The silent/non-silent settings affect how ports react to situations that cause unidirectional traffic. When a port is unable to transmit because of a failed physical interface or a broken fiber or cable, the neighbor port can still be left in an operational state. The partner continues to transmit data. But, data are lost because return traffic cannot be received. Spanning-tree loops can also form because of the unidirectional nature of the link.
Some fiber ports have the desired capability to bring the port to a nonoperational state when the port loses its receive signal (FEFI). This action causes the partner port to become nonoperational and effectively causes the ports at both ends of the link to go down.
When you use devices that transmit data (BPDUs), and you cannot detect unidirectional conditions, use non-silent mode in order to allow the ports to remain nonoperational until receive data are present and the link is verified to be bidirectional. The time that it takes PAgP to detect a unidirectional link is about 3.5 * 30 seconds = 105 sec. Thirty seconds is the time between two successive PAgP messages. Use UDLD, which is a more rapid detector of unidirectional links.
When you use devices that do not transmit any data, use silent mode. Use of silent mode forces the port to become connected and operational, regardless of whether received data are present or not. Additionally, for those ports that can detect the presence of a unidirectional condition, silent mode is used by default. Examples of these ports are newer platforms that use Layer 1 FEFI and UDLD.
