STP Loss of Root Bridge Example

The backup root bridge becomes the root bridge when something goes wrong with the primary root bridge: root bridge failure, root bridge malfunction, or administrative errors, and the backup root bridge looses connectivity with the root bridge.. If the priority of the root bridge is zero and backup root loses the connectivity to the root bridge, then automatic assignment of the priority value for the backup root becomes the initial configured value.

In the following example, Switch1 is the root bridge with priority 32,768. Switch2, Switch3, and Switch4 are configured with priority as 49,152, 36,864, and 40,960, respectively. When the physical link between Switch1 and Switch2 goes down, then Switch2 becomes the root bridge by changing its configured bridge priority, which will be less than Switch1's 32,768 (operational priority); in this case its priority will be 28,672 (32,768 - 4,096). When the priority value reaches 0, then automatic assignment of the priority value returns to the configured value.

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STP Root Bridge Loss Example