An ISL Not Being the Fastest Link

Over-utilization may occur when the speed of the ISL is slower than the speed of the edge links to end devices that exchange traffic via this ISL. Although the collective bandwidth after aggregating multiple ISLs may be higher than the bandwidth of an edge link, the slower operational speed of a member of an aggregated link (for example, port-channel) than the speed of the edge links causes the problem. Remember, speed and bandwidth are different. What matters here is the speed of a link. 当 ISL 的速度低于通过 ISL 交换流量的终端设备的边缘链路速度时，可能会出现过度使用的情况。虽然聚合多个 ISL 后的总带宽可能高于边缘链路的带宽，但聚合链路（如端口通道）成员的运行速度低于边缘链路的速度会导致问题。请记住，速度和带宽是不同的。这里重要的是链路的速度。

Refer to the example in Figure 2-20. The ISL has a bandwidth of 64 Gbps because of aggregating four physical links operating at 16GFC. Target-1 is an all-flash array that connects via a 32GFC link, and thus it sends frames at 32GFC speed. But Switch-2/Port-3 can send frames belonging to an I/O operation (mapped to a Fibre Channel Exchange) only via a single 16GFC link. This happens when Switch-2 uses a load-balancing scheme based on Source ID, Destination ID, and Exchange ID (the default setting on Cisco Fibre Channel switches). If there’s another I/O operation, frames belonging to it may take the other physical links in the aggregated link, but the frames belonging to an Exchange must traverse through the same physical link. 请参阅图 2-20 中的示例。由于聚合了四个以 16GFC 运行的物理链路，ISL 的带宽为 64 Gbps。目标-1 是通过 32GFC 链路连接的全闪存阵列，因此它以 32GFC 的速度发送帧。但 Switch-2/Port-3 只能通过一条 16GFC 链路发送属于 I/O 操作（映射到光纤通道交换）的帧。当 Switch-2 使用基于源 ID、目标 ID