Suppose that for a given number of buffer credits and for an given average FC frame size, the maximum FC link length is 50 km. If the average FC frame size is doubled, which of the following is correct?

The initial configuration has the exact number of buffer credits needed for a 50 km link with the original frame size. When the average frame size is doubled, the number of credits required to keep the link saturated is halved. Consequently, the original credit allocation is now excessive. Buffer credits are a finite, shared resource within a switch ASIC. Over-allocating credits to one port unnecessarily consumes this limited resource, potentially preventing other ports on the same ASIC from obtaining the credits they need to achieve their own maximum throughput over their respective distances. Therefore, to optimize the switch's overall performance and achieve maximum aggregate throughput, the credits on this link need to be adjusted down to the new, lower requirement.

A. The throughput is halved. Incorrect. The link has more than enough credits to remain fully saturated, so throughput is maintained at the line rate, not halved.
B. Throughput is doubled. Incorrect. Throughput cannot exceed the physical line rate of the Fibre Channel link.
C. The Throughput is the same. Incorrect. While true for this single link in isolation, this answer is incomplete. It ignores the critical network design principle of efficient resource management for the entire system (the switch).

1. Nokia 7750 SR OS Quality of Service Guide, Release 21.10.R1, Section: Buffer Pools. This document establishes the principle of finite, shared buffer resources on Nokia networking hardware. It states, "Each forwarding complex has a single large buffer pool that is available to all the queues on all the ports of the complex." This supports the reasoning that over-allocation on one port impacts the entire system. (Document ID: 3HE 17275 AAAB TQZZA, Edition 01, Page 105).

2. Kembel, R. (2000). Fibre Channel: A Comprehensive Introduction. "Chapter 10: Flow Control." This foundational text explains that buffer credits are a finite resource managed by the switch. The number of credits configured for a port is drawn from a pool available to a group of ports or the entire switch, making efficient allocation critical for overall fabric performance. (Specific reference to buffer pools on switch ASICs).

3. Brocade (2012). Buffer-to-Buffer Credit Management Technical Brief. While from another vendor, this document details the universal principle of FC switch architecture relevant here. It states: "Buffer credits are a finite resource within a switch that are shared by all ports on an ASIC... For this reason, it is important to allocate only the number of buffer credits required for a given distance." This directly supports the rationale for answer D. (Page 2, Paragraph 2).