Root Switch/Bridge Election Process

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Jul 19th, 2007
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Can someone be kind enough just to clarify this process for me.

I understand that the switch with the lowest Bridge ID or Priority Vlaue wins the election and that 32768 is the default value for all switches, is that the ID or Prioirity value?

I just need clarification on exactlym what these values are and how they are derived

Thanks guys and girls!


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glengregory Thu, 07/19/2007 - 09:33
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Bridge ID is composed of 2 parts; Bridge priority (2 bytes) & MAC address of switch (6 bytes).

The 32768 is the priority which can be changed.

For a real good explanation on this topic, I highly recommend reading the switching part in the ICND book.


mohammedmahmoud Thu, 07/19/2007 - 12:19
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The old original BID held only 2 fields; Priority and System ID (MAC address) -> This introduce a problem with per-VLAN STP (PVST+ and MST) as a new MAC address will be required for each VLAN BID -> to have unique BID per VLAN.

The solution was in the new BID format (MAC Address usage reduction), which introduced the VLANID (System ID Extension) into the BID -> A single MAC address can be used to represent all the VLAN BID (as the System ID Extension will differ per VLAN -> VLAN-ID).

The bridge priority (default 32768) in the new BID format is in increments of 4096 (0 -> 61440) -> 2^4=16 -> 16=0-15 -> 4096x15=61440


Mohammed Mahmoud.


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