PAgP modes and EtherChannel status

[Fuzz]

I've been wondering about PAgP modes, particularly regarding silent mode. As I understand it, if the channel on one end is set to on, the other end must also be set to on because no PAgP packets are sent in this mode to negotiate a link. However, does the silent mode affect this?

If in silent mode the link should form even if it doesn't receive PAgP packets, what's the purpose of having both desirable and auto silent modes? Surely they are the same thing, other than their active/passive states. That is of course unless the link doesn't form in auto silent mode unless it receives PAgP packets, in which case, what's the point of having silent mode when using auto?

So far I've got this:

On + On = Etherchannel
On + Desirable silent = Etherchannel
On + Desirable = No Etherchannel
On + Auto silent = Etherchannel
On + Auto = No Etherchannel
Desirable + Desirable = Etherchannel
Desirable + Auto = Etherchannel
Auto silent + Auto silent = Etherchannel
Auto + Auto = No Etherchannel

Is that correct or am I missing something here?

[lildeezul]

Fuzz, I have wondered the same thing. But i guess its just a design by cisco

but the only way i can think of is, the passive/active role. If you dont want negotiation packets beings sent across the links, maybe for security reason, maybe for link ultilization, maybe company policy, or for wahtever reason.

-Marcus

[lildeezul]

Fuzz,

I reread the BCMSN chapter on etherchannel, and supposely with the silent mode, the switch doesnt expect pagp packets, but i get your questions about, what is the point if the port is in auto mode. I guess, this comes in when one port is desirable, and one is auto. and the auto device isnt capapable to send pagp packets (bcmsn) therfore the switch would use the silent to NOT expect the pagp packets.


with silent mode, which i think is the best, DOES expect PAGP packets, which can detect UDLD (unidirection link detection) so that the switch can report the unreachable device to the STP process, and ect. (BCMSN study guide)

-HTH

Marcus

[Fuzz]

Surely auto mode sends PAgP packets in response to a desirable end packet, else why would you have both silent/non-silent desirable modes?

It's all too confusing, maybe I should just accept it and stop wondering why

[lildeezul]

Originally Posted by Fuzz

It's all too confusing, maybe I should just accept it and stop wondering why

Fuzz, i not going to lie, i have came to this conclusion on various subjects. lol... cisco is complex, i just got to accept it


but for instnace, maybe your tring to etherchannel with a device that that supports etherchannel, but maybe doesnt speak it, then the switch would need to have the command silent, so that it will form a port-channel, without expecting pagp packets from the device... ( i know wierd, and horrible design, but sometimes it happens)

[Fuzz]

I get that mate, but in that case why not use 'on'. As a network engineer you'd be expected to know the capabilities of the device you're connecting to and configure the switch accordingly, right? I mean, that's why we're learning this stuff now isn't it, so we can know what configurations need to be set up, and how that will affect the rest of the network.

As long as I have understood it correctly concerning when a link will or will not form, that'a all I'm worried about. Just a bit of a head scratcher as to why there are multiple ways of doing the same thing from the same perspective!