ITPEC FE Morning October 2018 Question 33
In the OSI basic reference model, which of the following is the layer where bridges operate?
Explanation
The data link layer — a bridge forwards frames between LAN segments by their destination MAC address.
A bridge learns which MAC addresses sit on each of its ports from the source addresses of the frames it sees, then forwards a frame only to the port where its destination lives, filtering local traffic. MAC addresses and frames belong to layer 2, the data link layer; a layer 2 switch is a many-port bridge.
Answer (c)
Why not others:
- •(a) Application layer — layer 7, where protocols such as HTTP and SMTP work; a device relaying up to this layer is a gateway
- •(b) Network layer — layer 3, where routers forward packets by IP address
- •(d) Transport layer — layer 4, which handles end-to-end delivery with TCP and UDP between hosts, not relaying between segments
Key rule: Repeaters and hubs work at layer 1, bridges and switches at layer 2, routers at layer 3, and gateways at layer 4 and above.
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