CCNP Exam Prep Tips and Must Knows

studentleader246016Things You Need to Know about Spanning Tree Protocol (STP)

STP Standards 

  • 802.1D – 1998: Legacy STP
  • CST: Assumes one Spanning Tree instance for the entire bridged network
  • PVST+: A Cisco enhancement of STP that provides a separate 802.1D Spanning Tree instance for each VLAN configured in the network
  • 802.1D-2004: An update to STP
  • 802.1s(MST): Maps multiple VLANs into the same Spanning Tree instance
  • 802.1w(RSTP): Improves convergence over 1998 STP by adding roles to ports and enhancing BPDU exchanges
  • PVRSTP+: A Cisco enhancement of RSTP using PVST+

STP performs three steps to provide loop-free topology

  • Elects one root bridge: Based on lowest Bridge ID 
  • Bridge ID made of Bridge priority and MAC address
  • Selects the root port on the nonroot bridges
  • Selects the designated port for each segment

Each Layer 2 port on a switch running 802.1D or PVST+ is in one of these four port states:

  • Blocking
  • Listening
  • Learning
  • Forwarding

Each Layer 2 port on a switch running 802.1D or PVST+ is in one of these port roles:

  • Root Port – Forwarding state
  • Designated Port – Forwarding state
  • Nondesignated Port – Blocking state
  • Disabled Port
  • Transition – Listening or Learning state

Each Layer 2 port on a switch running 802.1w or PVRSTP+ is in one of these three port states:

  • Discarding
  • Learning
  • Forwarding

802.1w and PVRSTP+ define these port roles:

  • Root Port – Forwarding state
  • Designated Port – Forwarding state
  • Alternate Port – Discarding state
  • Backup Port – Discarding state
  • Transition – Learning state

802.1w Edge Ports

  • An RSTP edge port is never intended to be connected to another switch
  • It immediately transitions to the forwarding state when enabled
  • The Cisco RSTP implementation maintains the PortFast keyword for edge port configuration
  • Switch(config)# spanning-tree Portfast default
  • Above command enables PortFast on nontrunking interfaces
  • Switch(config-if)# spanning-tree Portfast [trunk]
  • Above command configures PortFast on an interface

Cisco STP toolkit

  • PortFast: Configures access ports as edge ports, which transition directly to the forwarding state    
  • BPDUGuard: Disables a PortFast enabled port if a BPDU is received
  • BPDU Filter: Suppresses BPDUs on ports (not recommended)
  • When enabled globally, if BPDUs are detected:
    • Port loses its PortFast status
    • BPDU filter is disabled
    • STP sends and receives BPDUs on the port
  • When enabled on an interface:
    • It ignores all BPDUs it receives
    • It does not transmit any BPDUs
  • Root Guard: Prevents external switches from becoming Root Switch
  • Loop Guard: Prevents an alternate port or root port from becoming the designated ort if no BPDUs are received
  • UplinkFast*: Provides from 3 to 5 seconds of convergence after link failure (enabled on switches directly connected to the Root Switch)
  • BackboneFast*: Cuts the convergence time by max_age for an indirect failure

            * not required with PVRSTP+


Make sure you can configure:

  • RSTP – 802.1w
  • MSTP – 802.1s
  • Switch to become the Root Bridge or Secondary Root Bridge

Know how to:

  • Identify RootBridge
  • Identify which STP is being used

Make sure you are familiar with:

  • Show spanning-tree [vlan ]
  • Show spanning-tree summary totals
  • Spanning-tree toolkit
  • How a switch becomes the Root Bridge
  • Spanning-tree port roles
  • Spanning-tree port states
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