Which Bridge ID Cannot Appear in an STP Configuration BPDU?
Short answer
A A BID with priority 4096 and MAC 01-01-02-03-04-05 is invalid because that MAC is multicast, not a unicast bridge address usable in a configuration BPDU.
Question
Key Takeaways
The exam tests both BID fields—priority granularity and the MAC's I/G bit—with the trap of treating hexadecimal 10 as multicast or priority 0 as illegal.
STP Bridge IDs require a 4096-multiple priority and a unicast MAC address; this question shows why option A's multicast 01-... MAC is invalid.
Common mistake: Choosing D because 10 hex is mistaken for a multicast first octet; 0x10 = 00010000 has I/G=0 and is a valid unicast MAC, while A's 0x01 is the multicast address that cannot be a BID.
Expert Analysis
Bridge ID anatomy. An 802.1D/STP Bridge ID contains two fields: a 2-byte priority and a 6-byte MAC address. The priority is configurable only in 4096 increments, from 0 through 61440, and the MAC must be an individual/unicast address. A configuration BPDU carries the root identifier and the sender's bridge identifier, so any BID appearing there must satisfy both rules.
Why A is the exception. In 01-01-02-03-04-05, the first octet is 0x01 (0000 0001). The least-significant bit is the I/G bit and is set to 1, so this is a multicast MAC address. A multicast address cannot identify a single bridge, so it cannot serve as a Bridge ID in a configuration BPDU. The priority 4096 itself is valid, but the MAC makes option A invalid.
Checking the other options. B uses 32768 (a multiple of 4096) and 06-01-02-03-04-05; 0x06 is 0000 0110, with I/G=0, so the MAC is unicast. C uses priority 0, which is permitted—0 is a multiple of 4096—and 00-... is unicast. D also uses priority 0 and 10-...; 0x10 is 0001 0000, so the I/G bit is 0 and the MAC is unicast. A tempting misreading treats 10 as a multicast first octet, but the bit test is what matters; the multicast defect is in A's 01. If D's address were changed to 01-..., or if any priority were changed to a value such as 4097 or 1, that option would become invalid. Conversely, changing A's MAC to 02-... would make the BID legal.
Operational consequence. Misjudging the multicast bit can make you predict the wrong root bridge or accept an impossible BID. In a live network, a non-4096 priority or an invalid bridge MAC undermines deterministic root election; the result can be a suboptimal or unstable spanning tree, with blocked links, traffic tromboning, and slow convergence. The exam point is to validate both fields instead of focusing only on the numeric priority.
Exam Strategy
For STP questions, split every BID into priority and MAC before comparing. Verify the priority is a multiple of 4096 (0, 4096, 8192...61440), then check the least-significant bit of the first MAC octet: 1 means multicast and disqualifies it as a bridge ID. Do not assume a low or zero priority is wrong, and convert hexadecimal values such as 10, 06 and 00 carefully instead of treating them as decimal.
Frequently Asked Questions
Why can't 01-01-02-03-04-05 be used in a Bridge ID?
Is bridge priority 0 valid in an STP BID?
How can I tell if a MAC address is multicast?
Community Votes
50% of anonymous learners picked answer D. Votes are pick records left by other test-takers — they are not the verified answer.
Discussion 2 contributions
Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.
From public community threads 2
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Fmoura17 👍 1 Answered DD. 0 10-01-02-03-04-05 O endereço MAC 10-01-02-03-04-05 é inválido para um BID porque o primeiro octeto 10 (hex) = 00010000 (binário), onde o bit menos significativo (LSB) = 1, tornando-o um endereço MAC multicast. Endereços MAC multicast não podem ser usados como Bridge ID em BPDUs do STP. As outras opções possuem prioridades válidas (múltiplos de 4096) e endereços MAC unicast válidos.
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Heidorn 👍 1 Answered AThe correct answer is A due to the use of significant Multicast Bits
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