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
2
Topology Change
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