The bridge ID priority is combined with the system ID extension to create the bridge ID priority for the VLAN.
Bridge ID Priotirty = Priotirty(16bit) + Mac Address(48bit)
= Priority[first 4bit +12bit for VLAN] + Mac address(48bit)
so, first 4bit represents 4096, 8192, 12288, …. 32768… and next 12 bit represent VLAN numbers. (due to only 12bit, available VLAN will be 4096)
ex) Bridge ID Priority # below is 49162, b/c 49152 + 10 (VLAN #)
Access1#show spanning-tree vlan 10
VLAN0010
Spanning tree enabled protocol rstp
Root ID Priority 24586
Address 0015.63f6.b700
Cost 3019
Port 107 (FastEthernet3/0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 49162 (priority 49152 sys-id-ext 10)
Spanning tree enabled protocol rstp
Root ID Priority 24586
Address 0015.63f6.b700
Cost 3019
Port 107 (FastEthernet3/0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 49162 (priority 49152 sys-id-ext 10)
Address 000f.f794.3d00
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
UplinkFast enabled but inactive in rapid-pvst mode
Interface Role Sts Cost Prio.Nbr Type
Aging Time 300
UplinkFast enabled but inactive in rapid-pvst mode
Interface Role Sts Cost Prio.Nbr Type
—————- —- — ——— ——– ——————–
Fa3/0/1 Root FWD 3019 128.107 P2p
Fa3/0/2 Altn BLK 3019 128.108 P2p
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