• A Switch is permitted to maintain a local PTM context for use in fulfilling PTM Requests received on its Downstream Ports. • A Switch which is not acting as a PTM Root must invalidate its local context no more than 10 ms from the last PTM dialog on its Upstream Port. The Switch must then refresh its local PTM context prior to issuing further PTM ResponseD Messages on its Downstream Ports. This requirement for periodic refreshes is optional if it is guaranteed by implementation-specific means that the Switch’s local clock is phase locked with PTM Master Time. • Any Switch implementing a local clock for the purpose of maintaining a local PTM context must report the granularity of this clock as defined in the PTM Capabilities structure (Section 7.9.16 ). 翻译

• 交换机可以维护本地 PTM 上下文，用于满足其下行端口接收的 PTM 请求。
• 不作为 PTM 根的交换机必须在其上行端口上的最后一次 PTM 对话后不超过 10 毫秒无效化其本地上下文。然后，交换机必须在其下行端口上发出进一步的 PTM ResponseD 消息之前刷新其本地 PTM 上下文。如果实现特定的方法保证了交换机的本地时钟与 PTM 主时间相位锁定，则可以选择不定期刷新的要求。
• 为了维护本地 PTM 上下文而实现本地时钟的任何交换机都必须将该时钟的粒度报告为 PTM 能力结构（第 7.9.16 节）中定义的粒度。