• Order of Operations:
    • Elect the root bridge.
      • After a root bridge has been selected, all of it’s ports will be set into designated ports that are forwarding downstream.
    • Select root bridge interfaces.
      • Each non-root switch will look through ports and elect a root port that is pointed at the root bridge.
      • All other ports will be elected as designated ports that are pointing further downstream.
      • All ports not in designated will be in blocking.
  • Root Bridge Election:
    • Lowest Bridge ID wins.
      • Bridge ID contains:
        • Bridge priority (0-61440)
        • System ID (0-4095)
        • MAC address
  • Changing Root Bridge Election:
    • Change bridge priority manually
      • ‘spanning-tree vlan <vlan id> priority <priority number by 4096>
    • Use root bridge command
      • ‘spanning-tree vlan <vlan id> root <primary or secondary>
      • Will set local priority based on current Root Bridge priority
    • ‘show spanning tree root’ will show which switch is root.
  • Root Port Election:
    • Root Port is upstream pointing towards the Root Bridge.
    • Selected as RP based on lowest path cost (all link costs added up)
      • Higher bandwidth = lower cost – ie. Fa is 19, 1Gbps is 4
      • When a cost tie occurs:
        • Lowest upstream bridge ID
        • Lowest upstream port ID
Port ID under ‘show spanning-tree’ = Prio.Nbr
  • Root Port priority:
    • Per above, lowest wins.
    • Root Port priority can be changed locally, which will then impact the downstream switch.
      • Image above shows root port as Gi1/1 to SW1. If Gi1/1 receives the following config, then the root port is changed to Gi1/2 instead:
        • switch(config-int)#spanning-tree port-priority 240
  • ‘show spanning-tree vlan <id> detail’
    • Will display the local and remote port IDs.
    • Useful for modifying traffic flow in bridging domain.
Can help determine why one port is selected over the other.
  • Changing paths when not changing which switch is Root Bridge.
    • switch(config-int)#spanning-tree vlan <#> cost <#>
      • Command above will change the link cost locally on a switch.
      • By adding all link costs up towards the bridge, we can modify which port is a root bridge locally.
    • Changing paths can be done with the bandwidth command as well.
      • Bandwidth is connected to STP cost.

  • Plain Spanning Tree Protocol timers
    • Hello – How often BPDUs are sent out interfaces.
      • Defaults to 2 seconds
    • MaxAge – How long to wait in blocking without a BPDU
      • Defaults to 20 seconds.
    • Forward Delay – How long to wait in listening and learning
      • Defaults to 15 seconds
  • PortFast
    • Bypasses Forwarding Delay
    • For ports not running spanning tree
    • ‘switch(config-if)#spanning-tree portfast’
  • UplinkFast
    • Legacy for ‘Alternate port’ when root port dies.
    • Automatically switches alternative port to forwarding when root goes down.
    • ‘switch(config)#spanning-tree uplinkfast’
    • Automatically changes all port priorities when root port dies.
      • Way to tell all other switches that it is not the root.
  • BackboneFast
    • Indirect failures should start recalculating immediately.
  • BPDU Filter
    • Filters BPDUs both in and out.
    • Used on Edge ports to transition port to STP speaking member if it receives BPDU.
    • ‘switch(config)#spanning-tree portfast bpdufilter default’
  • BPDU Guard
    • Listens for BPDUs and if it receives one the interface will be put in err-disabled.
    • Can run an err-disabled cause command to bring interface back up after specific interval.
  • Root Guard
    • Shuts ports down if it receives an additional BPDU with better priority.
    • Can be configured with Portfast.
    • switch(config-int)#spanning-tree guard root
  • Loop Guard
    • Prevents unidirectional links by using BPDU keepalives.
    • ‘switch(config)#spanning-tree loopguard default’
    • ‘switch(config-if)#spanning-tree loopguard enable’
  • Unidirectional Link Detection
    • Prevents unidirectional links by using UDLD keepalives.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s