root bridge election in mixed environment

[sarahr202]

Hi everybody

Let say we have mixed set up where some switches uses bridge id as  Priority(2bytes)+Mac address, and some use extended system id as  Priority(4bits)+ VLANID(12 bits)+MAC

The question is " how  is the priority  considered by switches  using regualr bridge id? Let me elaborate on it.

     sw1------------------------sw2

    sw1 uses extended system id while sw2 uses regular bridge id.  Let assume there is only vlan 4.

   Sw1 mac address mac1  priority 32768, so  bridge id------------ 32768+4+mac1

   sw2  mac address mac2  priority  32768, so bridge id-----------------32768+mac2

   So when sw2 which is using regular bridge id receives a bpdu from sw1, will it consider priority as 32772(32768+4) or as 32768?

  Is it not correct that in such case where  a switch does not use extended system id such as sw2, is likely to become root bridge b/c it does not use vlan 

  to drive its bridge id.?

thanks and have a great weekend.

[maxime.gerges · Accepted solution]

If you use extend system id on one switch and not on the other, in that case SW2 will become root bridge. SW1 will put 3772 in its bridge ID BPDU and SW2 won't be aware that SW1 use extend system ID.

In such that case, it's true to say that a switch without extend system ID will be likely to become root bridge as it doesn't use the system ID (VLAN) in the its bridge ID calculation.