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Spanning Tree Protocol Topic Notes

STP Overview Summary

Spanning-Tree Protocol Overview Summary

  • Provides a loop-free Layer 2 topology
  • Uses the concept of a Root Bridge
    • All Layer 2 traffic is forwarded towards the Root Bridge

 

  Name  Standard   Resources  Convergence Time VLANs 
CST  Common Spanning Tree 802.1d Low Slow All VLANs
PVST+  Per-VLAN Spanning Tree Cisco High Slow Per VLAN
RSTP  Rapid Spanning Tree 802.1w Medium Fast All VLANs
PVRST+  Per-VLAN Rapid Spanning Tree Cisco Very High Fast Per VLAN
MSTP  Multiple Spanning Tree 802.1s Medium or High Fast VLAN List

 

STP Topology

STP Topology

  • Process to build the STP topology:
    • Elect a Root Bridge
    • Elect Non-Root Bridges Root ports
    • Elect Designated ports

 

Bridge Protocol Data Units (BPDUs)

Bridge Protocol Data Units

  • 2 types of BPDUs
    • Configuration BPDU
    • Topology Change Notification (TCN) BPDU
  • Both types sent in STP Hello BPDUs

 

Configuration BPDUs Topology Change Notification BPDUs
 Field Length(Bytes) Value  Field Length(Bytes) Value
 Protocol Identifier  2 0x0000  Protocol Identifier  2 0x0000
Protocol Version 1 0x00 Protocol Version 1 0x00
BPDU Type 1 0x00 BPDU Type 1 0x80
Flags 1  Indicates message information
Root Bridge ID 8 RBID
Root Path Cost 4 RPC
Sending Bridge ID 8 SBID
Sending port ID 2 SPID
 Message Age 2 Age since originated from RB
Max Age 2 When current configuration
message should be deleted
Hello Time 2  Time between Hello's
Forward Delay 2 Time to wait before transitioning
to a new state after topology change

 

  • Further field information:
  • Version ID:
    • 0x00 Config & TCN
    • 0x02 RST
    • 0x03 MSTP
    • 0x04 SPT
  • BPDU Type:
    • 0x00 Config BPDU
    • 0x80 TCN BPDU
    • 0x02 RST BPDU
  • Flags:
    • 1 :
      • 1 = Topology Change
    • 2 :
      • 1 = Proposal
    • 3-4 :
      • 01 = Alternate/Backup Port Role
      • 10 = Root Port Role
      • 11 = Designated Port Role
    • 5 :
      • 1 = Learning
    • 6 :
      • 1 = Forwarding
    • 7 :
      • 1 = Agreement
    • 8 :
      • 1 = Topology Change Acknowledgement
  • Sending Bridge ID:
    • CIST Regional Root ID in MST/SPT BPDU
  • Message Age:
    • 2 bytes in 1/256 secs
  • Max Age:
    • 2 bytes in 1/256 secs
  • Hello Time:
    • 2 bytes in 1/256 secs
  • Forward Delay:
    • 2 bytes in 1/256 secs

 

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Bridge ID (BID)

Bridge ID

Original 802.1d format Bridge ID

 

Priority System ID
(0 - 65535) MAC Address
 2 Bytes 6 Bytes

 

Current Format with System Extension Bridge ID

 

Priority System ID Extension System ID
Multiple of 4096 Contains VLAN 1-4096 MAC Address
 4 bits  12 bits 6 Bytes

 

  • Bridge ID consists of:
    • Bridge Priority
      • Value of 0 - 61440
        • Set in increments of 4096
      • Default is 32768
      • Lower value is preferred
      • 4 bits
    • System ID Extension
      • Value of 0 - 4095
      • VLAN ID
      • 12 Bits
    • MAC Address
      • 6 Bytes (48 bits)

 

  • Bridge ID (BID) can be set manually
    • With specific priority value
      • Command:
        • (config)#spanning-tree vlan <vlan> priority <1 - 61440>
    • Using root bridge macro
      • Command:
        • (config)#spanning-tree vlan <vlan> root [primary | secondary]
      • Sets the priority number based on the current Root Bridge priority
        • 4096 lower than current Root Bridge

 

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Root Bridge

STP Root Bridge
  • Used as a reference point for other Non-Root Bridges to determine path
  • Only a single Root Bridge is elected in Layer 2 network
  • Only the Root Bridge Generates BPDUs
    • All other bridges forward them on
  • Root Bridge sets timers
    • All other bridges use these timer settings
    • Can set manually on NON-Root bridges but unless that switch becomes the Root Bridge, the timers are ignored
    • Timers include:
      • Hello timer
      • Fordwarding Delay timer
      • Max_Age timer

 

Root Bridge Election
  1. All switches begin by sending STP Hello BPDUs claiming to be the Root Bridge
  2. If a switch receives a superior Hello BPDU (Lower BID), it stops claiming to be the Root Bridge by stopping originating Hello BPDUs
  3. Switch starts forwarding the superior Hello BPDUs received
  4. Eventually all switches stop forwarding Hello BPDUs except the Root Bridge's Hello BPUs
  5. Switch with lowest Bridge ID in the network becomes the Root Bridge.

 

STP Root Bridge

Figure 1 - STP Bridge Election

 

  • In the above diagram, all priorities being equal, SW1 will become the Root Bridge as it has the lowest MAC address,
    • Therefore the lowest Bridge ID

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Root Port Election

Root Port Election
  • Once Root Bridge elected, the following happens:
    • All Non-Root Bridges elect a Root Port
    • All Non-Root Bridges elect their Designated Ports
    • All Non-Root Bridges put all other ports as Alternate Ports

 

  •  Process:
  1. Each Non-Root Bridge adds the local port cost to the Root Path Cost (RPC) of received BPDUs
  2. Lowest or superior RPC is now set to Root Port
  3. Hellos received on Root Port are forwarded through Designated Ports
    1. Updates RPC, Sender Bridge ID, Sender Port ID and MessageAge
    2. Hellos received on other ports of Non-Root Bridge are processed but not forwarded
  4. Do not forward Hellos out Root Ports or blocking ports

 

  • Selection:
  • Prefer neighbor advertising lowest cost to root (RPC)
  • Equal cost tie breakers:
    1. Prefer neighbor with lowest Bridge ID (BID)
    2. Prefer the lowest Sender Port ID (SPID)
      1. Default value is 128 + port number
        1. 1st port will be 128.1
        2. 20th port will be 128.20 etc

 

STP Port Election 1

Figure 2 - Root Bridge Cost Advertisement

 

STP Port Election 2

Figure 3 - Port Priority used for tie-breaker

 

 

STP Port Election 3

Figure 4 - Port Role allocation

 

STP Port Election 4

Figure 5 - Port Role Allocation in 3 Switch Topology

 

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Designated Port Election

Designated Port Election
  • Only the Designated Port forwards Hellos on to a segment
  • On the Root Bridge, all ports are Designated Ports
  • Designated Ports face away from the Root Bridge
  • To become a Designated Port a switch must send superior BPDUs on a LAN segment
  • Uses same selection process as Root Port election:
    • Prefer neighbor advertising lowest cost to root (RPC)
    • Equal cost tie breakers:
      • Prefer neighbor with lowest Bridge ID (BID)
      • Prefer the lowest Sender Port ID (SPID)

 

  • Refer to Figure 4 and 5 for Port Role allocation on topology example

 

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STP Timers

Spanning Tree Timers
  • 3 main timers are used by Spanning Tree
    • Hello timer
      • Default is 2 seconds
      • Time between each BPDU sent on port
      • Originated by Root Bridge
        • Sent out all DP
        • Received on all RP
      • Can be configured manually on Root Bridge only
        • Command:
          • (config)#spanning-tree vlan <vlan-id> hello-time <1-10 seconds>
    • Forward Delay timer
      • Default of 15 seconds
      • Time to spend in Listening and Learning port states
      • Can be configured manually on Root Bridge only
        • Command:
          • (config)#spanning-tree vlan <vlan-id> forward-time <4-30 seconds>
    • Max Age timer
      • Default of 20 seconds
      • Length of time to wait before initiating a topology change if stops receiving Hllo BPDUs
      • Can be configured manually on Root Bridge only
        • Command:
          • (config)#spanning-tree vlan <vlan-id> max-age <6-40>

 

STP Topology Change

Spanning Tree Topology Change

  • Happens when:
    • Receives Topology Change Notification (TCN) BPDU on DP
    • Port moves from Learning or Forwarding to Blocking
    • A switch becomes the Root Bridge
    • Port moves to Forwarding and has at least 1 DP
  • When Topology Change happens, the CAM table needs to be flushed due to invalid entries
    • All switches notified to flush CAM
    • Each switch uses a short timer equivalent to Forward Delay to time out CAM table entries

 

STP TCN

Figure 6 – STP Topology Change Process

 

 

  1. Topology change event
  2. Each switch generates a TCN BPDU sent out its Root Port towards the Root Bridge for every Hello time until receives acknowledgement.
  3. Once received TCN BPDU, each switch sends back an acknowledgement through its next forwarded BPDU by setting the TCA bit in the flags field
  4. Once TCN reaches Root Bridge, RB floods throughout network with TC bit set, for MaxAge + Forward Delay. Informs other switches to reduce CAM age time to Forward Delay value

 

STP Path Cost

Spanning Tree Path Cost

  • STP Path Cost is accumulated based on bandwidth
  • The higher the bandwidth of a link, the lower the cost
  • The cost is all relative to the Root Bridge

 

Default Costs

802.1D-1998 802.1D-2004
Port Speed CST/RSTP MSTP
10Mbps 100 2000000
100Mbps 19 200000
1Gbps 4 20000
10Gbps 2 2000

 

  • 802.1d-2004 costs can be enabled manually for RSTP or PVST
    • Command:
      • (config)#spanning-tree pathcost method long
  • Default command is:
    • (config)#spanning-tree pathcost method short

 

STP Modes

STP Mode: Common Spanning Tree (CST)

Common Spanning Tree

  • Original STP specification
  • 802.1D standard
  • Legacy protocol
  • Not recommended for use
  • All VLANs under single instance
  • Flags field only uses 2 out of the 8 bits
    • Bit 1: Topology Change
    • Bit 8: Topology Change Acknowledgement

 

CST Port Roles

CST Port Roles

  1. Root Port
    • Forwarding state
    • Only on Non-Root Bridges
      • Only single port towards Root Bridge
    • Forwards traffic to Root Bridge
  2. Designated Port
    • Forwarding state
    • On Root and Non-Root Bridges
      • Ports facing away from Root Bridge
    • Receives traffic going towards Root
    • On Root Bridge all ports are Designated
  3. Non-Designated Port
    • Blocking state
    • Only on Non-Root Bridges
    • Receives BPDUs
    • Discards all other traffic
    • Unable to send traffic
  4. Disabled
    • Shutdown port
    • Doesn't participate in STP

 

STP Port Roles

Figure 7 - Port Role allocation with CST

 

    • As all priorities are equal and MAC addresses will be the same for both links, the selection is made on Port priority which is 128 plus the port number.
      • Lower value is preferred

 

  • To manually change a Port Role:
    • Modify the cost of the port on an interface
      • This changes the default cost added to BPDUs Root Path Cost in the inbound direction
      • Command:
        • (config-if)#spanning-tree vlan <vlan> cost
        • (config-if)#bandwidth <kbps>
    • Modify the Port ID
      • If the costs are equal, this will advertise a different port priority to neighbor.
      • Command:
        • (config-if)#spanning-tree vlan <vlan> port-priority <port-priority>
        • <port-priority> must be in increments of 64 (0-192)

 

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CST Port States

CST Port States

  1. Blocking
    • Receives BPDUs to determine location of Root Bridge
    • Would cause a loop if active
    • Time in state set by MaxAge timer
      • Default 20 seconds
  2. Listening
    • Receives and transmits BPDUs
    • Doesn't populate MAC table
    • Doesn't forward frames
    • Time in state set by Forward Delay timer
      • Default 15 seconds
  3. Learning
    • Prepares to participate in forwarding
    • Doesn't forward frames
    • Populates CAM table
    • Time in state set by Forward Delay time
      • Default 15 seconds
  4. Forwarding
    • Considered part of the active topology
    • Populates CAM table
    • Sends and receives BPDUs
    • Forwards frames
  5. Disabled
    • Doesn't participate in STP
    • Doesn't forward frames

 

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STP Mode: RSTP

Rapid Spanning-Tree (RSTP)

  • 802.1W standard
  • Single STP instance covering all VLAN
  • Automatically backwards compatible with CST
    • Shown by "P2P Peer (STP)" on Link Type
    • Will revert to legacy protocol process on this interface
  • Command:
    • (config)#spanning-tree mode rapid-pvst
  •  Flags field
    • Doesn't use TCA bit
    • Uses a Proposal/Agreement process
      • On P2P link
  • Each switch originates its own BPDUs
    • Contents based on Root Switch BPDU

 

RSTP Root Port Election

RSTP Root Port Election
  • Uses a Proposal/Agreement and Synchronisation operation
  • When electing a Root port, assume all other Non-Edge ports are Designated
    • Non-Edge ports are Discarding

 

STP RSTP Proposal-Agreement

Figure 8 - RSTP Proposal/Agreement and Sync process

 

 

  1. Sends proposal out all Designated ports
    • Proposal has Port role set to Designated
    • Contains Root Bridge information
    • Set to Discarding state
  2. Downstream switch reviews and synchronises information
    • If they don't have a better path to the Root Bridge, they agree
      • Elects a local Root Port
      • Blocks all non-edge Designated ports
      • Starts sync process on all Designated ports (Step 4)
    • If they have a better path, they announce their information
      • Local switch changes Root Port
  3. Downstream switch sends agreement to  upstream switch
    • When Designated port receives agreement
      1. Port is unblocked
      2. Moved straight to Forwarding state
  4. Proposal sent to next downstream switch
  5. Downstream switch reviews and synchronises information
  6. Downstream switch sends agreement to  upstream switch
  7. Process continues.....

 

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RSTP Port Roles

RSTP Port Roles
  1. Root Port
    • Does not use link-type parameter
    • Same role as 802.1d
    • Forwarding state
  2. Alternate
    • Does not use link type parameter in most cases
    • Equivalent of Uplinkfast
    • Fast Root path recovery
    • Replacement for Root Port
    • Discarding state
  3. Designated
    • Uses link type parameter
      • Rapid transition to forwarding only occurs if link type P2P
    • Same role as 802.1d
    • Forwarding state
  4. Backup
    • Replacement for Designated port
    • Activated if primary Designated Port fails
    • Discarding state
    • Not quick transition, driven by timers
  5. Edge port
    • Equivalent of Portfast
    • Straight to forwarding state
    • Doesn't generate TCN if changes state

 

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RSTP Port States

RSTP Port States
  1. Discarding
    • Outbound BPDUs have Proposal bit set
    • Default state when newly activated (unless Edge port)
    • Doesn't forward or receive frames
    • Processes BPDUs
    • Send and receives inter-switch signalling protocols such as LACP, DT< CDP, VTP, etc
  2. Learning
    • Outbound BPDUs have Proposal bit set
    • Same functionality as 802.1d Learning state
  3. Forwarding
    • Same functionality as 802.1d Forwarding state
    • Forwards frames

 

  • Ports are put into Designated Blocking state until they receive BPDU from counterpart

 

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RSTP Link Types

RSTP Link Types
  • Edge Ports
    • immediately transitions to Designated Forwarding state
    • Similar to PortFast
    • Never have a switch connection
    • When receives a BPDU
      • Looses Edge port status and becomes Non-Edge STP port
      • Generates a TCN
  • Non-Edge Ports
    • Default port type on Cisco switches
    • Point-to-Point
      • Full Duplex
      • Single RSTP switch connection
    • Shared
      • Half Duplex
      • Multiple RSTP switch connections

 

  • Link-type must be accurate
    • Can be configured manually
      • Command:
        • (config-if)#spanning-tree link-type <point-to-point | shared>
        • (config-if)#spanning-tree portfast [trunk]

 

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RSTP Timers

RSTP Timers
  • Hello timer
    • Each bridge generates own BPDUs
    • Every 2 seconds by default
    • If 3 hellos missed from neighbor, reconvergence begins
      • Information on port is aged out
      • 6 seconds vs. 20 seconds MaxAge
  • MaxAge used as hop count
    • Every bridge sends BPDUs on own
    • If BPDU MessageAge is equal to or higher than MaxAge, it is discarded
    • MaxAge also used on Shared ports for legacy CST backwards compatibility
  • Faults detected faster based on physical layer signalling

 

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RSTP Re-convergence

RSTP Re-convergence
  • Needs to re-converge if Root port is lost
  • If there is an Alternate port, it is selected in its place
    • New Root port then synchronised with downstream bridges
    • Same functionality as Uplinkfast
  • If there is no Alternate port and no better information available
    • Declare local bridge as Root bridge
    • Synchronise decision
    • Adapt to better information
  • Keep topology as small as possible
    • RSTP suffers from count-to-infinity depending on scale of design

 

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STP Mode: MSTP

Multiple Spanning-Tree

 

  • 802.1S standard
  • Convergence a lot quicker than PVST+
  • Backward compatible with
    • 802.1D (CST)
    • 802.1W (RSTP)
  • Inherits all RSTP functionality
  • If doesn't hear response from other bridges in MST, falls back to legacy protocol
    • Displayed as port type P2P Bound (STP)
  • CIST (Common and Internal Spanning Tree) Root must be within MST domain
    • Behaves like inter-region MST
    • Maps multiple VLANs to a single Spanning Tree instance
      • As opposed to 1 instance per VLAN (RSTP) or 1 instance for all VLANs (CST)
      • provides better scalability
  • Decouples VLAN and STP instance
  • Enables load balancing across multiple paths

 

MSTP Bridge ID

MSTP Bridge ID
  • Consists of:
    • Bridge Priority
      • 4 bits
      • Increments of 4096
    • Extended System ID
      • 12 bits
      • Carries MST instance number instead of VLAN number
    • MAC Address
      • 6 bytes (48 bits)

 

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MSTP Regions

MSTP Regions

 

STP MST Regions

Figure 9 - MST Regions

 

 

  • Each switch has a single MST Region configuration
    • All bridges must agree on configuration
  • Region Configuration consists of:
    • Name
      • 32 Bytes
    • Revision Number
      • 2 bytes
    • VLAN Association table
      • VLAN to STP instance mappings
      • Instance 0 used for CIST (Common Internal Spanning Tree) which is used for Inter-region root bridge election
  • This must be configured manually on each switch
  • VTPv3 supports MST including mappings
  • VLAN to instance mapping not propagated in BPDUs
  • If 2 switches differ on 1 or more attributes, they are part of 2 different regions
  • BPDUs contain only a digest of VLAN to instance mapping, revision number and name

 

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MSTP Intra vs. Inter Region

Intra Region vs Inter Region
  • Intra region
    • Details known within MST Region
    • VLAN to STP instances are manually defined
    • Undefined VLANs fall into CIST (MST 0)
  • Inter Region
    • Details between MST Regions are not known
    • Different regions see each other as Virtual Bridges (Figure 11)
      • Simplified Inter-Region calculation
      • Seen as a single switch
      • Intra-region MSTs are collapsed into CIST
    • A Regional Root Bridge is elected internally per Region
    • CIST Root bridge is elected for all Regions (Figure 10)
    • Unlikely to see in a real design as scalability/hardware issues
      • Too many MAC addresses on network
      • Not enough TCAM memory etc.

 

STP MST Multi-Region

Figure 10 - Multiple Regions

 

 

STP MST Virtual Bridges

Figure 11 - MST CIST 0 Virtual Bridges

 

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MSTP Configuration

MSTP Configuration
  • Real configuration would need to start on Root Bridge and work out

 

  1. Set the Spanning-Tree mode
    • Command:
      • (config)#spanning-tree mode mst
  2. Enable MST globally
    • Command:
      • (config)#spanning-tree mst configuration
  3. Define Region Name
    • Command:
      • (config-mst)#name <instance name>
  4. Define Revision Number
    • Command:
      • (config-mst)#revision number <1 - 65535>
  5. Define VLAN to instance mappings
    • Command:
      • (config-mst)#instance <instance> vlan <vlan IDs>

 

  • To change the Root Bridge manually
    • Command:
      • (config)#spanning-tree mst <instance> priority <priority>
  • To change Port cost manually
    • Command:
      • spanning-tree mst <instance> cost <cost>
  • To change Port ID manually
    • Command:
      • (config)#spanning-tree mst <instance> port-priority <priority>

 

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Cisco STP Toolkit

Cisco STP Toolkit

Portfast

Portfast
  • Configures interface type as Edge ports
  • Transitions directly to forwarding state
  • Effects TCN generation
  • Spanning Tree not enabled on these ports
  • Configured in Global mode:
    • Enables on all ports unless BPDUs received
      • Transitions to normal STP port
    •  Command:
      • (config)#spanning-tree portfast default
  • Configured in Interface mode
    • Command:
      • (config-if)#spanning-tree portfast
      • (config-if)#spanning-tree portfast trunk

 

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Uplinkfast

UplinkFast
  • Provides 3 to 5 seconds of convergence after a Root port link failure if alternate port available
  • Could potentislly change topology as other switches may avoid uplinkfast switch as metric set high
  • Not required with PVRST+ or Rapid Spanning Tree
  • Command:
    • (config)#spanning-tree uplinkfast

 

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BackboneFast

 

BackboneFast
  • Cuts convergence time by MAX_AGE for an indirect failure
  • When receives inferrior BPDU
  • Needs to be enabled everywhere
  • Not required with PVRST+
  • Command:
    • (config)#spanning-tree backbonefast

 

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BPDU Guard

BPDU Guard
  • Shuts down port if BPDU received
  • Configured in Global mode
    • Command:
      • (config)#spanning-tree portfast bpduguard default
  • Configured in Interface mode
    • Command:
      • (config-if)#spanning-tree bpduguard enable

 

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BPDU Filter

BPDU Filter
  • Configured in Global mode
    • Any Portfast port receiving BPDU becomes a standard port
    • Command
      • (config)#spanning-tree portfast bpduguard default
  • Configured in Interface mode
    • Ignores BPDUs and doesn't send any
    • Command:
      • (config-if)#spanning-tree bpdufilter enable

 

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Root Guard

RootGuard
  • If receives superior BPDU than current Root Bridge, port is moved to a Root-Inconsistent state (STP Listening)
  • Can not be used at the same time as LoopGuard
    • LoopGuard does opposite of RootGuard
  • Configure on downstream Designated ports
  • Command:
    • (config-if)#spanning-tree guard root

 

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Loop Guard

Loop Guard
  • Similar to UDLD
  • If link isn't receiving BPDUs, moves to a Loop-Inconsistent state (STP Blocking)
    • When receives BPDU, transitions normally
    • Only monitors Non-Designated ports and prevents them from becoming Designated
  • Used on unidirectional links
  • Can not be used at the same time as RootGuard
    • RootGuard does opposite of LoopGuard
  • Configured in Global mode:
    • Command:
      • (config)#spanning-tree loopguard default
  • Configured in Interface mode:
    •  Command:
      • (config-if)#spanning-tree guard loop

 

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STP UDLD

UniDirectional Link Detection

  • Cisco proprietary feature
  • UDLD detects when a link is unidirectional and shuts down the affected interface
  • Layer 2 protocol that works with Layer 1 mechanisms
  • Periodically transmits UDLD packets on enabled interface
  • If packets are not echo’d back, link is considered unidirectional
  • Devices both ends must support UDLD
  • UDLD uses well-known MAC address 0100.0CCC.CCCC to send frames
    • Each switch sends its own Device ID, Originator Port ID and timeout value to neighbor
    • Remote peer echos back ID of neighbor
    • If no frame received with device’s own ID for a certain amount of time, port considered unidirectional
  • Modes:
    • Normal
      • If stops receiving UDLD message but physical port is up, changes port state to Undetermined.
      • Doesn’t disable the port
      • More informational
      • Doesn’t prevent physical loops
    • Aggressive
      • Sends frames 8 times ievery seconds to attempt to re-establish UDLD
      • If no response port considered Unidirectional
      • Put in err-disable state
      • Not automatically recovered unless err-disable recovery is configured
      • Preferred method of UDLD
  • Subnetwork Access Protocol (SNAP)
  • High Level Data-Link Control (HDLC) protocol type 0x0111
  • Globally disabled by default
    • Per-interface Fiber enabled by default
    • Per-interface Copper disabled by default

 

Flex-Links

FlexLinks in access layer

  • Layer 2 availability feature
  • Can coexist with STP
  • Enhancement allows convergence time of less than 50 milliseconds
  • Active/Standby link pair is defined on a common access switch
    • Interface can belong to only one flexlink
    • Different interface types are allowed
  • Loops are not detected due to no STP
  • Failover is 1 to 2 seconds
  • Supported on 4500 and 6500 series switches
  • Supported on Layer 2 ports and port channels
    • Not supported on VLAN interface or L3 ports

 

STP Troubleshooting Commands

Troubleshooting Commands

#show spanning-tree vlan <vlan>

#show spanning-tree root – Displays Root Bridge

 #show spanning-tree detail – Shows end-to-end costs

#show spanning-tree interface detail – Shows detailed interface information on spanning tree

 

 

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