Pagp and silent mode

Question

Hello,
I am confused with the "silent" mode when used with PAGP "auto" and "desirable" modes.
"CCNP BCMSN exam certification guide" book, says that "silent" is used when peer is not capable of sending Pagp frames (does not participate in pagp), so that an etherchannel is built in our side anyway.
But everywhere(cisco's site,book), is reffered that if one peer is in "auto" mode and other in "on" mode OR one peer is in "desirable" and other in "on" mode, a channel is not formed.
Since the default submode for both - "desirable" and "auto" - is "silent"( meaning that our side does not expect any pagp frames) why a channel can not been established?
Thanks.

Answer 1

Hello Bob Grante,
I agree with you that the documentation is very confusing and vague about PaGP negotiation.
From what I understand an EtherChannel will form only if one side is configured to be non-silent. I cannot confirm this because I do not have access to a etherchannel capable switch at this time.
I quote the 3550 Software Configuration Guide 12.2(25)SE.
"Silent Mode" (Page 31-5)
"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."
The configuration examples te
"Configuring EtherChannels (Page 31-10)
"non-silent -- If your switch is connected to a partner that is
PAgP-capable, you can configure the switch interface for
nonsilent operation. You can configure an interface with the
non-silent keyword for use with the auto or desirable mode.
If you do not specify non-silent with the auto or desirable
mode, silent is assumed. The silent setting is for connections
to file servers or packet analyzers. This setting allows PAgP
to operate, to attach the interface to a channel group, and to
use the interface for transmission."
I also would like to quote Brian McGahan, CCIE #8593
"The description of silent and non-silent is hard to extrapolate from the brief description on CCO, but this is what it means. When in "silent" mode, the switch will generate (if in "desirable" mode) PAgP and respond to PAgP (if in "auto" mode) regardless of whether the link is "silent", meaning whether or not you are recieving traffic from your neighbor.
When in "non-silent" mode, the switch will only initiate or respond to a PAgP request if the link is "non-silent", meaning that you are receiving traffic in the interface.
Why this is even an option I'm not really sure, but by default the switch will run PAgP regardless of whether or not traffic is being received in the interface." -- http://forum.internetworkexpert.com/ubbthreads/showflat.php?Cat=&Number=3346&Main=3256
I think the only way to find out is to test it yourself in a lab environment. I will keep this in mind for future testing.
Best regards,
Leon