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.
VLAN 1
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
VLAN0001
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
EDIT:
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?
Thanks,
Adam
