1. Spanning Tree Protocol (STP)
What STP is, why do we need it, and how to configure STP (properly)
Frank Olshansky –

2. Introduction to the Spanning Tree Protocol
Also known as STP
Developed in 1990
IEEE 802.1D standard

3. Why do we need STP
STP was developed to allow for redundancy between switches and to prevent Layer 2 loops
No Layer 2 equivalent of Time To Live (TTL)
If there was no STP:
Broadcast Storms
Unstable MAC address table
Multiple copies of same frame

4. Without STP 2 1 3

5. Without STP 2 1 3

6. Without STP 2 1 3

7. Without STP 2 1 3

8. Without STP 2 1 3

9. Without STP 2 1 3

10. What STP does 2 1 3

11. What STP does 2 1 3

12. What STP does 2 1 3

13. Root bridge STP elects a single root bridge for the network
The root bridge is selected through an election process, and the switch that wins is the switch with the lowest Bridge ID (BID)
The bridge ID is an 8 byte field
2 byte bridge priority
32768 is the default
6 byte MAC address of the switch

14. Bridge Protocol Data Unit (BPDU)
Frames that contain information about STP
Sent as multicast to 01:80:C2:00:00:00
Configuration BPDUs are sent every 2 seconds by default
Three types
Configuration BPDU: Provides information to all switches.
TCN (Topology Change Notification): Lets switches know about changes in the STP topology.
TCA (Topology Change Acknowledgment): Acknowledges TCN.

15. Bridge Protocol Data Unit (BPDU)

16. STP port states
Root Port – Port on a non-root bridge that is closest to the root bridge, according to STP port cost
Designated Port – The port on a network link that is closest to the root bridge, according to STP port cost
Note: Every network link has a designated port
Non-designated port – Port that is blocked by STP, so that a loop-free Layer 2 topology is maintained
Disabled port – A port that is administratively down

17. STP port costs
10 Mbps = 100
100 Mbps = 19
1 Gbps = 4
10 Gbps = 2

18. Bridge Protocol Data Unit (BPDU)

19. STP port states 2 1 3

20. STP convergence times
When primary link to root bridge down, it takes 50 seconds for the switch to transition to the backup link
The switch links go through the following states:
Blocking – Switch listens for BPDUs from root
20 seconds
Listening – Listens to any BPDUs arriving on any interfaces
15 seconds
Learning – Populating MAC address table
Forwarding

21. STP Flavors
Common Spanning Tree (CST) – One spanning tree topology for all of the VLANs
Per-VLAN Spanning Tree plus (PVST+) – One spanning tree topology per VLAN; Cisco proprietary
PVST – Over ISL trunk
PVST+ – Over an 802.1Q trunk
Multiple Spanning Trees (MST) – One spanning tree topology for one or multiple vlans
IEEE 802.1s

22. STP Flavors The bridge ID is an 8 byte field
2 byte bridge priority
32768 is the default
6 byte MAC address of the switch
With PVST+ and MST, the bridge priority field is split
4 bit bridge priority
Increments of 4096
12 bit extended system id
1 to 4095 corresponding to the VLAN

23. Rapid Spanning Tree Protocol
Spanning tree topology convergence is much quicker
Convergence time goes from 50 seconds to 3 seconds

24. Rapid Spanning Tree Port Roles
Root port – Same as root STP port state in STP
Port on a non-root bridge that is closest to the root bridge, according to STP port cost
Designated Port – Same as designated port state in STP
The port on a network link that is closest to the root bridge, according to STP port cost
Note: Every network link has a designated port
Disabled Port – Same as disabled port state in STP
A port that is administratively down

25. Rapid Spanning Tree Port Roles
No such thing as non-designated ports
Alternate Port – Port that STP blocks, so that a loop-free Layer 2 topology is maintained
Can be changed to a designated port very quickly
Backup port – Only occurs when there are two connections going into a hub

26. Rapid Spanning Tree Port States
Disabled – Data is not forwarded on port
Disabled ports, Backup ports, and Alternate ports
Learning – Switch is learning MAC addresses through port
Seen when port is transitioning from disabled to forwarding
Forwarding – Data is being forwarded on port
Root and Designated ports

27. Rapid Spanning Tree Link Types
Point-to-Point – Inter-switch connections
Shared – Link going from a switch to a hub
Edge – Link going from switch to an edge device

28. Spanning Tree “Optional” Features
PortFast
Allows a switch port to go into forwarding state almost immediately
Configured on ports connecting to endpoints
Can set it up by default or on a per-interface basis
Turned off on trunk ports

29. Spanning Tree “Optional” Features
BPDU Guard
Causes a port to go into an error disabled state if a BPDU is received on that port
Can be enabled by default or on a per interface basis
Should be configured on any port that should not be connecting to other switches

30. Questions?