based on link speed)
When a bridge receives a BPDU, it adds its incoming port's cost to the BPDU's Root Path Cost
The Root Path Cost is the cumulative cost from this bridge to the root
The port with the lowest Root Path Cost becomes the Root Port

                        ROOT BRIDGE
                    Cost to root = 0
                            │
                    ────────┴────────
                   100Mbps  │  1Gbps
                   cost=19  │  cost=4
                            │
              ┌─────────────┼─────────────┐
              │             │             │
          SW-A (19)     SW-B (4)      SW-C (19)
      path cost = 19   path cost=4   path cost=19

IEEE 802.1D-1998 Port Costs (Original)
Link Speed	Port Cost	Notes
4 Mbps	250	Token Ring
10 Mbps	100	Original Ethernet
16 Mbps	62	Fast Token Ring
100 Mbps	19	Fast Ethernet
1 Gbps	4	Gigabit Ethernet
10 Gbps	2	10 Gigabit Ethernet

IEEE 802.1D-2004 Port Costs (Revised)
Link Speed	Recommended Cost	Notes
10 Mbps	2,000,000	More granularity for high speeds
100 Mbps	200,000	Proportional to bandwidth
1 Gbps	20,000	10x faster = 10x lower cost
10 Gbps	2,000	Standard for modern data centers
100 Gbps	200	Emerging standard
1 Tbps	20	Future-proofing

Path Cost Calculation Example:

                      ROOT (Priority 4096)
                            │
                     ┌──────┴──────┐
               1Gbps │             │ 100Mbps
               (4)   │             │ (19)
                     │             │
                   SW-A          SW-B
                     │             │
               1Gbps │             │ 1Gbps
               (4)   │             │ (4)
                     │             │
                     └──────┬──────┘
                          SW-C
                     (Access Layer)

SW-C's Path Cost Calculation:

Path through SW-A: 4 (to ROOT) + 4 (to SW-C) = 8
Path through SW-B: 19 (to ROOT) + 4 (to SW-C) = 23
Winner: Path through SW-A (cost 8)
SW-C's Root Port: Port connecting to SW-A

Administrative Cost Override

STP Timers and Their Critical Roles

STP relies on three configurable timers that control protocol behavior. Understanding these timers is essential for both troubleshooting and optimization.

The Three STP Timers:

Hello Time (Default: 2 seconds)

•Purpose: Interval between BPDU transmissions
•Range: 1-10 seconds
•Set by: Root bridge only (propagated in BPDUs)
•Impact of shorter values: Faster failure detection, more CPU/bandwidth usage
•Impact of longer values: Slower failure detection, lower overhead

Max Age (Default: 20 seconds)

•Purpose: Maximum time a bridge stores BPDU information
•Range: 6-40 seconds
•Set by: Root bridge only
•Failure detection: If no BPDU received for Max Age, port assumes link failure
•Determines convergence speed: Longer Max Age = slower reaction to failures

Forward Delay (Default: 15 seconds)

•Purpose: Time spent in Listening and Learning states
•Range: 4-30 seconds
•Set by: Root bridge only
•Port transition: Port waits Forward Delay twice (Listening → Learning → Forwarding)
•Total transition time: 2 × Forward Delay = 30 seconds by default

STP Timer Summary and Convergence Impact
Timer	Default	Range	Affects
Hello Time	2 seconds	1-10	BPDU frequency, overhead
Max Age	20 seconds	6-40	Failure detection time
Forward Delay	15 seconds	4-30	Port transition time
Total Convergence	30-50 seconds	—	Max Age + 2×Forward Delay

Why Such Long Default Timers?

The default timers were designed for networks with significant propagation delays and many bridges:

Network diameter consideration: BPDUs must propagate through the entire network before decisions are made
Message Age tracking: The Message Age field in BPDUs increments as they traverse bridges. Max Age must accommodate the maximum network diameter.
Stability priority: STP prioritizes stability over speed. Rushing port transitions risks temporary loops.

The 5-4-3-2-1 Rule Implication:

Timer Modification Warning

BPDU Processing and Decision Algorithm

BPDU Comparison: The Four-Part Tie-Breaker

When comparing two BPDUs (received vs. stored), bridges use this priority order:

BPDU Superiority Rules (in order)

•Lowest Root Bridge ID wins — The BPDU claiming a lower root is always superior
•Lowest Root Path Cost wins — If same root, the path with lower cost is better
•Lowest Sender Bridge ID wins — If same root and cost, the lower sending bridge ID wins
•Lowest Port ID wins — If all else equal, lower port ID (priority + number) wins

bpdu_comparison.pseudo
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// BPDU Comparison Algorithm
// Returns: true if received BPDU is superior to stored BPDU
 
function isReceivedBPDUsuperior(received: BPDU, stored: BPDU): boolean {
    // Step 1: Compare Root Bridge IDs
    if (received.rootBridgeId < stored.rootBridgeId) {
        return true;  // Lower root ID = superior
    }
    if (received.rootBridgeId > stored.rootBridgeId) {
        return false; // Higher root ID = inferior
    }
    
    // Root IDs equal, continue comparison
    
    // Step 2: Compare Root Path Costs
    let receivedCost = received.rootPathCost + thisPort.pathCost;
    if (receivedCost < stored.rootPathCost) {
        return true;  // Lower cost = superior
    }
    if (receivedCost > stored.rootPathCost) {
        return false; // Higher cost = inferior
    }
    
    // Path costs equal, continue comparison
    
    // Step 3: Compare Sender Bridge IDs
    if (received.senderBridgeId < stored.senderBridgeId) {
        return true;  // Lower sender ID = superior
    }
    if (received.senderBridgeId > stored.senderBridgeId) {
        return false; // Higher sender ID = inferior
    }
    
    // Sender IDs equal (same switch, different ports), compare Port IDs
    
    // Step 4: Compare Port IDs
    if (received.portId < stored.portId) {
        return true;  // Lower port ID = superior
    }
    
    return false; // Received BPDU is not superior
}

What Happens When a Superior BPDU Arrives:

Update stored information: The port stores the superior BPDU's data
Recalculate port role: Determine if this port should become Root Port
Propagate changes: If this bridge's topology information changed, update outgoing BPDUs
Trigger topology change: If port states change, initiate TCN if appropriate

What Happens When an Inferior BPDU Arrives:

If the received BPDU is inferior to stored information:

The BPDU is ignored for topology decisions
The port continues in its current role
If this port is Designated, it responds with its own superior BPDU

This mechanism ensures that "better" topology information always wins, and the entire network converges to agree on the same root and spanning tree.

The Beauty of Distributed Convergence

Port Role Determination

Every port on every bridge in the spanning tree is assigned one of three roles. Understanding these roles is fundamental to understanding STP behavior.

The Three Port Roles in 802.1D STP:

Root Port (RP)

•Definition: The port with the best (lowest cost) path to the root bridge
•Count: Exactly ONE root port per non-root bridge
•Direction: Points TOWARD the root bridge
•State: Always in Forwarding state (when topology stable)
•BPDU behavior: Receives BPDUs from upstream designated bridge

Designated Port (DP)

•Definition: The port responsible for forwarding traffic toward a segment
•Count: Exactly ONE designated port per network segment
•Direction: Points AWAY from the root bridge
•State: Always in Forwarding state (when topology stable)
•BPDU behavior: Sends BPDUs onto the segment

Blocked Port (Non-Designated / Alternate)

•Definition: A port that would create a loop if it forwarded traffic
•Count: All ports that aren't Root or Designated
•Direction: N/A (not actively forwarding)
•State: Blocking state (receives and processes BPDUs only)
•BPDU behavior: Receives but does not send Configuration BPDUs

Port Role Selection Algorithm:

For each bridge:

1. Am I the Root Bridge? (My Bridge ID is lowest in all received BPDUs)
   └── YES: All my ports are DESIGNATED PORTS
   └── NO: Continue...

2. Determine my ROOT PORT:
   - Compare all received BPDUs
   - Port receiving best BPDU = ROOT PORT
   - Best = lowest (Root ID, then Path Cost, then Sender ID, then Port ID)

3. For each remaining port, determine DESIGNATED vs BLOCKED:
   - Am I the best path to the root for this segment?
   - Compare my BPDU (from root port) against received BPDUs on this port
   - If my BPDU is better → This port is DESIGNATED
   - If my BPDU is worse → This port is BLOCKED

4. The Root Bridge's ports are always DESIGNATED (it is the best path by definition)

Port Role Summary
Role	Exists On	State	BPDU Behavior	Traffic Direction
Root	Non-root bridges	Forwarding	Receives	Toward root
Designated	All bridges	Forwarding	Sends	Away from root
Blocked	Non-root bridges	Blocking	Receives only	None (blocked)

Root Bridge Port Roles

BPDU Processing: Worked Examples

Let's trace through BPDU processing in a concrete example to solidify understanding.

Network Topology:

              SW1                    SW2                    SW3
    (32768.00:00:00:00:00:01)  (16384.00:00:00:00:00:02)  (32768.00:00:00:00:00:03)
              │                        │                        │
         Port 1                   Port 1                   Port 1
              │                        │                        │
              └─────────Link A─────────┴─────────Link B─────────┘
                                  (100 Mbps each, cost = 19)

Step 1: Initial State

When switches boot, each believes it is the root:

SW1 sends: Root = 32768.00:00:00:00:00:01, Cost = 0, Sender = SW1
SW2 sends: Root = 16384.00:00:00:00:00:02, Cost = 0, Sender = SW2
SW3 sends: Root = 32768.00:00:00:00:00:03, Cost = 0, Sender = SW3

Step 2: BPDU Comparison at SW1

SW1's Port 1 receives BPDU from SW2 claiming:

Root = 16384.00:00:00:00:00:02 (SW2)
Root Path Cost = 0
Sender Bridge = SW2

SW1 compares:

SW2's claimed root (16384.xxx) < SW1's claimed root (32768.xxx)
SW2 wins! SW1 now knows SW2 is the root.

SW1 updates:

Root Bridge = 16384.00:00:00:00:00:02
My Root Path Cost = 0 + 19 (Link A cost) = 19
Port 1 becomes ROOT PORT

Step 3: BPDU Comparison at SW3

SW3 receives SW2's BPDU (same as SW1):

SW2's claimed root (16384.xxx) < SW3's claimed root (32768.xxx)
SW2 wins! SW3 now knows SW2 is the root.

SW3 updates:

Root Bridge = 16384.00:00:00:00:00:02
My Root Path Cost = 0 + 19 (Link B cost) = 19
Port 1 becomes ROOT PORT

Step 4: Designated Port Decision on Link Between SW1-SW3

(Assume SW1 and SW3 also have a direct 100Mbps link between them)

Both SW1 and SW3 have:

Same Root: 16384.00:00:00:00:00:02
Same Root Path Cost: 19

Tie-breaker needed! Compare Sender Bridge IDs:

SW1: 32768.00:00:00:00:00:01
SW3: 32768.00:00:00:00:00:03
SW1's ID (00:00:00:00:00:01) < SW3's ID (00:00:00:00:00:03)

SW1 wins!

SW1's port toward SW3 becomes DESIGNATED
SW3's port toward SW1 becomes BLOCKED

Final State:

                    SW2 (ROOT)
                   /         \
              [RP]/           \[RP]
                 /             \
              SW1───[DP]  [BLK]───SW3
                  \              /
                   \            /
                    [DP]  [BLK]

Convergence Complete

Summary: Mastering STP Operation

We've covered the complete operational mechanics of the Spanning Tree Protocol—the message formats, identifiers, and algorithms that enable distributed loop prevention.

Key Takeaways

•BPDUs are the communication mechanism — Configuration BPDUs carry topology information; TCN BPDUs signal changes.
•Bridge ID determines root election — Priority (configurable) + MAC address; lowest wins.
•Path cost determines best route — Higher bandwidth = lower cost. Cumulative cost to root determines Root Port.
•Three timers govern behavior — Hello (2s), Max Age (20s), Forward Delay (15s); set only at root.
•Four-way tie-breaker for BPDUs — Root ID → Path Cost → Sender Bridge ID → Port ID.
•Three port roles — Root Port (toward root), Designated Port (forwarding to segment), Blocked Port (standby).

What's Next:

Page Complete

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