HOW TO CALCULATE THE BID VALUE WHERE THERE ARE MANY VLANs?

[okannteoman]

Hello friends,

The BID value consists of 32768 + VLAN ID/MAC address, but here the default VLAN ID is 1, but according to what is the VLAN ID value when different vlans are entered to the switches? Or is it given to Native Vlan? I will be very happy if you answer

[TotallyTodd]

It will totally use the actual VLAN that it is set on! This concept is totally a part of PVST, per VLAN spanning tree in which difference devices can actually play different roles in different VLANs! 

'  In PVST, each VLAN runs RSTP independently to maintain its own spanning tree without affecting the spanning trees of other VLANs. In this way, loops in each VLAN are eliminated and traffic of different VLANs is load shared over links.'

[rais]

BridgeID = Priority.MAC address. I am not sure if VLAN is part of BiD. But BiD can be configured per vlan.

Here is a useful prior posting:

https://community.cisco.com/t5/switching/why-does-the-bridge-id-for-each-vlan-on-a-switch-have-to-be/td-p/2446213

[Martin L]

Yes, BID is Priority value and MAC address. Priority value is also known as Extension/Extended value or priority. It includes VLan id and default priority of 32768.  Few years ago, 20-30 years, BID value did not included vlan ID. But now all switches are programmed with extension or extended BID.

Regards, ML
**Please Rate All Helpful Responses **

[rais]

Just to be clear BridgeID = Priority (4 bits) . Extended System ID (12 bits) . MAC Address (48 bits)

According to Wikipedia Spanning Tree Page: 

In the case of MST, the bridge system ID extension carries the MSTP instance number. Some vendors set the bridge system ID extension to carry a VLAN ID allowing a different spanning tree per VLAN, such as Cisco's PVST.

[Martin L]

First of all, you do not have to calculate anything; switch does it for you; just use show spanning-tree command to see values. Second, True, the BID value consists of 32768 + VLAN ID and MAC address; MAC is a tie-breaker when BIDs are equal so that Root Sw can be selected (lower MAC wins). You or switch adds vlan ID value to the priority value of 32768. this 32768 default value can be change by you if u wish;  and,  it should be changed when deciding which Sw should be Root switch.  Cisco uses per-vlan spanning tree so that each vlan instance has its own L2 domain with Root switch, BID priority values, ports, etc. One Root sw per L2 domain but same switch can be Root for all vlans.  So, for vlan 1 BID value is 32768 +1, for vlan 2, 32768 +2, for vlan 100, BID is 32768 +100, etc. For 2 switches with vlan 100 and connected via trunk link, default BID is 32868. Then, the lower MAC address decides which one is Root sw. Note they also run vlan 1 with BID 32769.

Vlan 1 is special vlan used for control traffic like STP, DTP, CDP, etc, and should not be used for end-user data traffic. By default VLan 1 is present on all switches and it is on (live) so that switch is plug and play (no configuration required at first boot).  you can just connect all devices to switch and they can communicate via vlan 1.  of course, you should add vlans, separate ports based on your topology.

Regards, ML
**Please Rate All Helpful Responses **