1. Spanning Tree protocol- Chapter 5
STP.ppt
17/04/2017
Spanning Tree protocol- Chapter 5
CCNA Exploration Semester 3
Modified by Profs. Ward
and Cappellino
S Ward Abingdon and Witney College

2. Topics Redundancy in a converged network
How Spanning Tree Protocol (STP) eliminates layer 2 loops
The STP algorithm and its 3 steps
Rapid spanning tree protocol

3. Semester 3 LAN Design Basic Switch Concepts Wireless STP- Ch 5 VLANs
Inter-VLAN routing
VTP

4. STP.ppt
17/04/2017
We want:
Redundancy accomplished at the ____________________ ______________________________________________
What is redundancy?
Multiple ________________ _________________
One link or device fails – another takes over.
Redundancy allows flexibility but does have some challenges
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5. Issues with Redundancy
STP.ppt
17/04/2017
Issues with Redundancy
Multiple ___________________
Create problems when all the links are active:
____________________
_____________________
See examples on following slides…
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6. Broadcast storm 3. And so on with nothing to stop it
STP.ppt
17/04/2017
Broadcast storm
3. And so on with nothing to stop it
2. Flood broadcast through non-source ports
There’s a switching loop B
1. Send ARP request A C D
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7. Duplicate Unicast Frame Transmissions
STP.ppt
Duplicate Unicast Frame Transmissions
17/04/2017 B
A is on port 3
Don’t know B
So flood A C
Send frame to B
Frame arrives A B
And again D
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8. STP.ppt
17/04/2017
Loops by mistake
Even if there are no deliberate loops for redundancy, there can be loops set up by mistake.
For example, by improper or no labeling of cables in the wiring closet
____________
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9. Etherchannel – the exception
STP.ppt
17/04/2017
Etherchannel – the exception
EtherChannel is a grouping of Ethernet ports on a switch that ____________________ ____________________________
Multiple connections do not make a loop where Etherchannel is used.
The _________________________________ with the combined bandwidth.
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10. Redundancy without loops
STP.ppt
17/04/2017
Redundancy without loops
There needs to be just ________________
_________________________, but _______ _________________ when they are needed.
This must be done quickly and automatically.
______________________ does this.
A blocked port does not include bridge protocol data unit (BPDU) frames that are used by STP to prevent loops.
S Ward Abingdon and Witney College

11. What is a spanning tree? A tree (extended star) topology
Spanning all devices
__________________________
Spanning tree is _____________ immediately __________________ _______________________

12. Not a spanning tree
Not a tree - it has loops.

13. Not a spanning tree
Not spanning. Device left out.

14. Spanning tree
No loops. Includes all devices.

15. Spanning tree protocol
Used by switches to __________________ _________________________________
________ unwanted links by _____________
STP defined by ________________
_____________ defined by _____________
Switches __________________ – ________________________________

16. Spanning tree algorithm
STP.ppt
17/04/2017
Spanning tree algorithm
The switches use this algorithm to configure the ports _______ ____________________
Choose ___________ to be “____________”
Choose a “_______” on ______ _____________ closest to the root bridge
Choose a “__________” which are all _______________ that are still ___________________________________________________________________________ known as “non-designated” ports
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17. Example of port designations
Root bridge
Designated port
Designated port
Root port
Root port
Non-designated port
Designated port
Root port
Designated port

18. STP.ppt
17/04/2017
1. Choose the Root Bridge
Root bridge serves as a __________________ ____________________________
______________ in the broadcast domain __________________________________
Each switch has a bridge ID (___) containing priority value, extended system ID, followed by the MAC address of the switch- more to come on BID …
A switch starts up. It sends out BPDU frames more to come on BPDU … containing the switch BID and the root ID every 2 seconds.
At first each switch identifies itself as the root bridge.
S Ward Abingdon and Witney College

19. Choose the root bridge (cont…)
STP.ppt
17/04/2017
Choose the root bridge (cont…)
___________ Bridge Protocol Data Units (______) to _______________________
The switch with the __________________________ ________________________
As a switch receives a BPDU, it ___________________ ________________________________________ through which the BPDU was received.
It passes on this information in its own BPDUs.
Eventually all switches agree that the switch with the lowest BID is the root bridge.
Administrator can set the priority to fix the selection
S Ward Abingdon and Witney College

20. STP.ppt
17/04/2017
BPDU
The _______________________________ ________________________________
The BPDU message is encapsulated in an Ethernet frame.
The ______________ MAC address in the BPDU frame is a _________________ for the ___________________________
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21. BPDU Frame contains 12 fields:
STP.ppt
17/04/2017
BPDU Frame contains 12 fields:
2 bytes
Protocol ID
Administrative uses
1 byte
Version
Message type
Flags
8 bytes
Root ID
BID and path information- used to identify the root bridge and the cost
4 bytes
Cost of path
Bridge ID
Port ID
Message age
Timer fields used to determine how frequently BPDU messages are sent
Max age
Hello time
Forward delay
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22. Bridge ID Used to ______________________ on a network
STP.ppt
17/04/2017
Bridge ID
Used to ______________________ on a network
The bridge ID consists of:
____________________________
Lowest priority- ie. Lowest BID- becomes root bridge
By default the priority is 32768
Value range is
___________________________________
Identifies the VLAN with which the BPDU is associated
________________________
MAC address used as ___________________________
MAC address with the lowest hexadecimal value has the lower BID
Best to configure the desired root bridge and not count on MAC address
S Ward Abingdon and Witney College

23. STP.ppt
17/04/2017
Configure priority
Set a _____________________ if a specific switch is to become the root bridge
Method 1:
SW1#spanning-tree vlan 1 root primary
Sets value to or 4096 less than lowest priority detected.
SW1#spanning-tree vlan 1 root secondary
Sets value to This switch should become the root bridge if the primary root bridge fails.
Method 2:
SW1#spanning-tree vlan 1 priority 24576
command gives more granular control over the bridge priority value by allowing the NA to configure the priority
How would we verify bridge priority?
S Ward Abingdon and Witney College

24. STP.ppt
17/04/2017
2. Select root ports
After root bridge designation _________________________ ___________________________________________________________________________ in the broadcast domain.
Every non-root bridge (Switch) selects a root port
Only _________________________________
This is the _____________________________ to the root bridge
Path information is determined by _______________________ _________ along the path from the destination to the root bridge.
What is cost?...
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25. Finding the cost of a link
STP.ppt
17/04/2017
Finding the cost of a link
Default port costs are defined by the speed at which the port operates
Set by IEEE.
Costs may change as newer, faster Ethernet is developed.
Port cost is configurable by the NA
Link speed
Revised cost
Previous cost
10 Gbps 2 1
1 Gbps 4
100 Mbps 19 10
10 Mbps
100
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26. Changing the cost of a link
STP.ppt
17/04/2017
Changing the cost of a link
SW1(config)#int fa0/1
SW1(config-if)#spanning-tree cost 25
SW1(config-if)#end
To revert back to default value use “no”…
SW1(config-if)#no spanning-tree cost
S Ward Abingdon and Witney College

27. STP.ppt
17/04/2017
What is the best path?
In the example, the path cost from switch S2 to the root bridge switch S1, over path 1 is 19, while the path cost over path 2 is 38.
Because path 1 has a lower overall path cost to the root bridge, it is the preferred path.
STP then configures the redundant path to be blocked, preventing a loop from occurring.
S Ward Abingdon and Witney College

28. What if root ports have the same cost?
STP.ppt
17/04/2017
What if root ports have the same cost?
Switch uses the customizable port priority value. If those are the ____________?
_____________________________
The port ID is ________________________________ port. This gives each port a unique number
Example: by default F0/1 has port priority
value of 128 and .1 as port ID giving: 128.1
F0/2 has 128.2 X
F0/1
F0/2
F0/1 would be the root port…
S Ward Abingdon and Witney College

29. Configure port priority
STP.ppt
17/04/2017
Configure port priority
SW2(config-if)#spanning-tree port-priority 112
Priority values range from , in increments of 16.
The ____________________________
________________________
Becomes ________________________
____________________________________ _____________________________ by STA to prevent looping
S Ward Abingdon and Witney College

30. 3. Select designated/non-designated ports
STP.ppt
17/04/2017
3. Select designated/non-designated ports
____________ ports - All _______________ __________________________________
For _____ bridges, ______________________ ports.
For ___________bridges, a designated port is the _______________________________________________________________ as needed.
Only ___________________________________
______________ ports - All ports configured to be in a _______________ to prevent loops
S Ward Abingdon and Witney College

31. Select designated/non-designated ports cont…
STP.ppt
17/04/2017
Select designated/non-designated ports cont…
If two switches are connected to the same LAN segment, which switch port will be the designated and which the non-designated port?
Switches on the LAN segment in question exchange BPDU frames, which contain the switch BID.
Lowest path cost to the root bridge will be the designated port. If _____________________?
Switch with the ____________ has its port configured as a _______________________
Switch with the higher BID- non-designated port
See example on next slide…
S Ward Abingdon and Witney College

32. Example…

33. Summary of Port Roles STP makes: Root ports- forwarding
Designated ports- forwarding
Non-designated ports- shut down or blocked

34. Five Port states in traditional STP
STP.ppt
17/04/2017
Five Port states in traditional STP
Blocking – _______________ BPDU frames. Non-designated port which _________________ ___________________________________
Listening - ____________________ BPDU frames.
Learning - _____________________ BPDU frames. _____________________ in preparation of participating in frame forwarding
Forwarding – Fully active, _________________
Disabled – ___________________________
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35. STP.ppt
17/04/2017
States and timers
Amount of time that a port stays in the various port states depends on the ___________
During a topology change
A port temporarily __________________________________________ for a specified period called the "forward delay interval.“
Possibly forward or remain in blocked state
Up to ____________________ __________ again- full convergence
S Ward Abingdon and Witney College

36. BPDU timers Timers are optimised for a 7-switch diameter network.
STP.ppt
17/04/2017
BPDU timers
Timers are optimised for a 7-switch diameter network.
______________________________________________________________ to travel from the ________________ on the broadcast domain
The network has __________________ before switches forward user data.
Typically timers and the diameter should not be adjusted though they are configurable
S Ward Abingdon and Witney College

37. STP.ppt
17/04/2017
Cisco PortFast
An access port leading to a workstation or server does not need to go through the STP modes because it will not be closed down.
The switch port can be configured with _____________ ____________________________________________
If a switch is connected later and the port receives a BPDU, STP can put the port into a blocking state and then through the modes.
Feature called ______________________
To configure portfast, enter interface config mode for the port where portfast is going to be used and then the command:
spanning-tree portfast
S Ward Abingdon and Witney College

38. Verify spanning tree
Root bridge
This switch

39. Topology change notification (TCN)
STP.ppt
17/04/2017
Topology change notification (TCN)
After the network converges, the root bridge sends out BPDUs, but the other switches do not normally send BPDUs back.
If there is a ______________________________ ______________ called the topology change notification (________) ______________________
Each switch that receives the TCN sends an ____________________ and sends a TCN towards the root bridge until the root bridge receives it.
The ____________________________ with the topology change (TC) bit set into the whole network
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40. STP.ppt
17/04/2017
STP developments
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41. PVST+ ________________ can be implemented.
STP.ppt
17/04/2017
PVST+
________________ can be implemented.
Separate instance of spanning tree runs for each VLAN
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42. PVST+ cont… The VLAN needs to be identified, so each BID has 3 fields:
STP.ppt
17/04/2017
PVST+ cont…
The VLAN needs to be identified, so each BID has 3 fields:
Bridge Priority
_____________________________________
MAC address.
Original BID just had bridge priority and MAC address
PVST+ is the default spanning-tree configuration for a Catalyst 2960 switch.
S Ward Abingdon and Witney College

43. STP.ppt
17/04/2017
PVST+ cont…
When the priority and extended system ID are prepended to the switch MAC address, each VLAN on the switch can be represented by a _________
S Ward Abingdon and Witney College

44. Rapid Spanning Tree Protocol
STP.ppt
17/04/2017
Rapid Spanning Tree Protocol
__________________________________ with it.
RSTP (IEEE 802.1w) is an evolution of STP (IEEE 802.1D)
Much _____________________
____________________________
2 in version field indicates it is RSTP
Sends BPDUs with its current information every 2 seconds.
Does not use timers in the same way as STP
3 missed BPDUs taken to mean loss of the link. (6 seconds)
_____________________________
RSTP does not have a __________________________
S Ward Abingdon and Witney College

45. _______________ in RSTP
STP.ppt
17/04/2017
_______________ in RSTP
A port that will ______________________ __________________________
Immediately goes to forwarding state.
Same idea as Cisco’s PortFast.
An edge port becomes a normal spanning-tree port if it receives a BPDU
Configuring an edge port uses the PortFast keyword as before.
spanning-tree portfast
S Ward Abingdon and Witney College

46. STP.ppt
17/04/2017
Link types
___________ are categorized into 2 link types, ____________________________
A _________________ between two switches is regarded as a _________________
A link ____________________ is regarded as a ____________________
Ports on a ________________ are able to _________________________
Designated ports
Make the most use of the link type parameter.
S Ward Abingdon and Witney College

47. STP.ppt
17/04/2017
Port states
There are only _____________ in RSTP that correspond to the three possible operational states.
The 802.1D ________________ states are ________ into a unique 802.1w ________________________
Operational
STP
RSTP
Enabled
Blocking
Discarding
Listening
Learning
Forwarding
Disabled
S Ward Abingdon and Witney College

48. STP.ppt
17/04/2017
RSTP Port Roles
____ - A ___________ that has been elected for the spanning-tree topology
_______________ - A ____________________________________
Alternate - An _______ ___________________ This path is different than using the root port.
Backup - A ___________________ to a segment where _____________________________________
_________ - Not strictly part of STP, a network administrator can ___________________ 48
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49. Design considerations
STP.ppt
17/04/2017
Design considerations
Root bridge should be a ______________ in the ______________ of the network.
________ the number of _______________
Use VTP ____________.
Use ________________________
_________________ even if no blocked ports or physical redundancy exist
S Ward Abingdon and Witney College

50. Troubleshooting Suggestions
STP.ppt
17/04/2017
Troubleshooting Suggestions
Before you troubleshoot a bridging loop, you need to know at least these items:
Topology of the bridge network
Location of the root bridge
Location of the blocked ports and the redundant links
How the network looks when it works correctly
Use the “_________” command as needed
S Ward Abingdon and Witney College