Electing an STP Root Bridge
Electing an STP Root Bridge
The first step in the STP convergence process is electing a Root Bridge, which is the central reference point for the STP topology. As a best practice, the Root Bridge should be the most centralized switch in the STP topology.
A Root Bridge is elected based on its Bridge ID, comprised of two components in the original 802.1D standard:
• 16-bit Bridge priority
• 48-bit MAC address
The default priority is 32,768, and the lowest priority wins. If there is a tie in priority, the lowest MAC address is used as the tie-breaker.
Consider the following example:
Switches exchange BPDU’s to perform the election process, and the lowest Bridge ID determines the Root Bridge:
• SwitchB, SwitchC, and SwitchE have the default priority of 32,768.
• SwitchA and SwitchD are tied with a lower priority of 100.
• SwitchA has the lowest MAC address, and will be elected the Root Bridge.
By default, a switch will always believe it is the Root Bridge, until it receives a BPDU from a switch with a lower Bridge ID. This is referred to as a superior BPDU. The election process is continuous – if a new switch with the lowest Bridge ID is added to the topology, it will be elected as the Root Bridge.
The first step in the STP convergence process is electing a Root Bridge, which is the central reference point for the STP topology. As a best practice, the Root Bridge should be the most centralized switch in the STP topology.
A Root Bridge is elected based on its Bridge ID, comprised of two components in the original 802.1D standard:
• 16-bit Bridge priority
• 48-bit MAC address
The default priority is 32,768, and the lowest priority wins. If there is a tie in priority, the lowest MAC address is used as the tie-breaker.
Consider the following example:
Switches exchange BPDU’s to perform the election process, and the lowest Bridge ID determines the Root Bridge:
• SwitchB, SwitchC, and SwitchE have the default priority of 32,768.
• SwitchA and SwitchD are tied with a lower priority of 100.
• SwitchA has the lowest MAC address, and will be elected the Root Bridge.
By default, a switch will always believe it is the Root Bridge, until it receives a BPDU from a switch with a lower Bridge ID. This is referred to as a superior BPDU. The election process is continuous – if a new switch with the lowest Bridge ID is added to the topology, it will be elected as the Root Bridge.
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