Thursday, 27 February 2020

Bridge ID

What is Bridge ID?

It is a 8-byte field which is a combination of bridge priority (2 bytes), extended system ID  and Mac address (6 bytes) of a device.

Bridge ID = Bridge Priority (2 bytes)+ Extended System ID (vlan No)+Mac Address (6 bytes)
  • By default the priority value is 32768
  • extended system id represent the VLAN no.
  •  for example :- 
    • for vlan 1 it would be 1 .....so the Bridge ID would be 32768+1=32769
    • for vlan 2 it would be 2 .....so the Bridge ID would be 32768+2=32770
    • for vlan 5 it would be 5......so the Bridge ID would be 32768+5= 32773
    • for vlan 10 it would be 10..so the Bridge ID would be 32768+10= 32778 and so on.
  • priority value could be assign manually from 1 to 65536 in the multiple of 4096
  • But remember as already disused switch with lowest Bridge-id will become root Bridge.
  • If there is a tie on bridge priority then the MAC address added in Bridge id  and it is well known that mac address is the physical address of the system and couldn't be same of two devices, so the system with lowest value MAC address become a Rood Bridge
          Note : STP could be run for every VLAN called as PVSTP( Per VLAN STP)

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