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PVST alternate port selection question

hi folks,

I have 3   2960 CISCO switches, the default STP mode is PVST. 

Ports F0/2 and F0/4 on all 3 switches are enabled and configured as trunk ports, all other ports are in shutdown state. 


The priority on all 3 equals to the default of 32768 + the extended system  ID 1 (for VLAN 1 ) = 32769 

I've converted the hexadecimal MAC representation of the corresponding VLAN 1 interface on each switch to decimal 

S1   965034079591
S2   413110024370
S3      5416955065

During BPDUs exchange it is determined that S3 will be the ROOT BRIDGE for the given topology ( because the priority and extended system ID on all 3 switches contains the same value, each MAC address is examined, and the lowest value is selected to determine the ROOT BRIDGE). In this case, it's the S3. 

Because S3 is the ROOT BRDIGE it will be referenced for all future path/cost calculations. 

Because S3 is the ROOT BRDIGE I can tell that it's F0/2 and F0/4 ports are DESIGNATED PORTS. 

I can also tell, that the F0/4 port on S1 and F0/4 on S2 are ROOT PORTS. 

What I don't understand why was the F0/2 port on S1 elected to be an ALTERNATE PORT thus in a BLOCKING STATE:

S1#show spanning-tree


Spanning tree enabled protocol ieee

Root ID Priority 32769

Address 0001.42E0.2CB9

Cost 19

Port 4(FastEthernet0/4)

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)

Address 00E0.B083.7567

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Aging Time 20

Interface Role Sts Cost Prio.Nbr Type

---------------- ---- --- --------- -------- --------------------------------

Fa0/2 Altn BLK 19 128.2 P2p

Fa0/4 Root FWD 19 128.4 P2p


I think I've figured it out. So to reach the ROOT BRIDGE from S1 F0/2 port I would have to go first through S2 and then to S3, because this are interconnected via FastEthernet ports it gives a total cost of 38 (S1 to S2 = 19 and S2 to S3 = 19). On the other hand F0/2 on S2 has a lower cost (19) because it can reach S3 directly via the F0/4 to F0/2 link on S3, correct? 



Hall of Fame Guru


That is not how it is worked out for designated ports.

S1 and S2 both receive a BPDU from S3 and both switches add the cost of the interface the BPDU was received on ie. 19 before transmitting a BPDU onto the segment connecting S1 to S2. So both S1 and S2 each receive a BPDU from each other with a cost of 19.

If there is a tie then the next check is the same as for the root election and as you can see from your hex representations it is S2 that is the next lowest.

Does this make sense ?



Thank you Jon. 

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