1. STP – Spanning Tree Protocol A closer look
CIS 187 Multilayer Switched Networks
CCNP SWITCH version 7
Rick Graziani

2. Five-Step STP Decision Sequence
STP Convergence Step 1 Elect one Root Bridge Step 2 Elect Root Ports Step 3 Elect Designated Ports
When creating a loop-free topology, STP always uses the same five-step decision sequence:
Five-Step decision Sequence
Step 1: Lowest root bridge ID - Determines the root bridge
Bridge ID = priority (4 bits) + locally assigned system ID extension (12 bits) + ID [MAC address] (48 bits); the default bridge priority is 32768, and
Step 2: Lowest cost to the root bridge - Favors the upstream switch with the least cost to root
Step 3: Lowest sender bridge ID - Serves as a tie breaker if multiple upstream switches have equal cost to root
Step 4: Lowest sender Port ID (port priority then port ID) - Serves as a tie breaker if a switch has multiple (non-Etherchannel) links to a single upstream switch, where:
Port ID = priority (4 bits, default 128) + ID (Interface number)
Rick Graziani

3. Other notes
The port that receives the best BPDU from the root switch is assigned root port status and is put into forwarding state.
Rest of the active ports of that switch send this BPDU.
If the switch receives a better BPDU on another port, it stops transmitting its BPDU, and send the better BPDU.
If the port transmits a BPDU but doesn't receive any, it assumes a role of a designated port and is put into forwarding state.
If a port is not transmitting any BPDU, then it is put in a blocking state. (It still receives BPDUs.)
Best BPDU is stored on per-port basis.
Rick Graziani

4. What is the BID of this switch?
Core# show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority
Address E.7EBB
Cost
Port (GigabitEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority (priority sys-id-ext 1)
Address C945.A573
Aging Time 20
Rick Graziani

5. Elect one Root Bridge Lowest BID wins!
Rick Graziani

6. Its all done with BPDUs!
Rick Graziani

7. Root Bridge Selection Criteria
My BID is C945.A573 I’m the root!
Who wins?
My BID is B I’m the root!
My BID is E0D.9315 I’m the root!
My BID is E461.46EC I’m the root!
My BID is E.7EBB I’m the root! I win!
At the beginning, all bridges assume and declare themselves as the Root Bridge, by placing its own BID in the Root BID field of the BPDU.
Rick Graziani

8. Root Bridge Path Cost Root bridge sends out BPDU
Root ID, Root Path Cost, Sender ID, Sender Port ID
Path Cost
Root bridge sends out BPDU
Adjacent switches receive and store the best BPDU for each port
The Root Path Cost is not incremented (only when BPDU is transmitted.
Any BPDU received from another switch will be "inferior" to this BPDU.
BPDU received
BPDU received
Fa0/3: A2, 0, A2, Fa0/1
Fa0/3: A2, 0, A2, Fa0/3
Root Bridge
BPDUs sent
BPDU received
Fa0/1: A2, 0, A2, Fa0/1
Fa0/3: A2, 0, A2, Fa0/3
Fa0/5: A2, 0, A2, Fa0/5
Fa0/5: A2, 0, A2, Fa0/5
Rick Graziani

9. Root Bridge Path Cost The best BPDU is sent to other switches
BPDU ( * Best BPDU)
Root ID, Root Path Cost, Sender ID, Sender Port ID
Path Cost
The best BPDU is sent to other switches
The Root Path Cost = Root Path Cost of best BPDU + Path Cost of the port the BPDU was received on
Best BPDUs per port
Best BPDUs per port
* Fa0/3: A2, 0, A2, Fa0/1
G0/2: A2, 19, A1, G1/2
* Fa0/3: A2, 0, A2, Fa0/3
G0/1: A2, 19, A1, G1/1
BPDU received (not best)
BPDU received (not best)
G0/2: A2, 19, A1, G1/2
G0/1: A2, 19, A1, G1/1
Best BPDUs per port
Root Bridge
* Fa0/5: A2, 0, A2, Fa0/5
BPDUs sent
BPDU sent 19
Fa0/1: A2, 0, A2, Fa0/1
Fa0/3: A2, 0, A2, Fa0/3
Fa0/5: A2, 0, A2, Fa0/5
G1/2: A2, 19, A1, G1/2
G1/1: A2, 19, A1, G1/1

10. Root Bridge Path Cost The best BPDU is sent to other switches
Root ID, Root Path Cost, Sender ID, Sender Port ID
Path Cost
The best BPDU is sent to other switches
The Root Path Cost = Root Path Cost of best BPDU + Path Cost of the port the BPDU was received on
Root Bridge: All BPDUs from other switches are considered "inferior"
Best BPDUs per port
Best BPDUs per port
* Fa0/3: A2, 0, A2, Fa0/1
G0/2: A2, 19, A1, G1/2
* Fa0/3: A2, 0, A2, Fa0/3
G0/1: A2, 19, A1, G1/1
BPDU sent
BPDU sent
Fa0/5: A2, 19, D1, Fa0/5
Fa0/3: A2, 19, D1, Fa0/3
G0/1: A2, 19, D1, G0/1
G0/2: A2, 19, D1, G0/2
Fa0/5: A2, 19, D2, Fa0/5
Fa0/3: A2, 19, D2, Fa0/3
G0/1: A2, 19, D2, G0/1 19 19
Root Bridge
Best BPDUs per port
Best BPDUs per port (inferior)
* Fa0/5: A2, 0, A2, Fa0/5
G0/2: A2, 19, D1, G0/2
G01/2: A2, 19, D2, G0/1
Best BPDUs per port
Fa0/3: A2, 19, D2, Fa0/3
Fa0/1: A2, 19, D1, Fa0/1
Fa0/5: A2, 19, A1, Fa0/5
Best BPDUs per port
Rick Graziani

11. Root Bridge Path Cost The best BPDU is sent to other switches
Root ID, Root Path Cost, Sender ID, Sender Port ID
Path Cost
The best BPDU is sent to other switches
The Root Path Cost = Root Path Cost of best BPDU + Path Cost of the port the BPDU was received on
BPDU received
BPDU received
Best BPDUs per port
G0/1: A2, 19, D1, G0/1
G0/2: A2, 19, D2, G0/2
* G0/1: A2, 19, D1, G0/1
G0/2: A2, 19, D2, G0/2
Best BPDUs per port
Best BPDUs per port
* Fa0/3: A2, 0, A2, Fa0/1
G0/2: A2, 19, A1, G1/2
* Fa0/3: A2, 0, A2, Fa0/3
G0/1: A2, 19, A1, G1/1
Root Bridge
Best BPDUs per port
* Fa0/5: A2, 0, A2, Fa0/5
G0/2: A2, 19, D1, G0/2
G1/2: A2, 19, D2, G0/1
Best BPDUs per port
Rick Graziani

12. Root Bridge Path Cost The best BPDU is sent to other switches
Root ID, Root Path Cost, Sender ID, Sender Port ID
Path Cost
The best BPDU is sent to other switches
The Root Path Cost = Root Path Cost of best BPDU + Path Cost of the port the BPDU was received on
Best BPDUs per port
BPDU sent
BPDU sent
* G0/1: A2, 19, D1, G0/1
G0/2: A2, 19, D2, G0/2
G0/1: A2, 23, Co, G0/1
G0/2: A2, 23, Co, G0/2 4 4
BPDU received
BPDU received
G0/1: A2, 23, Co, G0/2
G0/2: A2, 23, Co, G0/2
Best BPDUs per port
Best BPDUs per port
* Fa0/3: A2, 0, A2, Fa0/3
G0/1: A2, 19, A1, G1/1
G0/2: A2, 23, Co, G0/2
* Fa0/3: A2, 0, A2, Fa0/1
G0/2: A2, 19, A1, G1/2
G0/1: A2, 23, Co, G0/2
Root Bridge
Best BPDUs per port
* Fa0/5: A2, 0, A2, Fa0/5
G0/2: A2, 19, D1, G0/2
G1/2: A2, 19, D2, G0/1
Best BPDUs per port
Fa0/1: A2, 0, A2, Fa0/1
Fa0/3: A2, 0, A2, Fa0/3
Fa0/5: A2, 0, A2, Fa0/5
Rick Graziani

13. Root Bridge Path Cost After all best BPDUs are sent and received.
Root ID, Root Path Cost, Sender ID, Sender Port ID
Path Cost
After all best BPDUs are sent and received.
Root port selection (smallest BPDU on the switch)
The port that receives the best BPDU from the root switch is assigned root port status and is put into forwarding state.
Best BPDUs per port
* G0/1: A2, 19, D1, G0/1
G0/2: A2, 19, D2, G0/2 RP
Best BPDUs per port
Best BPDUs per port
* Fa0/3: A2, 0, A2, Fa0/1
G0/2: A2, 19, A1, G1/2
G0/1: A2, 23, Co, G0/1
Fa0/5: A2, 19, D2, Fa0/5
* Fa0/3: A2, 0, A2, Fa0/3
G0/1: A2, 19, A1, G1/1
G0/2: A2, 23, Co, G0/2
Fa0/5: A2, 19, D2, Fa0/5 RP RP
Root Bridge
Best BPDUs per port
* Fa0/5: A2, 0, A2, Fa0/5
G0/2: A2, 19, D1, G0/2
G1/2: A2, 19, D2, G0/1
Best BPDUs per port
Fa0/1: A2, 0, A2, Fa0/1
Fa0/3: A2, 0, A2, Fa0/3
Fa0/5: A2, 0, A2, Fa0/5 RP
Rick Graziani

14. BPDU
Root ID, Root Path Cost, Sender ID, Sender Port ID
Path Cost
Designated port is port with “best” path to root (smallest BPDU) on link
Root Bridge: All BPDUs from other switches are considered "inferior"
Best BPDUs per port
Best BPDUs per port
* Fa0/3: A2, 0, A2, Fa0/1
G0/2: A2, 19, A1, G1/2
G0/1: A2, 23, Co, G0/1
Fa0/5: A2, 19, D2, Fa0/5
* Fa0/3: A2, 0, A2, Fa0/3
G0/1: A2, 19, A1, G1/1
G0/2: A2, 23, Co, G0/2
Fa0/5: A2, 19, D2, Fa0/5 RP
BPDU sent
BPDU sent
Fa0/3: A2, 19, D1, Fa0/3
Fa0/3: A2, 19, D2, Fa0/3 RP RP
Best BPDUs per port
* Fa0/5: A2, 0, A2, Fa0/5
G0/2: A2, 19, D1, G0/2
G1/2: A2, 19, D2, G0/1
Best BPDUs per port
Root Bridge DP DP
BPDU sent
Fa0/1: A2, 0, A2, Fa0/1
Fa0/3: A2, 0, A2, Fa0/3
Fa0/5: A2, 0, A2, Fa0/5
BPDU sent RP DP
Fa0/5: A2, 19, A1, Fa0/5
Rick Graziani

15. BPDU
Root ID, Root Path Cost, Sender ID, Sender Port ID
Path Cost
Designated port is port with “best” path to root (smallest BPDU) on link
Best BPDUs per port
BPDU sent
BPDU sent
* G0/1: A2, 19, D1, G0/1
G0/2: A2, 19, D2, G0/2
G0/1: A2, 23, D1, G0/1
G0/2: A2, 23, D2, G0/2 RP
BPDU sent DP DP
BPDU sent
G0/1: A2, 19, D1, G0/1
G0/2: A2, 19, D2, G0/2
Best BPDUs per port
Best BPDUs per port
* Fa0/3: A2, 0, A2, Fa0/1
G0/2: A2, 19, A1, G1/2
G0/1: A2, 23, Co, G0/1
Fa0/5: A2, 19, D2, Fa0/5
* Fa0/3: A2, 0, A2, Fa0/3
G0/1: A2, 19, A1, G1/1
G0/2: A2, 23, Co, G0/2
Fa0/5: A2, 19, D2, Fa0/5 RP RP DP DP
Root Bridge RP DP
Rick Graziani

16. BPDU
Root ID, Root Path Cost, Sender ID, Sender Port ID
Path Cost
Designated port is port with “best” path to root (smallest BPDU) on link
Best BPDUs per port
* Fa0/3: A2, 0, A2, Fa0/1
G0/2: A2, 19, A1, G1/2
G0/1: A2, 23, Co, G0/1
Fa0/5: A2, 19, D2, Fa0/5 RP DP DP
BPDU sent
G0/2: A2, 19, D1, G0/2 RP
E0D.9315 RP
BPDU sent
G1/2: A2, 19, A2, G1/2 DP DP DP
E461.46EC
Best BPDUs per port
Root Bridge
* Fa0/5: A2, 0, A2, Fa0/5
G0/2: A2, 19, D1, G0/2
G1/2: A2, 19, D2, G0/1
Best BPDUs per port RP DP
Rick Graziani

17. BPDU
Root ID, Root Path Cost, Sender ID, Sender Port ID
Path Cost
Designated port is port with “best” path to root (smallest BPDU) on link
Best BPDUs per port
Best BPDUs per port
* Fa0/3: A2, 0, A2, Fa0/1
G0/2: A2, 19, A1, G1/2
G0/1: A2, 23, Co, G0/1
Fa0/5: A2, 19, D2, Fa0/5 RP
* Fa0/3: A2, 0, A2, Fa0/3
G0/1: A2, 19, A1, G1/1
G0/2: A2, 23, Co, G0/2
Fa0/5: A2, 19, D2, Fa0/5 DP DP DP
BPDU sent
BPDU sent
G0/2: A2, 19, D1, G0/2
Fa0/5: A2, 19, D2, Fa0/5 RP
E0D.9315 RP
B0.5850 DP DP DP
Root Bridge RP DP
Rick Graziani

18. BPDU
Root ID, Root Path Cost, Sender ID, Sender Port ID
Path Cost
Designated port is port with “best” path to root (smallest BPDU) on link
Best BPDUs per port RP
* Fa0/3: A2, 0, A2, Fa0/3
G0/1: A2, 19, A1, G1/1
G0/2: A2, 23, Co, G0/2
Fa0/5: A2, 19, D2, Fa0/5 DP DP DP
BPDU sent
Fa0/5: A2, 19, D2, Fa0/5 RP RP
B0.5850
BPDU sent
G1/2: A2, 19, A2, G1/2 DP DP DP
E461.46EC DP
Root Bridge
Best BPDUs per port
Best BPDUs per port RP DP
* Fa0/5: A2, 0, A2, Fa0/5
G0/2: A2, 19, D1, G0/2
G1/2: A2, 19, D2, G0/1
Rick Graziani

19. BPDU
Root ID, Root Path Cost, Sender ID, Sender Port ID
Path Cost
If not a Root Port or Designated Port then becomes a Non-Designated (Alternate) Port and put in blocking mode. RP X
NDP DP DP DP X
NDP X X RP RP
NDP
NDP
Root Bridge DP DP DP DP RP DP
Rick Graziani

20. Root Bridge Sender's Port ID (lowest) Path Cost BPDU
Root ID, Root Path Cost, Sender ID, Sender Port ID
Path Cost
Sender's Port ID (lowest) RP
Best BPDU
A2, 0, D1, Fa0/3 DP DP DP
NDP
BPDU received
Fa0/3: A2, 0, A2, Fa0/1
BPDU received RP RP
Fa0/1: A2, 0, A2, Fa0/2
NDP
NDP
Fa0/1
NDP
Root Bridge DP DP DP
BPDU sent DP DP
Fa0/2
Fa0/1: A2, 0, A2, Fa0/1 RP DP
BPDU sent
Fa0/2: A2, 0, A2, Fa0/2
Rick Graziani

21. show spanning-tree detail
Path Cost
This is the root path cost that will be advertised in the BPDU
Access1# show spanning-tree detail
VLAN0001 is executing the ieee compatible Spanning Tree Protocol
Bridge Identifier has priority of 32768, sysid 1, 0003.E461.46EC
Configured hello time 2, max age 20, forward delay 15
Current root has priority 32769
Root port is 5 (FastEthernet0/5), cost of root path is 19
Topology change flag not set, detected flag not set
Number of topology changes 0 last change occurred 00:00:00 ago
from FastEthernet0/1
Times: hold 1, topology change 35, notification 2
hello 2, max age 20, forward delay 15
Timers: hello 0, topology change 0, notification 0, aging 300
Rick Graziani

22. show spanning-tree Which port is the Root Port? Path Cost
Core# show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority
Address E.7EBB
Cost
Port (GigabitEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority (priority sys-id-ext 1)
Address C945.A573
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
Gi0/ Root FWD P2p
Gi0/ Altn BLK P2p
Path Cost
Rick Graziani

23. show spanning-tree detail
Which port is the Root Port?
Core# show spanning-tree detail
VLAN0001 is executing the ieee compatible Spanning Tree Protocol
Bridge Identifier has priority of 32768, sysid 1, 0001.C945.A573
Configured hello time 2, max age 20, forward delay 15
Current root has priority 32769
Root port is 25 (GigabitEthernet0/1), cost of root path is 4
Topology change flag not set, detected flag not set
Number of topology changes 0 last change occurred 00:00:00 ago
from FastEthernet0/1
Times: hold 1, topology change 35, notification 2
hello 2, max age 20, forward delay 15
Timers: hello 0, topology change 0, notification 0, aging 300
Path Cost
Rick Graziani

24. Access 1 has Lower Sender BID
Distribution1# show spanning-tree detail
Port 26 (GigabitEthernet0/2) of VLAN0030 is designated blocking
Port path cost 4, Port priority 128, Port Identifier
Designated root has priority 128, address 000C.CF0B.1503
Designated bridge has priority 32769, address 0003.E461.46EC
Designated port id is , designated path cost 4
Timers: message age 16, forward delay 0, hold 0
Number of transitions to forwarding state: 1
Link type is point-to-point by default
Access1# show spanning-tree detail
Port 26 (GigabitEthernet1/2) of VLAN0001 is designated forwarding
Designated root has priority 128, address 0001.C746.B605
E461.46EC
Note: PT does not show proper BID
Rick Graziani

25. Distribution 1 has Lower Sender BID
Distribution1# show spanning-tree detail
Port 5 (FastEthernet0/5) of VLAN0030 is designated forwarding
Port path cost 19, Port priority 128, Port Identifier 128.5
Designated root has priority 128, address 000C.CF0B.1503
Designated bridge has priority 32769, address E0D.9315
Designated port id is 128.5, designated path cost 19
Timers: message age 16, forward delay 0, hold 0
Number of transitions to forwarding state: 1
Link type is point-to-point by default
Distribution2# show spanning-tree detail
Port 5 (FastEthernet0/5) of VLAN0001 is designated blocking
Note: PT does not show proper BID
Rick Graziani

26. Access 1 has Lower Sender BID
Distribution2# show spanning-tree detail
Port 25 (GigabitEthernet0/1) of VLAN0001 is designated blocking
Port path cost 4, Port priority 128, Port Identifier
Designated root has priority 128, address 00D0.BCC1.2603
Designated bridge has priority 32769, address 0003.E461.46EC
Designated port id is , designated path cost 4
Timers: message age 16, forward delay 0, hold 0
Number of transitions to forwarding state: 1
Link type is point-to-point by default
Access1# show spanning-tree detail
Port 25 (GigabitEthernet1/1) of VLAN0001 is designated forwarding
Designated root has priority 128, address 0001.C746.B605
Note: PT does not show proper BID
Rick Graziani

27. Core# show spanning-tree Interface Role Sts Cost Prio.Nbr Type
Gi0/ Root FWD P2p
Gi0/ Altn BLK P2p
Distribution1# show spanning-tree
Interface Role Sts Cost Prio.Nbr Type
Fa0/ Root FWD P2p
Fa0/ Desg FWD P2p
Gi0/ Desg FWD P2p
Gi0/ Altn BLK P2p
Distribution2# show spanning-tree
Interface Role Sts Cost Prio.Nbr Type
Fa0/ Root FWD P2p
Fa0/ Altn BLK P2p
Gi0/ Altn BLK P2p
Gi0/ Desg FWD P2p
Access1# show spanning-tree
Interface Role Sts Cost Prio.Nbr Type
Fa0/ Root FWD P2p
Gi1/ Desg FWD P2p
Gi1/ Desg FWD P2p
Access2# show spanning-tree
Interface Role Sts Cost Prio.Nbr Type
Fa0/ Desg FWD P2p
Fa0/ Desg FWD P2p
Fa0/ Desg FWD P2p
Rick Graziani