etherchannel,pagp mode desirable and on

Answered Question
Jul 18th, 2009
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Hi Every body!


please consider the following config:


Sw1

interface range f0/1-12

channel group 1 mode desirable


Sw2

interface range f0/1-12

channel group 1 mode on


Will etherchannel form?

My book says the mode " on" disable the pagp / lacp negottation.

Based on the above , my reasoning is as follows:

Because of mode" desirable" sw1 actively asks sw2 to form etherchannel, sw1 does not send any response as pagp is disabled. Since sw1 does not receive any response from sw2, therefore sw1 can not form etherchannel.

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thanks have a nice weekend!

Correct Answer by Edison Ortiz about 7 years 9 months ago

No it will not form.


Valid combinations:


PAgP:


Desirable <-> Desirable

Desirable <-> Auto


LACP:


Actice <-> Active

Active <-> Passive


Unconditional Etherchannel:


On <-> On


http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_50_se/configuration/guide/swethchl.html


Correct Answer by Istvan_Rabai about 7 years 9 months ago

Hi Sarah,


When the Etherchannel is configured in PAgP desirable mode, by default it operates in "silent" submode.

"Silent" submode means that the ports will be added to the Etherchannel even if the other end of the link is "silent" , i.e. it does not transmit PAgP packets.

In "silent" submode, Sw1 will wait 15 seconds to hear inbound PAgP packets, and if no PAgP packets arrive, it adds the ports to the Etherchannel anyway.


If the command you use is the following:

channel group 1 mode desirable non-silent


then the switch expects PAgP packets from the other end on each port and will NOT add the ports to the Etherchannel without actual negotiations.


Cheers:

Istvan

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Correct Answer
Istvan_Rabai Sat, 07/18/2009 - 10:31
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Hi Sarah,


When the Etherchannel is configured in PAgP desirable mode, by default it operates in "silent" submode.

"Silent" submode means that the ports will be added to the Etherchannel even if the other end of the link is "silent" , i.e. it does not transmit PAgP packets.

In "silent" submode, Sw1 will wait 15 seconds to hear inbound PAgP packets, and if no PAgP packets arrive, it adds the ports to the Etherchannel anyway.


If the command you use is the following:

channel group 1 mode desirable non-silent


then the switch expects PAgP packets from the other end on each port and will NOT add the ports to the Etherchannel without actual negotiations.


Cheers:

Istvan

sarahr202 Sat, 07/18/2009 - 12:08
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Thanks Istvan.


Based on your response, two switches configured with " channel-group 1 mode auto silent, should form etherchannel.

Why?

Because each switch ,because of option " silent " will wait for 15 seconds for inbound pagp packets if none received, switch will form etherchannel anyway. Since both switches are configured with " silent" option, both switches will form etherchannel after 15 seconds even if no pagp packet is received from each other.

Could you explian this please?


Here is my concept:


pagp can also help detect uni directional problem link which is commonly encountered in fiber links. In electrical link such as copper, if tranceiver is faulty say transmitter is not up , it will cause the other port to go down as well. The end result is link will go down if one of the tranceivers on the link is faulty.

However in fiber link, it is possible for a link to be up even if one of the tranceiver is down( say transmitter is not working)which results link being uni directional.

Pagp can help detect unidirectional link by " nonsilent" option.


If i configure the switch as:

channel-group 1 mode dynamic desirable nonsilent.


Then above command instructs the switch first check the if the link is unidirectional or not. Check the link for unidirectional by waiting for any traffic such as bpdu, pagp frames on RX( receiver). If none is received, do not include this in etherchannel.

Here "nonsilent" will help detect the uni directional link before the link can be included in bundle( etherchannel).


The option" silent" mode does not provide such a check. If a switch is configured with" channel-group 1 mode desirable silent" , the switch will not wait for any packet be received on RX first before that link can be included in bundle , rather it will act according to what option ( auto,desirable) is configured. In our case , we configured the command " channel-group 1 mode desirable silent", so the switch will actively asks the other end to form a etherchannel.

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If i consider your point :

"In "silent" submode, Sw1 will wait 15 seconds to hear inbound PAgP packets, and if no PAgP packets arrive, it adds the ports to the Etherchannel anyway."


Then i expect two switches configured with " channel-group 1 mode auto silent", to form etherchannel. Because after 15 seconds, these switches should be able to form etherchannel.

But we know they will not form etherchannel.


Sorry to for the long- winded reply.


You have a nice weekend!




Istvan_Rabai Sat, 07/18/2009 - 12:39
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Hi Sarah,


I don't have switches at hand this time, but I would suggest you to try it on real switches.


I may have mistaken with this and Edison may be right.

If so, in this case I have to eat my words.


But real switches will tell you the exact answer to this.


Thanks:

Istvan

sarahr202 Sat, 07/18/2009 - 13:19
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Thanks Istvan.

I don't have real switches either. All i got are few books and Cisco net pro.



thanks and have a nice day!

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