Switchport mode Trunk

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Jan 23rd, 2008
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Hi Experts,

I would like to understand..If a gigabit port is trunk to another switch from Core Switch, and I dun wan to put the command switchport mode trunk,

what is the different ? I have tried not putting it and it is still ok.


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paolo bevilacqua Wed, 01/23/2008 - 17:23
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The port can be in "auto" mode that will cause it to go trunk as necessary.

shrikar.dange Wed, 01/23/2008 - 20:03
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What is the OS of gig interface switch?

On catos for all ethernet interfaces the default is dynamic auto and for cisco IOS it is dynamic desirable (default for all ethernet interfaces).

The dyanmic auto (in case of cat os) puts the interface in trunk mode if the neighbouring interface is set to trunk or desirable (no auto mode here).

The dynamic desirable mode (in case of NATIVE IOS) puts the interface in trunk mode if the opposite end is set to trunk,desirable OR AUTO mode.



shri :)

mohammedmahmoud Thu, 01/24/2008 - 00:11
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Hi Cindy,

First you need to check the default status of your interface using "sh interfaces g0/1 switchport" for example, which varies according to the platform, for example the 3550 uses dynamic desirable by default, while the 3560 uses dynamic auto by default.

Check the administrative and operational modes in the output of the command.

As Shri pointed out, you need to take care of the operational combination:

Desirable (start negotiation - active) > The other side can be: Trunk / Desirable / Auto

Auto (waits for negotiation - passive) > The other side can be: Trunk / Desirable.

Rack1SW1#sh interfaces f0/13 switchport

Name: Fa0/13

Switchport: Enabled

Administrative Mode: dynamic auto

Operational Mode: trunk

Administrative Trunking Encapsulation: negotiate

Operational Trunking Encapsulation: isl

Negotiation of Trunking: On

Access Mode VLAN: 1 (default)

Trunking Native Mode VLAN: 1 (default)

Administrative Native VLAN tagging: enabled

Voice VLAN: none

Administrative private-vlan host-association: none

Administrative private-vlan mapping: none

Administrative private-vlan trunk native VLAN: none

Administrative private-vlan trunk Native VLAN tagging: enabled

Administrative private-vlan trunk encapsulation: dot1q

Administrative private-vlan trunk normal VLANs: none

Administrative private-vlan trunk private VLANs: none

Operational private-vlan: none

Trunking VLANs Enabled: ALL

Pruning VLANs Enabled: 2-1001

Capture Mode Disabled

Capture VLANs Allowed: ALL


Mohammed Mahmoud.


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