PAgP: Silent vs Non silent mode

During my review for the CCNP SWITCH v1 test, I ran into a feature that doesn’t seem to logically make sense based on what I have read so far.

If you have Any Cisco SWITCH Exam books you will read about an option for PAgP configuration called [non silent]

In the Portable Command Guide by Empson the option is not even listed.

If you read the FLG  in Table 2-7 non silent mode is described as:

“If a switch is connected to a partner that is PagP-capable, configure the switch interface for non-silent operation.. The non-silent keyword is always used with the auto or desirable mode. If you do not specify non-silent with auto or desirable mode, silent is assumed. The silent setting is for connections to file servers or packet analyzer.: this setting enables PAgP to operate, to attach the interface to a channel group, and to use the interface for transmission.”

Then there is the explanation from the OCG in chapter 6 page 116:

“By default, PAgP operates in silent submode with the desirable and auto modes, and allows ports to be added to an EtherChannel even if the other end of the link is silent and never transmits PAgP packets. this might seem to go against the idea of PAgP, in which two endpoints are supposed to negotiate a channel. After all, how can two switches negotiate anything if no PAgP packets are received?

The key is in the phrase if the other end is silent. The silent submode listens for any PAgP packets from the far end, looking to negotiate a channel. If none is received, silent submode assumes that a channel should be built anyway, so no more PAgP packets are expected from the far end.”

So after reading this I ask two questions:

1. How can two endpoints negotiate if both ends are in silent mode (by default)?

2. Why connect a file server or network analyzer to an EtherChannel? What is the benefit?

 

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