At first basic operations of STP will be briefly described and then specific scenarios will be presented.
Cisco PVST+ version of Spanning Tree Protocol is covered in this article.
Spanning Tree Protocol (STP) is used to create loop free network topologies. It will elect one root swith and block redundant path's towards it in the network. The STP decisions are based on multiple factor's like switch priority or the speed of the interfaces. STP is constanty running in the network - so if used network path would become unavailable (link failure for example), STP would rebuild it's topology (to be able to use previously blocked link).
The network switch'es which are participating in STP operations are exchanging BPDU's.
In stable PVST+ environment, only Root switch is generating BPDU's - other switche's are resending received BPDU's.
Let's assume that all network switch'es in the environment are powered on in the same moment. Every device will assume at the beginning that it is the STP Root. All switches will start generating the BPDU's. Devices by analysing the information present in the received BPDU are deciding if they should keep the Root role or not (and if they should generate BPDU, or to resend received one).
The switch with the lowest priority (default value 32768) will be the root. If the priority of two switches are the same - the one with lowest MAC address will become the root.
By default BPDU's are send every 2 seconds.
After it is decided which device will be STP Root - the port roles are being determined:
- Root port (the best path to the root switch)
- Designated port (the port on the designated switch for LAN segment, which have the best path to root switch)
- Alternate Port (alternate path to root bridge)
Every connection/path between switches has it's cost in STP topology. The cost is being determined by the speed of the port, where BPDU is being received. The switch which has received the BPDU, is adding the cost of receiving port before resending it through other port's. The path with lowest cost, is being considered the best one.
The best BPDU received by the switch on particular port, is saved by the device.
If switch has two path's with the same cost, these are the tie breaker's:
- lowest sender bridge ID
- lowest sender port ID
The port's in STP topology can be in several states:
- Blocking (BPDU's receive, Frames are not forwarded)
- Listening (BPDU's send and receive, MAC address table is not populated, Frames are not forwarded)
- Learning (BPDU's send and receive, MAC address table is populated, Frames are not forwarded)
- Forwarding (BPDU's send and receive, Frames are forwarded)
For the port to change the state from listening to learning - forward delay timer (default 15 seconds) need to pass. The rule is also applicable for the learning to forwarding change.
Port moves through states as follows:
- From blocking to listening or to disabled
- From listening to learning or to disabled
- From learning to forwarding or to disabled
- From forwarding to disabled
If the non root switch stop receive BPDU's (on the root port, or not designated port), it will wait for the 'Max Age time' (default 20 seconds) decremented by 'Message Age' - to pass before making any changes (like changing the port role from Alternate to Designated or considering himself as the Root Switch) .
Note: Every BPDU contains the field 'Message Age', which is being incremented by 1 by every switch which is present on the BPDU path. When Root switch generates BPDU - Message Age value is set to zero.
If switch will receive BPDU which contains better information than previosuly received BPDU [superior BPDU] (for example receiving on Alternate port BPDU which has root priority value lower than BPDU received on the current Root port) - switch will take the action immediatelly (without waiting for Max Age time to pass).
Topology Change Notification:
In normal operations, switch is receiving BPDU's which are generated by the Root device. It is not sending BPDU's towards Root. To notify Root about the topology change, special kind of BPDU was introduced - Topology Change Notification (TCN). When device needs to signal topology change - it starts to send out TCN out of root port.
What is considered to be topology change for sending out TCN:
- when forwarding port is going down
- when a port moves to forwarding, and switch has designated port
The designated switch receives BPDU, ackonwledge it - and generate another one out of its root port. The process continues till TCN reach STP root.
When Root is aware that there as been a topology change, it starts to send configuration BPDU with
Topology Change bit set. As soon as the other bridge is receiving this BPDU - it is changing its aging time of mac address table to forward delay time (default 15 seconds).
TC bit is set by the Root for Max Age + forward delay time (default 35 seconds).
On the Diagram1 used topology is presented. All the links are 100 Mbps. The STP priorities are set as follows:
 |
| Diagram1 - startup topology |
sw1 - STP priority 8192, STP Root
sw2 - STP priority 12288
sw3 - STP default settings (STP priority 32768)
Spanning-tree information from used devices:
sw1#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 8193
Address 000f.2399.2a80
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 8193 (priority 8192 sys-id-ext 1)
Address 000f.2399.2a80
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1 Desg FWD 19 128.1 P2p
Fa0/2 Desg FWD 19 128.2 P2p
sw2#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 8193
Address 000f.2399.2a80
Cost 19
Port 1 (FastEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 12289 (priority 12288 sys-id-ext 1)
Address 0013.7f2f.3c80
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1 Root FWD 19 128.1 P2p
Fa0/3 Desg FWD 19 128.3 P2p
sw3#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 8193
Address 000f.2399.2a80
Cost 19
Port 2 (FastEthernet0/2)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0014.6aa0.8400
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/2 Root FWD 19 128.2 P2p
Fa0/3 Altn BLK 19 128.3 P2p
In capture1 and capture2 sample BPDU's content is presented :
- capture1 presents BPDU send by device sw1 from port Fa0/1
- capture2 presents BPDU send by device sw3 from port Fa0/20 (port is not used in topology, it was only used to capture sample BPDU to present different BPDU content)
 | |||||
| capture1 - BPDU from sw1 port Fa0/1 |
 |
| capture2 - BPDU from sw3 port Fa0/20 |
BPDU's generated by sw1 has Bridge ID and Root Bridge ID values the same set to sw1 parameters. BPDU's send by sw3 has this values different (Root Bridge ID pointing to sw1, Bridge ID value pointing to sw3).
Scenario 1:
In that scenario, interface Fa0/1 on sw1 device will be shutdowned.
It can simuluate the cabling problem, or the port issues on the connection between sw1 and sw2.
Diagram 2 presents situation which will be described in scenario 1.
 |
| Diagram 2 - scenario 1 condition |
From STP perspective, on sw1 there were no modifications.
Jan 9 15:21:54.019: %SPANTREE-5-TOPOTRAP: Topology Change Trap for vlan 1
Jan 9 15:21:55.923: %LINK-5-CHANGED: Interface FastEthernet0/1, changed state to administratively down
Jan 9 15:21:56.923: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to down
Jan 9 15:22:11: STP: VLAN0001 Topology Change rcvd on Fa0/2
Jan 9 15:22:41: STP: VLAN0001 Topology Change rcvd on Fa0/2
On sw2 situation looks quite diferent. From STP perspective definietly more happened than on sw1.
List of events :
- due to the fact that interface FastEthernet0/1 on sw2 is down, sw2 assumed that it is STP root immediatelly ("STP: VLAN0001 we are the spanning tree root" message). sw2 was receiving BPDU's only from sw1 (sw3 was not sending BPDU's to sw2, since the port on sw3 was in Alternate role). Since the interafce FastEthernet0/1 on sw2 (towards sw1) is down - device did not wait for Max Age to expire.
- at 15:22:11 sw2 received BPDU from sw3 (after Max Age time on port FastEthernet0/3 on sw3 passed, and port on sw3 changed the state to listening). As a result sw2 stoped considering itself as the STP root ("STP: VLAN0001 heard root 8193-000f.2399.2a80 on Fa0/3", "supersedes 12289-0013.7f2f.3c80" messages), and changed the role of the port FastEthernet0/3 to be root port.
sw2#
Jan 9 15:21:53: STP: VLAN0001 we are the spanning tree root
Jan 9 15:21:54.934: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to down
Jan 9 15:21:55.946: %LINK-3-UPDOWN: Interface FastEthernet0/1, changed state to down
Jan 9 15:22:11: STP: VLAN0001 heard root 8193-000f.2399.2a80 on Fa0/3
Jan 9 15:22:11: supersedes 12289-0013.7f2f.3c80
Jan 9 15:22:11: STP: VLAN0001 new root is 8193, 000f.2399.2a80 on port Fa0/3, cost 38
Jan 9 15:22:11.949: %SPANTREE-5-ROOTCHANGE: Root Changed for vlan 1: New Root Port is FastEthernet0/3. New Root Mac Address is 000f.2399.2a80
Jan 9 15:22:11: STP: VLAN0001 sent Topology Change Notice on Fa0/3
Chain of events on sw3:
- at 15:21:53 sw3 start to receive BPDU's on port FastEthernet0/3 from sw2, which was considering itself to be STP Root ("STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/3" message).
The priority of that BPDU was higher (inferior BPDU), than previously received BPDU on port FastEthernet0/3 from sw2 (when sw2 was receiving the BPDU's from sw1)
- after Max Age has passed (at 15:22:11), sw3 moved the port FastEthernet0/3 to listening state ("STP: VLAN0001 Fa0/3 -> listening" message). In the final stage the port FastEthernet0/3 move to forwarding state, and started to be Designated port.
Around 15:22:41 communication towards sw2 was restored (when port FastEthernet0/3 on sw3 changed state to forwarding).Communication to sw2 was lost around 15:21:53 - when interface Fa0/1 was shutdown. Device sw2 was isolated from the network for allmost 50 seconds in summary.
sw3#
Jan 9 15:21:53: STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/3
Jan 9 15:21:55: STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/3
Jan 9 15:21:57: STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/3
Jan 9 15:21:59: STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/3
Jan 9 15:22:01: STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/3
Jan 9 15:22:03: STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/3
Jan 9 15:22:05: STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/3
Jan 9 15:22:07: STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/3
Jan 9 15:22:09: STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/3
Jan 9 15:22:11: STP: VLAN0001 Fa0/3 -> listening
Jan 9 15:22:11: STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/3
Jan 9 15:22:11: STP: VLAN0001 Topology Change rcvd on Fa0/3
Jan 9 15:22:11: STP: VLAN0001 sent Topology Change Notice on Fa0/2
Jan 9 15:22:26: STP: VLAN0001 Fa0/3 -> learning
Jan 9 15:22:41: STP: VLAN0001 sent Topology Change Notice on Fa0/2
Jan 9 15:22:41: STP: VLAN0001 Fa0/3 -> forwarding
Situation looks also quite interesting, when interface FastEthernet0/1 was enabled on sw1 (restoring the connection between sw1 and sw2).
- after enabling interface FastEthernet0/1 on sw1, the interface went through listenening, learning stage to finally move to forwarding status. After 30 seconds (two times forward delay) interface was ready to process the traffic.
sw1(config-if)#no sh
sw1(config-if)#
Jan 9 15:27:26.055: %LINK-3-UPDOWN: Interface FastEthernet0/1, changed state to up
Jan 9 15:27:28: set portid: VLAN0001 Fa0/1: new port id 8001
Jan 9 15:27:28: STP: VLAN0001 Fa0/1 -> listening
Jan 9 15:27:28: STP: VLAN0001 Topology Change rcvd on Fa0/2
Jan 9 15:27:29.595: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to up
Jan 9 15:27:43: STP: VLAN0001 Fa0/1 -> learning
Jan 9 15:27:58.595: %SPANTREE-5-TOPOTRAP: Topology Change Trap for vlan 1
Jan 9 15:27:58: STP: VLAN0001 Fa0/1 -> forwarding
Jan 9 15:27:58: STP: VLAN0001 Topology Change rcvd on Fa0/1
Chain of events observed on sw2 - were simmilar to the ones which happened on sw1.
Jan 9 15:27:27.586: %LINK-3-UPDOWN: Interface FastEthernet0/1, changed state to up
Jan 9 15:27:28: set portid: VLAN0001 Fa0/1: new port id 8001
Jan 9 15:27:28: STP: VLAN0001 Fa0/1 -> listening
Jan 9 15:27:28: STP: VLAN0001 new root port Fa0/1, cost 19
Jan 9 15:27:29.602: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to up
Jan 9 15:27:43: STP: VLAN0001 Fa0/1 -> learning
Jan 9 15:27:58.602: %SPANTREE-5-TOPOTRAP: Topology Change Trap for vlan 1
Jan 9 15:27:58: STP: VLAN0001 sent Topology Change Notice on Fa0/1
Jan 9 15:27:58: STP: VLAN0001 Fa0/1 -> forwarding
On sw3 port FastEthernet0/3 moved to blocking state, immediatelly after sw2 resend to sw3 BPDU received from sw1 (from timestamps it is visible, that it was the moment when port FastEthernet0/1 on sw1 and sw2 moved to listening state).
Jan 9 15:27:28: STP: VLAN0001 sent Topology Change Notice on Fa0/2
Jan 9 15:27:28: STP: VLAN0001 Fa0/3 -> blocking
After restoring the connection between sw1 and sw2 - communication towards sw2 was disturbed for 30 seconds (two times forward delay).
Scenario 2:
In that scenario - BPDU's will stop being send out from interface Fa 0/3 on SW2 (while the link remains up). This might be caused by STP operations issues or misconfiguration.
As a result, SW3 will transit port 3 to forwarding state (in normal operations port is in Alternate Role, Blocking state). This will cause loop being created in topology. For the network this is catastrophical situation which results in traffic disturbances.
Let's see - how long it would take with default STP timers, for port 3 on SW3 move to forwarding state, from the moment when BPDU would stop being send out by SW2.
To stop sending out BPDU's from port Fa 0/3 on SW3 bpdu filter feature was used (this is misconfiguration from production network persepctive).
Diagram 3 presents situation which will be described in scenario 2.
 |
| Diagram 3 - scenario 2 condition |
configure terminal
int fa 0/3
spanning-tree bpdufilter enable
Chain of events observed on sw3:
Last BPDU was received on FA0/3 by sw3 at 15:31:51 .
Oct 30 15:31:51: STP: VLAN0001 rx BPDU: config protocol = ieee, packet from FastEthernet0/3 , linktype IEEE_SPANNING , enctype 2, encsize 17
Oct 30 15:31:51: STP: enc 01 80 C2 00 00 00 00 13 7F 2F 3C 83 00 26 42 42 03
Oct 30 15:31:51: STP: Data 00000000002001000F23992A8000000013300100137F2F3C8080030100140002000F00
Oct 30 15:31:51: STP: VLAN0001 Fa0/3:0000 00 00 00 2001000F23992A80 00000013 300100137F2F3C80 8003 0100 1400 0200 0F00
Oct 30 15:31:51: STP(1) port Fa0/3 supersedes 0
After 19 seconds (Max Age - Message Age) since last BPDU was received on Fa0/3, Fa 0/3 moves to listening state (it was Altn/Blck)
Oct 30 15:32:10: STP: VLAN0001 Fa0/3 -> listening
After 15 seconds (Forwald Delay), Fa 0/3 moves to learning
Oct 30 15:32:25: STP: VLAN0001 Fa0/3 -> learning
After 15 seconds (Forwald Delay), Fa 0/3 moves to Forwarding
Oct 30 15:32:40: STP: VLAN0001 Fa0/3 -> forwarding
After 49 seconds from the moment when SW2 stopped sending out BPDU's from port Fa0/3 - loop was created in the network.
After issuing the command "no spanning-tree bpdufilter enable" on SW2 (misconfiguration was removed) - these are the events observed on sw3:
BPDU was received on interafce Fa 0/3.
Oct 30 15:32:49: STP: VLAN0001 rx BPDU: config protocol = ieee, packet from FastEthernet0/3 , linktype IEEE_SPANNING , enctype 2, encsize 17
Oct 30 15:32:49: STP: enc 01 80 C2 00 00 00 00 13 7F 2F 3C 83 00 26 42 42 03
Oct 30 15:32:49: STP: Data 00000000012001000F23992A8000000013300100137F2F3C8080030100140002000F00
Oct 30 15:32:49: STP: VLAN0001 Fa0/3:0000 00 00 01 2001000F23992A80 00000013 300100137F2F3C80 8003 0100 1400 0200 0F00
Oct 30 15:32:49: STP(1) port Fa0/3 supersedes -80
Interface Fa 0/3 moves to Blocking state.
Oct 30 15:32:49: STP: VLAN0001 Fa0/3 -> blocking
At this moment - the network started functioning properly again - after 1 second when misconfiguration was removed.
Scenario 3:
In that scenario, STP priority on sw2 will be changed to 4096 - so this device would become STP root.
Diagram 4 presents the final topology, after STP priority will be set to 4096 on sw2 device. .
 |
| Diagram 4 - topology after setting STP priority to 4096 on sw2 |
After changing the STP priority on sw2 device, switch immediatelly took over the role of STP root functionality, and started generating BPDU's.
sw2(config)#spanning-tree vlan 1-1000 priority 4096
Oct 25 09:41:26: setting bridge id (which=1) prio 4097 prio cfg 4096 sysid 1 (on) id 1001.0013.7f2f.3c80
Oct 25 09:41:26: STP: VLAN0001 we are the spanning tree root
Oct 25 09:41:26: STP: VLAN0001 Topology Change rcvd on Fa0/3
As soon the sw1 device received the superior BPDU (with lower priority) from sw2, sw1 accepted that sw2 is the new STP root. The role of the port FastEthernet0/1 changed to root (previously it was designated). The port was already in forwarding state - no forward delay timers were needed to pass ("%SPANTREE-5-ROOTCHANGE: Root Changed for vlan 1: New Root Port is FastEthernet0/1. New Root Mac Address is 0013.7f2f.3c80" message).
sw1#
Oct 25 09:41:26: STP: VLAN0001 heard root 4097-0013.7f2f.3c80 on Fa0/1
Oct 25 09:41:26: supersedes 8193-000f.2399.2a80
Oct 25 09:41:26: STP: VLAN0001 new root is 4097, 0013.7f2f.3c80 on port Fa0/1, cost 19
Oct 25 09:41:26.602: %SPANTREE-5-ROOTCHANGE: Root Changed for vlan 1: New Root Port is FastEthernet0/1. New Root Mac Address is 0013.7f2f.3c80
After sw3 received superior BPDU from sw3 on interface FastEthernet0/3 at 09:41:26, the port changed the role to root port and transitioned to listening state (it was in Alternate state before).
After 15 seconds the port moved to listening state, and after 15 seconds more finally to forwarding state.
Additionally - immediatelly when sw3 received superior BPDU from sw2 at 09:41:26, interface FastEthernet0/2 (connection to sw1) - transitioned to blocking state ("STP: VLAN0001 Fa0/2 -> blocking" message).
sw3#
Oct 25 09:41:26: STP: VLAN0001 heard root 4097-0013.7f2f.3c80 on Fa0/3
Oct 25 09:41:26: supersedes 8193-000f.2399.2a80
Oct 25 09:41:26: STP: VLAN0001 new root is 4097, 0013.7f2f.3c80 on port Fa0/3, cost 19
Oct 25 09:41:26: STP: VLAN0001 Fa0/3 -> listening
Oct 25 09:41:26: STP: VLAN0001 sent Topology Change Notice on Fa0/3
Oct 25 09:41:26: STP: VLAN0001 Fa0/2 -> blocking
Oct 25 09:41:41: STP: VLAN0001 Fa0/3 -> learning
Oct 25 09:41:56: STP: VLAN0001 Fa0/3 -> forwarding
After changing STP priority on sw2 to 4096, sw3 device was isolated from the network for 30 seconds - while STP was adapting to new conditions.
While STP priority was set back on sw2 to previous value 12288 (sw1 would becaome STP root then again) - situation looks even more interesting.
Chain of events on sw2:
- priority was set to 12288 on sw2 at 09:43:36 - after that sw2 was still STP root in the network, and was generating BPDU's (no other device is generating BPDU's at the moment).
- sw2 received BPDU from sw1 at 09:43:56 ("VLAN0001 heard root 8193-000f.2399.2a80 on Fa0/1" message) - this happened 20 seconds after setting priority on sw2 to 12288 due to Max Age timer which needed to pass on sw1. After receiving superior BPDU from sw1 - sw2 release the STP root role and port FastEthernet0/2 became new root port ("%SPANTREE-5-ROOTCHANGE: Root Changed for vlan 1: New Root Port is FastEthernet0/1") .
- what is worth to mention, at 09:43:56 sw2 received inferior BPDU from sw3, whic was claiming that sw3 is the STP root at the moment ("VLAN0001 heard root 32769-0014.6aa0.8400 on Fa0/3")
We will take closer look on that while analysing sw3 operations.
sw2(config)#spanning-tree vlan 1-1000 priority 12288
Oct 25 09:43:36: setting bridge id (which=1) prio 12289 prio cfg 12288 sysid 1 (on) id 3001.0013.7f2f.3c80
Oct 25 09:43:56: STP: VLAN0001 heard root 8193-000f.2399.2a80 on Fa0/1
Oct 25 09:43:56: supersedes 12289-0013.7f2f.3c80
Oct 25 09:43:56: STP: VLAN0001 new root is 8193, 000f.2399.2a80 on port Fa0/1, cost 19
Oct 25 09:43:56.604: %SPANTREE-5-ROOTCHANGE: Root Changed for vlan 1: New Root Port is FastEthernet0/1. New Root Mac Address is 000f.2399.2a80
Oct 25 09:43:56: STP: VLAN0001 heard root 32769-0014.6aa0.8400 on Fa0/3
Oct 25 09:43:56: STP: VLAN0001 Topology Change rcvd on Fa0/3
Oct 25 09:43:56: STP: VLAN0001 sent Topology Change Notice on Fa0/1
Chain of events observed on sw1:
- after setting the new priority on sw2 to 12288, sw1 started to receive BPDU's with new priority value on port FastEthernet0/1 ("STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/1").
Since this was inferior BPDU to previous one received on that interface (previous BPDU has priority set to 4096) - sw1 was waiting for Max Age time to pass before taking any actions. Newly received BPDU's were not resend to sw3.
- at 09:43:56, after Max Age timer passed - sw1 took over STP root role while it has lower priority 8192 ("VLAN0001 we are the spanning tree root") and started generating BPDU's (at the same time sw2 received BPDU from sw1).
- at 09:43:56 also BPDU from sw3 was received which was claiming that sw3 is STP root in the network ("STP: VLAN0001 heard root 32769-0014.6aa0.8400 on Fa0/2" message). sw1 did not take any action based on that BPDU - since it was inferior BPDU (higher priority). sw1 was still generating BPDU's and sending them out through ports FastEthernet0/1 and FastEthernet0/2.
sw1#
Oct 25 09:43:38: STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/1
Oct 25 09:43:40: STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/1
Oct 25 09:43:42: STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/1
Oct 25 09:43:44: STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/1
Oct 25 09:43:46: STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/1
Oct 25 09:43:48: STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/1
Oct 25 09:43:50: STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/1
Oct 25 09:43:52: STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/1
Oct 25 09:43:54: STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/1
Oct 25 09:43:56: STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/1
Oct 25 09:43:56: STP: VLAN0001 we are the spanning tree root
Oct 25 09:43:56: STP: VLAN0001 heard root 32769-0014.6aa0.8400 on Fa0/2
Oct 25 09:43:56: STP: VLAN0001 Topology Change rcvd on Fa0/1
Oct 25 09:43:58: STP: VLAN0001 Topology Change rcvd on Fa0/2
Let's analyse what happened on sw3 device and why during the reconvergence it assumed it is STP root :
- simmilar to sw1, sw3 at 09:43:38 started to receive from sw2 BPDU's with new priority set ("STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/3"). Since this was inferior BPDU than previously received from sw2 on that port (previous priority was set to 4096) - this BPDU was not saved and resend by the switch (Max Age time need to pass before).
- after Max Age time passed- at 09:43:56 - sw3 transitioned interface FastEthernet0/2 (connection to sw1) to listening state (sw1 stopped resending inferior BPDU's, which sw2 was generating).
At this point in time - sw3 did not have any BPDU's saved on any interface (BPDU's received from sw2 not saved yet, no BPDU's from sw1) - so sw3 claimed itself to be STP root and started
generating BPDU's (this is when sw1 and sw2 received BPDU from sw3 which was claiming it is STP root).
- immediatelly after, sw3 saved BPDU's received from sw2 ("VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/3", "supersedes 32769-0014.6aa0.8400"). Device sw3 relesaed the STP root role, and marked interface FastEthernet0/3 (towards sw2) - as the new root port.
- this was also the moment - when sw1 started to generate BPDU's. Device sw3 received superior BPDU from sw1 ("VLAN0001 heard root 8193-000f.2399.2a80 on Fa0/2", "supersedes 12289-0013.7f2f.3c80"). Interface FastEthernet0/2 became root port.
- interface FastEthernet0/3 (towards sw2) moved to blocking state at 09:43:57 ("STP: VLAN0001 Fa0/3 -> blocking" message).
- at 09:44:10 interface FastEthernet0/2 (towards sw1) moved to learning state, and after 15 seconds to forwarding state.
For 28 seconds - communication towards sw3 was disturbed (09:43:57 Fa0/3 moved to blocking, Fa 0/2 was then already in listening state). At 09:44:25 Fa0/2 moved to forwarding.
sw3#
Oct 25 09:43:38: STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/3
Oct 25 09:43:40: STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/3
Oct 25 09:43:42: STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/3
Oct 25 09:43:44: STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/3
Oct 25 09:43:46: STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/3
Oct 25 09:43:48: STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/3
Oct 25 09:43:50: STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/3
Oct 25 09:43:52: STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/3
Oct 25 09:43:54: STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/3
Oct 25 09:43:55: STP: VLAN0001 Fa0/2 -> listening
Oct 25 09:43:56: STP: VLAN0001 we are the spanning tree root
Oct 25 09:43:56: STP: VLAN0001 heard root 12289-0013.7f2f.3c80 on Fa0/3
Oct 25 09:43:56: supersedes 32769-0014.6aa0.8400
Oct 25 09:43:56: STP: VLAN0001 new root is 12289, 0013.7f2f.3c80 on port Fa0/3, cost 19
Oct 25 09:43:56: STP: VLAN0001 sent Topology Change Notice on Fa0/3
Oct 25 09:43:56: STP: VLAN0001 heard root 8193-000f.2399.2a80 on Fa0/2
Oct 25 09:43:56: supersedes 12289-0013.7f2f.3c80
Oct 25 09:43:56: STP: VLAN0001 new root is 8193, 000f.2399.2a80 on port Fa0/2, cost 19
Oct 25 09:43:57: STP: VLAN0001 Fa0/3 -> blocking
Oct 25 09:43:58: STP: VLAN0001 sent Topology Change Notice on Fa0/2
Oct 25 09:44:10: STP: VLAN0001 Fa0/2 -> learning
Oct 25 09:44:25: STP: VLAN0001 Fa0/2 -> forwarding
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