What's the difference between silent & non-silent in PAgP

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Jun 25th, 2007
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Hello!


I am puzzled about silent & nonsilent when configuring PAgP. What is the difference, please?

"desirable" & "desirable non-silent"

"auto" & "auto non-silent"


I have looked up several stuff/guides, but still not clearly understand it.


Thank you!

Correct Answer by Edison Ortiz about 10 years 3 days ago

I agree it's a bit confusing, but my understanding is that when silent mode is used, it allows PAgP to operate when the neighbor device isn't PAgP capable.


If you have non-silent setting, it will send PAgP packets and it won't form the etherchannel until it receives a reply from the neighbor device.


As for using mode on, Yes - this should be the ideal mode for link partners that do not support PAgP nor LACP.


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Edison Ortiz Mon, 06/25/2007 - 18:24
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Switch ports exchange PAgP packets only with partner ports configured in the auto or desirable modes. Ports configured in the on mode do not exchange PAgP packets.


Both the auto and desirable modes enable ports to negotiate with partner ports to form an EtherChannel based on criteria such as port speed and, for Layer 2 EtherChannels, trunking state and VLAN numbers.


Ports can form an EtherChannel when they are in different PAgP modes as long as the modes are compatible. For example:


?A port in the desirable mode can form an EtherChannel with another port that is in the desirable or auto mode.


?A port in the auto mode can form an EtherChannel with another port in the desirable mode.


A port in the auto mode cannot form an EtherChannel with another port that is also in the auto mode because neither port starts PAgP negotiation.


If your switch is connected to a partner that is PAgP-capable, you can configure the switch port for nonsilent operation by using the non-silent keyword. If you do not specify non-silent with the auto or desirable mode, silent mode is assumed.


Use the silent mode 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 port to a channel group, and to use the port for transmission.


http://www.cisco.com/univercd/cc/td/doc/product/lan/cat3560/12225see/scg/swethchl.htm



taoleiccie Tue, 06/26/2007 - 17:09
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Thank you, but I still don't quite understand.


I understand this:

=============================================

In this case, running PAgP on a physical port connected to a silent partner prevents that switch port from ever becoming operational.

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This means the partner cannot negotiate using PAgP, so the EthernetChannel cannot established.


But I don't quite understand this:

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However, the silent setting allows PAgP to operate, to attach the port to a channel group, and to use the port for transmission.

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If so, when using "silent" setting, the "auto" & "desirable" are the same, aren't they?

Further, why don't we use "trunk on" to achieve this?

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Edison Ortiz Tue, 06/26/2007 - 18:02
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I agree it's a bit confusing, but my understanding is that when silent mode is used, it allows PAgP to operate when the neighbor device isn't PAgP capable.


If you have non-silent setting, it will send PAgP packets and it won't form the etherchannel until it receives a reply from the neighbor device.


As for using mode on, Yes - this should be the ideal mode for link partners that do not support PAgP nor LACP.


taoleiccie Tue, 06/26/2007 - 20:00
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In my previous post, I typed "trunk on", it is a clerical error. I meant "mode on". Sorry.


It seems that even if "auto (silent)" on both sides, the etherchannel won't be established, but links within that channelgroup can forward traffic; on the other hand, if both sides are in "auto non-silent", neither channelgroup nor links can be used to forward traffic.

glen.grant Wed, 06/27/2007 - 06:07
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that is correct if both sides are auto it will not come up . The sides have to both be desirable or one side has to be desirable and the other side can be auto . Auto on both sides is not a valid configuration . If you have cisco devices on each end the best config will be to set them as "desirable non-silent" .

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