A network engineer deploys four switches in a redundant enterprise topology running standard IEEE 802.1D Spanning Tree Protocol. The switches are configured with the following bridge priorities and MAC addresses:
• Switch-Alpha: Priority 4096, MAC 00:11:22:33:44:55
• Switch-Beta: Priority 8192, MAC 00:11:22:33:44:AA
• Switch-Gamma: Priority 4096, MAC 00:11:22:33:44:11
• Switch-Delta: Priority 32768, MAC 00:11:22:33:44:00
Which switch will be elected as the Root Bridge for this STP domain?
- Switch-GammaAnswer
- BSwitch-Alpha
- CSwitch-Delta
- DSwitch-Beta
Answer
Switch-Gamma will be elected as the Root Bridge because it shares the lowest bridge priority value (4096) and has the lowest MAC address among the tied switches.
In Spanning Tree Protocol (STP), the Root Bridge election evaluates the Bridge Identifier (BID), which consists of a 2-byte Bridge Priority and a 6-byte MAC address. The switch with the lowest numerical BID becomes the Root Bridge. Priority is evaluated first; Switch-Alpha and Switch-Gamma tie with the lowest priority of 4096. Evaluating the MAC address as the tie-breaker reveals that Switch-Gamma's MAC address (00:11:22:33:44:11) is lower than Switch-Alpha's MAC address (00:11:22:33:44:55), making Switch-Gamma the Root Bridge.
Step-by-Step Solution
Key Concept
STP Root Bridge Election Mechanics