Switching Topic 6 Rapid spanning tree protocol. Agenda RSTP features – Port states – Port roles – BPDU format – Edge ports and link types – Proposals.

Switching

Topic 6Rapid 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

2Topology ChangeProposalPort RoleLearningForwardingAgreementTopology 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 6Rapid spanning tree protocol