This is a pretty good explanation of what it does, out of a cisco doc.
Silent/Non-Silent Mode
When you deal with fiber connections, there is a possibility that, even if a receive (Rx) transceiver dies, the transmit (Tx) transceiver on the other end is still up. In a similar scenario, packets can get black holed.
It is important for the switch that transmits to remove this port from the EtherChannel bundle. In order to do so on the Catalyst 5500/5000, you set PAgP in non-silent mode. Non-silent mode means that, if the Rx does not receive traffic, the port is not put into the channel. However, use of non-silent mode is not enough because this detection happens only when the channel is formed.
In order to prevent the black holing of traffic when the channel is already formed, this occurs:
1.
PAgP detects that the Rx port does not receive any traffic.
2.
PAgP resets the Tx transceiver of the port that does not receive traffic. PAgP resets it for 1.6 seconds so that the switch on the other end also resets the port.
3.
The faulty port does not join the channel anymore because no traffic is received on that port.
On the Catalyst 5500/5000, set non-silent mode on fiber strands and silent mode on copper strands. This is both the default and recommended setting because, on fiber connections on the Catalyst 5500/5000, the negotiation is usually not available, so there is no way to detect the problem at a physical layer.
Default PAgP Settings on the Catalyst 4500/4000 and 5500/5000
By default, PAgP is auto for a plug-and-play implementation. Disable PAgP manually from the ports where there is no need to have it.
By default, the silent mode is on. Non-silent is acceptable as well. However, because a port can be connected to a device that does not send traffic (for example, a sniffer), it is more general to have silent enabled.
Recommendations
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Use the non-silent keyword when you connect to a device that transmits bridge protocol data units (BPDUs) or other traffic. Use this keyword with the auto or desirable mode. PAgP non-silent adds an extra level of link state detection because it listens for BPDUs or other traffic in order to determine if the link functions properly. This adds a form of UniDirectional Link Detection (UDLD) capability that is not available when you use the default silent PAgP mode.
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Use the silent keyword when you connect to a silent partner (which is a device that does not generate BPDUs or other traffic). An example of a silent partner is a traffic generator that does not transmit packets. Use the silent keyword with auto or desirable mode. If you do not specify silent or non-silent, silent is assumed.
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The silent mode does not disable the PAgP ability to detect unidirectional links. However, when you configure a channel, non-silent prevents a unidirectional port from even joining the link.
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A PAgP configuration (the set port channel {desirable | auto} command) is safer than a non-PAgP configuration (the set port channel on command). A PAgP configuration provides protection for unidirectional links and also avoids misconfigurations that can arise when there are ports channeling on one side of the link and not on the other side.
