Circumstances for Transmission of Multiple Copies of Unicast Frames in a Switched LAN

B

In a switched LAN environment, unicast frames are transmitted only to the intended recipient's port, thus avoiding unnecessary network congestion. However, multiple copies of the same unicast frame can be transmitted in certain circumstances, such as:

A. After broken links are re-established: When a link between two switches goes down and then comes back up, the Spanning Tree Protocol (STP) is used to prevent loops in the network by blocking redundant paths. When the link comes back up, STP reconverges and unblocks the previously blocked redundant path, which can result in multiple copies of the same unicast frame being transmitted.

B. In an improperly implemented redundant topology: When a network is designed with redundant links, the goal is to provide alternate paths in case of link failures. However, improper implementation can lead to loops in the network, which can cause multiple copies of the same unicast frame to be transmitted.

C. When upper-layer protocols require high reliability: Some upper-layer protocols, such as certain database or file transfer protocols, require high reliability and may implement their own mechanisms to ensure delivery. These mechanisms can result in multiple copies of the same unicast frame being transmitted.

D. During high traffic periods: When the traffic load on a switch is high, the switch may use multiple paths to transmit the same unicast frame, resulting in multiple copies of the same frame being transmitted.

E. When a dual ring topology is in use: In a dual ring topology, unicast frames are transmitted in both directions to ensure redundancy. However, this can result in multiple copies of the same unicast frame being transmitted.

In summary, multiple copies of the same unicast frame can be transmitted in a switched LAN under certain circumstances, such as after broken links are re-established, in an improperly implemented redundant topology, when upper-layer protocols require high reliability, during high traffic periods, or when a dual ring topology is in use. These situations can lead to network congestion, increased latency, and other performance issues. Proper network design and configuration can help prevent these issues.