1. Topic 5 Spanning tree protocol
Switching
Topic 5
Spanning tree protocol

2. Agenda Redundancy Spanning tree concepts Spanning tree evolves BPDUs
Root bridge and elections
Port roles
Port states
Timers
PortFast and BPDU guard
Spanning tree evolves

3. Redundancy Networks need redundancy to be highly available
Redundancy is achieved by having alternate devices and alternate links
In a switching environment, whenever multiple paths exist layer 2 loops can occur
Layer 2 loops escalate to broadcast storms which very quickly overwhelm switches and the network is down
In complex wiring closets, connections can be accidentally duplicated resulting in lost connectivity

4. Issues with alternate links
Broadcast frames circulate and cause MAC address tables to constantly update and fill causing the switch to flood on all ports
Duplicate unicast frames arrive at the destination and are dropped

5. Issues with redundant links

6. Issues with redundant links

7. Issues with redundant links

8. STP topology
Redundancy protects the network from a single point of failure
STP protects the network from layer 2 loops
STP:
Only one logical path between switches
Blocks alternate links
Blocked links do not forward data frames
Blocked links receive STP frames
If a cable or switch fails, STP unblocks the link to provide an alternative path

9. STA spanning tree algorithm
IEEE 802.1d standard
Determines which ports to block
Single switch is elected as a root bridge
On each other switch, STA calculates which link has the shortest path to the root bridge
STA assigns roles to switchports on the switch:
Root port (non-root bridges)
The port with the shortest path to the root bridge
Designated port
Non-root port that is allowed to forward data
Non-designated port
Put into a blocked state to prevent loops

10. Bridge ID BID is a unique number to identify switches Consists of :
Bridge priority, defaults to (1 to 65526)
MAC address of switch
Extended system ID (VLAN ID)
BID can be set by admin by changing the priority value (in increments of 4096)
The switch with the lowest BID is elected as the root bridge

11. BPDU frame structure

12. Root bridge election process
The root bridge is the STP reference point for the broadcast domain
Election process:
Each switch sends out BPDUs every 2 secs
BPDUs contain the switch BID and the root BID
Switch receives BPDUs from other switches
IF the root BID in the update < root BID of the switch
Switch updates its rootBID to the new value
Switch forwards BPDUs with new root BID
BPDUs circulate and converge to one root BID – that of the elected root bridge

13. Best paths BPDUs are sent out by the root bridge
BPDU contain a field for path cost which is updated by each switch that receives it
Path cost is calculated by adding port cost of the receiving port to the path cost in the BPDU
Port costs are based on the speed of the port
10gig = 2, gig = 4, fa = 19, eth = 100 (to set IEEE values)
Port cost can be manually set by admin
spanning-tree cost value
no spanning-tree cost (to set IEEE values)
The lowest cost path forwards and the other alternate paths are blocked

14. STP process
On start, each switch sets the root BID with its own BID and sends out BPDUs every two seconds (hello interval) on all switchports
Switch records its own BID, root BID and path cost to root bridge
Switch receives BPDUs
If root BID < local BID (I am not root bridge)
Update root BID on switch
Update the path cost by adding cost of port receiving BPDU (0 +19 = 19)
Send BPDUs with the updated rootBID and path cost values to other switches
If root BID = local BID (I am root bridge)
Received BPDUs are dropped
No values are updated

15. Port roles Root port Designated port
One root port per non-root switch
If there two equal cost paths from switch to root bridge
Which port has lowest port priority? 128 is the default
Which port has lowest interface ID?
Port with lowest value becomes root port, other becomes alternate port and is blocked
Designated port
One designated port per segment
Designated port receives and forwards frames
All ports on the root bridge are designated
If a segment has equal cost paths from each end of the segment
Switches send each other BPDUs to determine which switch has the lower BID
Switch with lower BID places its port into designated role, the port is blocked
Non-designated port (alternate port)
Is blocked to prevent loops
Does not forward frames or populate its MAC address table
Does continue to receive BPDUs
If a link fails, the non-designated port may transition to a forwarding state

16. Port states Switchports transition through five states to forward data
Blocking
The port receives BPDUs to determine the root bridge location and its STP role
Listening
Port is receiving and transmitting BPDUs to inform adjacent switches that it is preparing to transition to forwarding state
Learning
Port populates its MAC address table to prepare to forward data
Forwarding
Port is active and forwards frames and sends and receives BPDUs
Disabled
Administratively shutdown

17. Port timers
Port timers determine the time the port spends in each STP state
Forward delay
time spent in listening and learning states, by default secs
Max age
length of time the switch saves BPDU information
after 20 seconds of receiving no BPDUs, the link to root is considered down
Hello time
time between each BPDU frame sent – two second default
Default values allow for convergence on a network of diameter 7 (number of switches that separate hosts at far ends of the network)
Reconfigure timers by configuring the network diameter (do this with caution)
Only the root bridge can send information to adjust timers

18. How does it all work?

19. How does it all work?

20. How does it all work?

21. How does it all work?

22. How does it all work?

23. How does it all work?

24. How does it all work?

25. How does it all work

26. How does it all work

27. How does it all work?

28. How does it all work?

29. PortFast
Allows a port to transition from blocking to forwarding without the listening and learning delay
Supports DHCP by allowing the DHCP request to go out immediately and avoids the DHCP timeout due to switch transitioning time
Cisco® proprietary
(config-if)#spanning-tree portfast

30. BPDU guard
BPDU guard places a PortFast port into blocking state if a BPDU is received on that port
Protects a port configured with PortFast
If a switch is attached to a port configured with PortFast a layer 2 loop may occur, followed by a broadcast storm

31. Topology change notification
When any switch has a topology change it sends a topology change notification BPDU to the root bridge
The root bridge sets the TC flag on BPDUs it sends to all switches
Switches reduce the aging time on STP information to flush out stale information and speed up convergence

32. Configure the STP root STP is enabled by default Method 1 Method 2
Spanning-tree VLAN VID root primary
Spanning-tree VLAN VID root secondary (backup root)
Method 2
Spanning-tree VLAN VID priority value
Verify configuration
Show spanning tree

33. STP variants PVST PVST+ RSTP Rapid PVST+ MSTP
Spanning tree for each VLAN (using ISL)
Different STP root bridges for each VLAN
BackboneFast, UplinkFast and PortFast
PVST+
Spanning tree for each VLAN (using 802.1q)
RSTP
Version 2 with faster convergence
Rapid PVST+
Cisco® RSTP
MSTP
Multiple VLANs mapped to the same spanning tree instance
Multiple paths and load balancing

34. Configure PVST+
Select the switches for primary and secondary root bridges for each VLAN
Configure root bridges
(config)#spanning-tree VLAN VLANID root primary
(config)#spanning-tree VLAN VLANID root secondary
Verify configuration
#Show spanning-tree active
#Show run to see priority values

35. Default settings Default settings for Cisco® 2960 VLAN 1 PVST+
Priority 32768
Port priority 128
Port cost Gig = 4, fa = 19, eth = 100
Hello time = 2 secs
Forwarding delay = secs
Max age = 20 secs

36. Agenda Redundancy Spanning tree concepts Spanning tree evolves BPDUs
Root bridge and elections
Port roles
Port states
Timers
PortFast and BPDU guard
Spanning tree evolves

37. Topic 5 Spanning tree protocol
Switching
Topic 5
Spanning tree protocol