Trunk mode and method

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Feb 22nd, 2010
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Hi,

In trunking encapsulation method 802.1q we can configure each end of trunk as per different modes available ( trunk, dynamic desirable, auto,non-negotiate ). and finaly if it matches the crieteria trunk is formed.

In case, if trunking encapsulation is ISL instead of 802.1q do we have all the above modes viz  trunk, dynamic desirable, auto,non-negotiate.

or these modes are available for 802.1 q only. I wish if i could try it on test setup but practically not possible.

Please share experience.

Thanks

Subodh

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Lei Tian Mon, 02/22/2010 - 06:41
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Hi Subodh,


You have same options with ISL trunking.


1, configure "sw mode dynamic des"

Switch(config-if)#do sh int tru


Port        Mode         Encapsulation  Status        Native vlan
Gi0/3       on           802.1q         trunking      1
Gi0/16      desirable    isl            trunking      1

2, configure "sw mo dynaic auto"

Switch(config-if)#do sh int tru


Port        Mode         Encapsulation  Status        Native vlan
Gi0/3       on           802.1q         trunking      1
Gi0/16      auto         isl            trunking      1

3, configure "sw mo trunk"

Switch(config-if)#do sh int trun


Port        Mode         Encapsulation  Status        Native vlan
Gi0/3       on           802.1q         trunking      1
Gi0/16      on           isl            trunking      1


HTH,

Lei Tian

KARUPPUCHAMY MA... Mon, 02/22/2010 - 06:43
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Hi,


when we are speaking about TRUNK, there are two words.The first one is MODES and other one is encapsulation.


In encapsulation persfective there are two protocols. One is ISL(cisco) and other one is 802.1q(open standard)


In Trunk Mode perspeftive there are 5 modes.


on  

Puts the port into permanent trunking mode and negotiates to convert the link into a trunk link. The port becomes a trunk port even if the neighboring port does not agree to the change.


off

Puts the port into permanent nontrunking mode and negotiates to convert the link into a nontrunk link. The port becomes a nontrunk port even if the neighboring port does not agree to the change.


desirable 

Makes the port actively attempt to convert the link to a trunk link. The port becomes a trunk port if the neighboring port is set to on, desirable, or auto mode.


auto  

Enables the port to convert the link to a trunk link. The port becomes a trunk port if the neighboring port is set to on or desirable mode. This is the default mode for Fast and Gigabit Ethernet ports.


nonegotiate  

Puts the port into permanent trunking mode but prevents the port from generating DTP frames. You must configure the neighboring port manually as a trunk port to establish a trunk link.



i hope this wil clear your doubt.


regards

karuppu

Giuseppe Larosa Mon, 02/22/2010 - 06:51
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Hello Subodh,


the various states dynamic desirable, auto refers to DTP, dynamic trunking protocol, role of the switch port.


So all of them applies to either 802.1Q or ISL trunking protocol, actually the protocol to be used can be part of the negotiation



Hope to help

Giuseppe

Ganesh Hariharan Mon, 02/22/2010 - 06:59
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Hi,

In trunking encapsulation method 802.1q we can configure each end of trunk as per different modes available ( trunk, dynamic desirable, auto,non-negotiate ). and finaly if it matches the crieteria trunk is formed.

In case, if trunking encapsulation is ISL instead of 802.1q do we have all the above modes viz  trunk, dynamic desirable, auto,non-negotiate.

or these modes are available for 802.1 q only. I wish if i could try it on test setup but practically not possible.

Please share experience.

Thanks

Subodh

Hi Subodh,


The modes are genrally come in picture when ever you want to make trunk between ports by using trunk mechanism either 802.1q or ISL.


Folowing are the modes which are configured to make the link as trunk link

on   Forces the link into permanent trunking, even if the neighbor doesn't agree

off   Forces the link to permanently not trunk, even if the neighbor doesn't agree

desirable Causes the port to actively attempt to become a trunk, subject to neighbor agreement (neighbor set to on, desirable, or auto )

auto  Causes the port to passively be willing to convert to trunking. The port will not trunk unless the neighbor is set to on or desirable . This is the default mode Note that auto-auto (both ends default) links will not become trunks.

nonegotiate Forces the port to permanently trunk but not send DTP frames. For use when the DTP frames confuse the neighboring (non-Cisco) 802.1q switch. You must manually set the neighboring switch to trunking.


check out the below link for more information hopt to help


http://faculty.valenciacc.edu/wyousif/CCNA/Semester%203%20Presentations/Student%20Resources/DTP.pdf


Ganesh.H

bapatsubodh Mon, 02/22/2010 - 08:35
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Hi,

I was wondering if all the modes auto,desirable, on , non-nego as available even if we encap. the frame in ISL instead of .1q.

And as pointed out yes all those are available.

Thanks

subodh

francisco_1 Mon, 02/22/2010 - 09:01
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The states below whether you are using ISL or 802.1Q


This list describes the five different states for which you can configure DTP:

  • auto: The port listens for DTP frames from the neighbor switch. If the neighbor switch indicates that the switch would like to be a trunk, or that the switch is a trunk, auto state creates the trunk with the neighbor switch. The auto state does not propagate any intent to become a trunk; auto state is solely dependent on the neighbor switch to make the trunking decision.

  • desirable: DTP is spoken to the neighbor switch to which you want to establish an ISL trunk. The switch with desirable configuration communicates that the switch is able to be an ISL trunk and wants the neighbor switch to also be an ISL trunk.

  • on: DTP is spoken to the neighbor switch. The on state automatically enables ISL trunking on the port, regardless of the state of the neighbor switch. The port remains an ISL trunk unless the port receives an ISL packet that explicitly disables the ISL trunk.

  • nonegotiate: DTP is not spoken to the neighbor switch. The nonegotiate state automatically enables ISL trunking on the port, regardless of the state of the neighbor switch.

  • off: There can be no use of ISL on this port, regardless of the configuration of DTP mode on the other switch port.

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