3

I am having trouble on how should I identify which port will be blocked and which will be the designated, since both BID and Cost are the same for the ports.

Now for first port, the output of show spanning-tree is,

Spanning tree enabled protocol rstp
Root ID Priority 24984
Address 00D0.58AC.C607
Cost 9
Port 28(Port-channel 2)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 33176 (priority 32768 sys-id-ext 408)
Address 0001.43C6.4D81  
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20

Interface Role Sts Cost Prio.Nbr Type
--------- ---- --- ---- -------- ----
Fa0/1     Desg FWD  19  128.1    P2p
Po1       Desg FWD  9   128.27   Shr
Po2       Root FWD  9   128.28   Shr

and on the second, the output is,

Spanning tree enabled protocol rstp
Root ID Priority 24984
Address 00D0.58AC.C607
Cost 9
Port 28(Port-channel 2)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 33176 (priority 32768 sys-id-ext 408)
Address 0002.16C9.4B1D 
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20

Interface Role Sts Cost Prio.Nbr Type
--------- ---- --- ---- -------- ----
Fa0/1     Desg FWD  19  128.1    P2p
Po1       Altn BLK  9   128.27   Shr
Po2       Root FWD  9   128.28   Shr

The cost in both the cases is 9 and the BID of the ports are 33176 because both are using same VLAN numbers, but how come the first one is assigned as designated and the other is blocked ?

What I understand is that it depends on the lower MAC address but how come show spanning-tree commands show two address in both ports i.e

Address 00D0.58AC.C607  (same in both ports),

and the other one which is different on both ports,

Port1,

Bridge ID Priority 33176 (priority 32768 sys-id-ext 408)
Address 0001.43C6.4D81  

Port2,

Bridge ID Priority 33176 (priority 32768 sys-id-ext 408)
Address 0002.16C9.4B1D 

Can someone please explain why there are two different addresses for each port and spanning tree is really calculating the designated port using the address if cost and BID are the same ?

  • 00d0 is the mac of the root switch. The others are the bridge id of each switch. (so there's a 3rd switch somewhere) A port connected to the root switch will also be fwd'ing. – Ricky Beam May 15 '15 at 6:05
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As Ricky says, the first address is the root ID:

Root ID Priority 24984 
Address 00D0.58AC.C607

the second the local bridge ID:

Bridge ID Priority 33176 (priority 32768 sys-id-ext 408)
Address 0001.43C6.4D81

or

Bridge ID Priority 33176 (priority 32768 sys-id-ext 408)
Address 0002.16C9.4B1D

The full bridge ID is formed of the Priority and the MAC Address, so it's 33176.0001.43C6.4D81 or 33176.0002.16C9.4B1D (with apologies for mixing radices). So, there should never be a tie with different bridges.

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