In a network of LANs connected by bridges, packets are sent from one LAN to another through intermediate bridges. Since more than one path may exist between two LANs, packets may have to be routed through multiple bridges. Why is the spanning tree algorithm used for bridge-routing?
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Answer: B
The correct option is B: For avoiding loops in the routing paths.
Explanation:
In a bridged local area network (LAN), redundant paths are often introduced to ensure reliability. However, these redundant paths can create physical loops. When loops exist in a bridged network, several issues arise:
- Broadcast Storms: Broadcast frames (such as ARP requests) circulate endlessly around the loop, consuming all available bandwidth.
- MAC Table Instability: Bridges update their forwarding tables based on the source addresses of incoming frames. If a frame circulates in a loop, its source address appears on different ports repeatedly, causing the MAC address table to constantly flap.
To prevent these issues, the Spanning Tree Protocol (STP) is used. STP dynamically detects loops and disables redundant links (putting them into a blocking state) to create a single, loop-free logical path (a spanning tree) between any two LAN segments.
Why other options are incorrect:
- Option A: While STP calculates paths based on port costs relative to a root bridge, its primary objective is loop prevention, not finding the absolute shortest path between all arbitrary pairs of LANs.
- Option C: Although STP provides fault tolerance (by unblocking redundant links if an active link fails), the core reason for using a spanning tree specifically is to eliminate loops.
- Option D: Collisions are managed at the physical and MAC layers (e.g., using CSMA/CD or full-duplex links), not by the spanning tree algorithm.
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