Ok, I have 3 switches in a triangular shape. S1 at the top, S2 at the lower left, S3 at the bottom right. They're all connected to each using a rollover cable. I'm studying for CCNA and I just ran across the Designated Ports section.
Ok, S2 is the root switch. Here is the output of show-spanning tree on S1.
S1#sh span
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 24577
Address 0060.479A.2816
Cost 19
Port 1(FastEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 00D0.BA78.5980
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1 Root FWD 19 128.1 P2p
Fa0/2 Altn BLK 19 16.2 P2p
Why is Fa0/1 the root port when the costs are the same, but FA0/2 has a lower priority?
I think I may be misunderstanding something.
Also, S3 clearly has lower priority to the root than S1, so why isn't S1 using the interface between S1-S3 as the root port?
S3(config)#do sh span
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0060.479A.2816
Cost 19
Port 1(FastEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0090.21CC.9485
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1 Root LSN 19 16.1 P2p
Fa0/2 Desg FWD 19 16.2 P2p
