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Refer to the exhibit. For STPs VLAN1 instance, why is Switch _SW3 a non root bridge?
It has a higher Bridge ID _BID compared to the elected Root Bridge.
It is running STP 802.1d while the root bridge is running an RSTP instance.
It has a higher MAC address portion on the BID compared to the root bridge.
It is using Port-channel which has multiple bundled interfaces.
When STP is converging on the LAN, the Switches need to elect a Root Bridge. This is done by comparing the BPDUs (Bridge Protocol Data Units) sent and received by the switches participating in the STP. The switch receiving the BPDU will compare its BID among others. The Switch with lowest BID will be elected the Root Bridge. The BID consists on Priority, MAC address and Sys-id (VLAN info). In this case the priority is the same for both 32768, this switch and the root bridge, so they will compare the MAC address. When comparing MAC addresses, the lowest numerical MAC address is that of the Root Bridge. For more information refer to: [http://www.cisco.com/en/US/tech/tk389/tk621/technologiestech_note09186a008009482f.shtml]
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