OSI model fundamentals: Do network bridges operate at the MAC sublayer of Layer 2 (the data link layer)?
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ATrue
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BFalse
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COnly when VLANs are configured
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DOnly with full-duplex links
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ENone of the above
Answer
Correct Answer: True
Explanation
Introduction / Context:Bridges (and modern Ethernet switches) forward frames based on MAC addresses and maintain forwarding tables to separate collision domains. Understanding the precise OSI placement helps clarify capabilities and limitations, such as lack of IP-layer routing.
Given Data / Assumptions:
- Bridge = device that forwards frames on MAC addresses.
- OSI Layer 2 has two sublayers: Logical Link Control (LLC) and Media Access Control (MAC).
- We consider standard Ethernet bridging behavior.
Concept / Approach:Bridges operate at Layer 2 and primarily at the MAC sublayer. They learn source MAC addresses, populate forwarding tables, and filter/forward frames accordingly. They do not interpret IP headers (Layer 3) and therefore do not route between subnets unless additional capabilities are present (that would make them routers or Layer-3 switches).
Step-by-Step Solution:
Recognize the function: MAC-based forwarding, flooding unknown unicasts, and STP for loops.Map to OSI: these are all MAC sublayer tasks at Layer 2.Conclude that the statement is true.Note: managed switches may expose higher-layer features, but pure bridging remains Layer 2.Verification / Alternative check:Protocol analyzers show bridges do not modify IP headers; they act on Ethernet frame headers (destination/source MAC).
Why Other Options Are Wrong:
- 'False' contradicts the well-established OSI placement of bridges.
- Other qualifiers (VLANs, duplex) are unrelated to the bridge's OSI layer.
Common Pitfalls:Confusing Layer-3 switches (which route) with pure Layer-2 bridging.
Final Answer:True