EtherChannel in Silent mode - What is correct?

Unanswered Question
Sep 13th, 2010

According to the Cisco Book:

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 allows a switch to form an EtherChannel with a device such as a file server or a network analyzer that doesn’t participate in PAgP
Are the following tips really correct?
(Shouldn't we add: "Except if the channel is in PAgP/silent mode"? )

■ EtherChannel on mode does not send or receive PAgP or LACP packets. Therefore, both ends should be set to on mode before the channel can form except....

EtherChannel desirable (PAgP) or active (LACP) mode attempts to ask the far end to bring up a channel. Therefore, the other end must be set to either desirable or auto mode except....

EtherChannel auto (PAgP) or passive (LACP) mode participates in the channel protocol, but only if the far end asks for participation. Therefore, two switches in the auto or passive mode will not form an EtherChannel except....

Because EtherChannel/PAgp can comes up even if the other end of the link is silent
What do you think?

I have this problem too.
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Nagaraja Thanthry Mon, 09/13/2010 - 06:10


You are correct in that the PAgP etherchannel will come up in certain cases

when you use the silent keyword along with "auto" or "desirable" keywords.

However, when you set the channel mode to "ON", you are configuring a layer

2 only etherchannel without any protocols managing the etherchannel. Hence

the PAgP does not apply here. When you are setting the LACP features, again,

since PAgP has no significance here, the "silent" feature is not required

here as well. In both "ON" mode and "desirable/active (LACP)" mode, we do

not need to talk about the silent feature. Only in the PAgP specific line,

you can mention "the except" clause.



Jeetiz95110 Tue, 09/14/2010 - 02:39

Thanks for you answer.

So by default and according to the "defaut/silent mode", this combination could works?

PAgP/ON <==> PAgP/desirable: OK (it work because "desirable" or "auto" mode don't expect PAgP negociation packet in silent mode)

PAgP/ON <==> PAgP/auto: OK (same..)

PAgP/desirable <==> PAgP/auto: OK

PAgP/auto <==> PAgP/auto: OK

If it's true, this is in conflict with the Cisco rule wich saying:

  • Only the combination auto-desirable, desirable-desirable and on-on will allow a channe l to be formed.


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