What is the technical reason for PaGP Silent mode and why is it a default?

Question

So I have done a little reading around PaGP silent mode, and I don't understand why it exists. My understanding is from:

The silent mode is used when the switch is connected to a device that is not PAgP-capable and seldom, if ever, sends packets. An example of a silent partner is a file server or a packet analyzer that is not generating traffic. In this case, running PAgP on a physical port connected to a silent partner prevents that switch port from ever becoming operational; however, the silent setting allows PAgP to operate, to attach the interface to a channel group, and to use the interface for transmission. http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3550/software/release/12-1_13_ea1/configuration/guide/3550scg/swethchl.html#wp1028480

If you know the partner device is never going to negotiate why not just force the channel with no negotiation protocol (neither PaGP or LACP) and just set it to on?

Also I tried to test this with the following:

Switch 1           Switch 2               Results
auto (silent)      auto (silent)          ports suspended
auto (silent)      on                     sw1 ports suspended
des  (silent)      on                     sw1 ports suspended

Am I fundamentally missing something here?

Answer 1

PAgP silent permits the channel to be formed, but PAgP packets can still be interpreted by the device that is set to silent. Imagine the case with some devices that do support PAgP but may not enable it on a "secondary" LAG, but you still want this LAG to be online in the event of a failure so you can switch over to it.

You could force the channel on, which would have the same function in terms of getting the channel up but may have other side effects. In networks with spanning tree configured, PAgP silent will also prevent spanning tree from reporting the port to be down. Since LACP/PAGP frames are normally received only over one link in the bundle, if they are not sent or received on a port configured in a bundle and set to "on" then the port will be marked as down.

See these two links.

It is important to understand that there are a lot of other features such as loop guard and UDLD that may cause your ports to be suspended.