802.1t Extended System ID

Unanswered Question
Sep 20th, 2007
User Badges:

If a switch is configured with the following:

SwitchB(config)# spanning-tree extended system-id

SwitchB(config)# spanning-tree vlan 5 root primary

If SwitchA is the root bridge with a priority of 4200, i suppose SwitchB has no chance of becoming the new root.(As the new priority of SwB must be 2400-2096=104, but since the priority is in multiples of 4096 so the priority of swB infact remains unchanged)

What is the lower threshold priority for avoiding this situation? I mean what is the minimum root priority value of SwitchA so that SwitchB can take over as the new root bridge with the configuration above? Should it be 8192?

PS: I have no equipment or simulations to test the situation for the time being.

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 3 (1 ratings)


If both switches have the same priority set their MAC address will be the tiebreaker so if switch B has a lower MAC address that will win the election.

The bridge priority (if extended system ID is enabled) is built up from 4 bit priority and 12 VLAN ID.

Therefore the Bit 16, Bit 15, Bit 14, Bit 13 have the values of 32768, 16384, 8192, 4096.

I think the 8192 for secondary and 4096 for the primary would be OK.

Hope it helps, rate if does



This Discussion