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Switch Trunk

Posted on 2014-11-16
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Last Modified: 2014-11-22
If I understand a Trunk link can be formed between 2 Switches if the ports are:

Trunk-Trunk
Trunk-Auto,Desirable
Auto,Desirable-Auto-Desirable

However sometimes , in addition to configuring the port as Trunk, I have seen the command Nonegotiate added.

ports cannot also form a Trunk when they are in 2 different Domains.

I wonder if there is a list that covers all the situations and combinations when 2 switch ports can form a Trunk and when they cannot ?

Thanks
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Question by:jskfan
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Expert Comment

by:Don Johnston
ID: 40446485
Not sure if I'm interpreting your post correctly, but auto-auto will not result in a trunk.
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Author Comment

by:jskfan
ID: 40447470
Don,
I know Auto - Auto does not form Trunk
Sometimes they use Switchport Nonegotiate, I wonder what impact does it have in forming a Trunk
Switchport Nonegotiate can be used in conjunction with Switchport mode Access and Switchport mode Trunk.
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Assisted Solution

by:Predrag Jovic
Predrag Jovic earned 250 total points
ID: 40447548
Switchport nonegotiate disables DTP (dynamic trunk negotiation), so you can still form trunk if on other switch ports is set to trunk (with or without nonegotiate), but port won't do negotiation on that port.
I dont see any point using it with switchport mode access.
And also, if you put switchport nonegotiate you can form working trunk between different VTP domains, since trunk is hard coded.
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Author Comment

by:jskfan
ID: 40447718
I do not understand the usage of trunk nonegotiate other than disabling DTP. what 's the impact of disabling DTP ?

So Trunk will still form regardless if nonegotiate is typed or not as long as you type:
Switch mode Trunk
Which means the Trunk is ON
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Assisted Solution

by:Predrag Jovic
Predrag Jovic earned 250 total points
ID: 40447771
switchport mode trunk still send DTP frames although it is in trunk mode.
switchport nonegotiate prevents port from sending DTP frames.

First case (without nonegotiate)
switchport mode trunk will be still able negotiate trunk with other side if other side is configured as auto or dynamic desirable.
Second case (with nonegotiate)
If switchport nonegotiate is issued on port configured with switchport mode trunk command and other side is configured as auto or dynamic desirable, one side will be trunk and other side will be access port.
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Author Comment

by:jskfan
ID: 40458896
If switchport nonegotiate is issued on port configured with switchport mode trunk command and other side is configured as auto or dynamic desirable, one side will be trunk and other side will be access port.

Which means the Trunk will not form ?
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Assisted Solution

by:Don Johnston
Don Johnston earned 250 total points
ID: 40458922
That is correct. There will not be a trunk between the two switches.  However, that does not mean that traffic can't flow.

One side will treat the link as a trunk and tag all VLANs except the native VLAN.  The other side will treat the link as an access link and send/receive frames for the configured VLAN.

Which means that the native VLAN on the trunking side will be able to communicate with the access VLAN on the other.
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Author Comment

by:jskfan
ID: 40459457
SO that 's what I concluded, if I am not wrong :
The Trunk will form if both sides are :
-Trunk-Trunk, Trunk-Auto, Desirable-Auto, Desirable-Desirable. Trunk-Desirable.
-if both side are Auto-Auto, the trunk will not form.
-If one port on one side is configured  with Switchport mode Trunk/NoNegotiate command and the other side with Switchport mode Trunk, This will form a Trunk.
- If one port on one side is configured  with Switchport mode Trunk/NoNegotiate command and the other side with Switchport mode Trunk/Nonegotiate, This will
form a Trunk.
-If one port on one side is configured  with Switchport mode Trunk/NoNegotiate command and the other side with Dynamic Desirable, This will Not form a Trunk.
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Accepted Solution

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Don Johnston earned 250 total points
ID: 40459459
You are correct on all counts.
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Author Closing Comment

by:jskfan
ID: 40459523
Thank you Guys!
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