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					       Switching

          Topic 6
Rapid spanning tree protocol
                   Agenda
• RSTP features
  – Port states
  – Port roles
  – BPDU format
  – Edge ports and link types
  – Proposals and agreements
  – Topology changes
  – Configuration and issues
                RSTP features
• Rapid spanning tree protocol
• IEEE 802.1w standard
• New port states
  – discarding, learning and forwarding
• Much faster convergence after topology change
  – An alternate or backup port can immediately change
    to forwarding state
  – Does not use 802.1d timers
  – All switches sends BPDUs every two seconds (hello
    interval)
  – Fast aging if three consecutive hello BPDUs are missed
• Same BPDU format as STP
  – uses version 2, backwardly compatible with 802.1d
               RSTP port states
• Discarding
  – Prevents layer 2 loops
  – Does not forward data frames or learn MAC addresses
  – Equivalent to blocking and listening in STP
• Learning
  – Accepts frames to learn MAC addresses and populates
    the MAC address table
  – Does not forward data frames
• Forwarding
  – Does forward data frames
  – Proposal and agreement process to transition to
    forwarding
                   RSTP port roles
• Root root
   – The port on each non-root switch which receives the best BPDU
   – The best BPDU has the lowest path cost to the root bridge
   – Forwards data
• Designated port
   – The designated port is the port that sends the best BPDU
   – Forwards data (receives frames that are destined for the root
     bridge)
• Alternate port
   – Offers an alternative path to the root bridge
   – A better BPDU has been received from a port on another switch
   – The alternate port blocks
• Backup port
   – An alternative path on the same switch
   – If the forwarding port fails, the backup port on the same switch
     takes over
                  BPDU format
• BPDU version = 2
• Flag fields include the port role
• Flag fields include a status for learning and forwarding


                                          Topology Change
              2                           Proposal
                                          Port Role
                                          Learning
                                          Forwarding
                                          Agreement
                                          Topology Change ACK
   Rapid transition to forwarding
• BPDUs are sent by all switches every two seconds
  – STP BPDUs are generated by the root bridge and
    relayed by the non root bridges
• Faster aging of information
  – three missed hellos
  – STP waits 20 seconds for max age timer
• Accepts inferior BPDUs
  – Primary link goes down and hellos fail three times
  – Designated switch sends a BPDU with a path to root
  – BPDU is accepted and link transitions immediately
                    Edge ports
• The PortFast feature
• Access ports connected to hosts will never create
  layer 2 loops
• Edge port can transition immediately to
  forwarding
• If an RSTP edge port receives a BPDU it becomes
  a normal spanning tree port
• To configure an edge port
  – (config-if)# spanning-tree portfast
• Important to support DHCP
                      Link types
• Rapid transitioning on root ports and designated
  ports if they are point-to-point links or edge ports
• Two link types for non-edge ports:
   – Point-to-point
      • Full duplex
   – Shared
      • Half duplex
        RSTP proposal and agreement
• A topology change occurs
   – On STP the port waits forward delay before transitioning to forwarding
• On RSTP
   – A switch sends a proposal BPDU to the other switch
   – If the proposal contains a lower path cost to the root the receiving port
      still blocks, but is assigned the root port role
   – The switch syncs its ports – edge ports keep forwarding, designated
      and alternate ports block
   – As soon as ports are in sync the root port comes up and starts
      forwarding
        • No timers
   – Switch sends a proposal to downstream switches on designated ports
      and the process repeats
RSTP proposals
RSTP proposals
RSTP proposals
RSTP proposal
RSTP proposals
RSTP proposals
RSTP proposal
RSTP topology
     Topology Change Notification
• When the topology changes and ports change state, paths to
  hosts change and MAC address tables will contain wrong
  information
   – If a switch receives a topology change notification BPDU
   – Switch flushes MAC addresses associated with designated and
     root ports (not edge ports)
   – Switch starts a timer = twice the hello time and while this timer is
     on it sends BPDUs with TC bit set, to the root bridge and to
     neighbour switches
   – Neighbours flush MAC addresses associated with designated and
     root ports
   – Neighbours start the TC while timer and send BPDUs with TC bit
     set to the root bridge and to their neighbours while the TC while
     timer is running
• Faster flushing of MAC address tables than 802.1D and does
  not rely on the root bridge initiating the process
                 Rapid PVST+
• Cisco® version of RSTP
• Set Rapid PVST+ globally for the switch
     (config)#spanning-tree mode rapid-pvst
     (config)# Int fa0/1
     (config-if)# spanning-tree link-type point-to-point
     (config-if)# spanning-tree bpduguard enable
     (config-if)#exit
     (config)# clear spanning-tree detected-protocols
     (config)# show spanning-tree vlan VID
           Selecting the root bridge
• For each VLAN, identify which switch will be the root
    – Choose a powerful switch in centre of network with direct connection to
      routers and servers
    – Configure the root bridge primary switch and select and configure the
      backup root bridge secondary
• Decide which links will be redundant blocked links
    – Minimise the number of blocked links so there is less surface for errors
      (two redundant links is enough)
• Prune any VLANS not needed off trunks
    – Trunk all VLANS over core but prune VLANS in single locations at
      distribution switches
• Use layer 3 switching to avoid having STP blocked ports
    – Layer 3 switching is routing at switch speed
    – Redundancy by routing protocols
• Don’t turn off STP – its there to protect from layer 2 loops if cables or
  switches are added in error
              Troubleshooting
• Keep good documentation
  – Topology of bridge network
  – Location of root bridge
  – Location of blocked ports and redundant links
• Port fast configured on a trunk link will create
  looping traffic and delay
• STP is optimised for a network diameter of 7 –
  more than 7 and max age timer which is
  incremented in the BPDU by each switch that is
  crossed causes BPDU to be discarded too soon
                   Agenda
• RSTP features
  – Port states
  – Port roles
  – BPDU format
  – Edge ports and link types
  – Proposals and agreements
  – Topology changes
  – Configuration and issues
       Switching

          Topic 6
Rapid spanning tree protocol

				
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posted:3/21/2012
language:English
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