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etherchannel issues 3750

Unanswered Question
Jan 24th, 2006
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Hello

what are the main reason's not to use mode "on" instead of the other options like active, passive or desirable?

Are there problems with distributing the traffic if one link goes donw?

best regeards

Oliver


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arvindchari Tue, 01/24/2006 - 02:34
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I will try to explain to my best knowledge


1. " what are the main reason's not to use mode "on" instead of the other options like active, passive or desirable? "


the various options such as on / auto / passive / desirable affect the formation of the etherchannel itself


the possible combinations which can be used to form an etherchannel successfuly are:


desirable + auto = successful etherchannel formation (auto = initiate etherchannel discovery and desirable = attempt to find initiating packets on the current port (s) )

on + on = successful etherchannel formation (on = always form etherchannel)


My guess is that on + on would elimitate the slight delay that occurs while negotiation of packets.


as far as i know passive is only applicable for etherchannels that use LACP (default is PAgP). passive puts the port into a state where it only forms and etherchannel if it recieves and LACP packet.



2. "Are there problems with distributing the traffic if one link goes donw? "


If one of the links in the bundle fail, there are no other issues other than the temporary loss of bandwidth. Etherchannels dynamically reconfigure themselves to adjust to that (i.e adjust to both the interface shutting down and coming back up)


One question that I do have is regarding uni directional links inside etherchannels and what could be the consequences of the same. Any inputs would be appreciated?




HTH

Arvind

twojciac Tue, 01/24/2006 - 09:45
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If your hardware supports PAgP, definitely use desirable mode on all interfaces you wish to channel. When you use the on mode, it will attempt to unconditionally channel instead of negotiating the channel. The result depends on what code you are running and what features you have enabled.


It's possible that BPDUs can get lost down these unidirectional paths and in turn 802.1d will unblock a link that should be blocking. This can create a loop in the network where any broadcast traffic traversing the "good" link in the etherchannel will melt the network.


Here's a good reference with more detail:

http://www.cisco.com/en/US/tech/tk389/tk213/technologies_tech_note09186a008009448d.shtml

arvindchari Tue, 01/24/2006 - 20:07
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Thanks for that excellent article!

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