Credit Loss Recovery Using Link Reset Protocol
An FC port invokes the Link Reset protocol (Figure 2-43) to attempt to recover the credits when it has zero remaining-Tx-B2B-credits for an extended duration. The Fibre Channel standard refers to this duration as Error Detect Timeout Value (E_D_TOV). This duration must be long enough for a port to give up on the normal mechanism of the return of B2B credits. Cisco MDS switches set E_D_TOV to 2 seconds by default. However, for initiating Link Reset Protocol MDS switches use the default values of 1.5 seconds for an E_Port and 1 second for a F_Port. 当 FC 端口的剩余 Tx-B2B 信元为零且持续时间较长时，FC 端口会调用链路重置协议（图 2-43）尝试恢复信元。光纤通道标准将这一持续时间称为错误检测超时值（E_D_TOV）。该持续时间必须足够长，以便端口放弃 B2B 信元返回的正常机制。Cisco MDS 交换机默认将 E_D_TOV 设置为 2 秒。但是，对于启动链路重置协议，MDS 交换机使用 E_Port 1.5 秒和 F_Port 1 秒的默认值。
Figure 2-43 Credit loss recovery by Link Reset protocol
Note:提示
Cisco MDS switches call this ‘Credit Loss Recovery’. Cisco MDS 交换机称之为 "信元损失恢复"。
The Link Reset Protocol starts when an FC port sends a Link Reset (LR) primitive sequence. The neighbor, after receiving LR, returns a Link Reset Response (LRR) primitive sequence. Before returning LRR, the neighbor processes the frames in its ingress buffers to make the buffers available for re-use. If, however, it’s unable to process the frames, the neighbor may drop the frames before sending LRR or not send an LRR at all resulting in a link failure. 当 FC 端口发送链路重置（LR）原始序列时，链路重置协议开始。邻居收到 LR 后，会返回一个链路重置响应 (LRR) 原始序列。在返回 LRR 之前，邻居会处理其入口缓冲区中的帧，以使缓冲区可重新使用。但是，如果邻居无法处理这些帧，它可能会在发送 LRR 之前丢弃这些帧，或者根本不发送 LRR，从而导致链路故障。
When Link Reset Protocol completes, both FC ports have reset their remaining Tx and Rx B2B credits to the same value as they had learned from each other during link initialization (FLOGI or ELP). 当链路重置协议完成时，两个 FC 端口已将其剩余的 Tx 和 Rx B2B 信元重置为与它们在链路初始化（FLOGI 或 ELP