How to configure trunk on Cisco Catalyst Switch

Lessons Discussion
108

Hello Hemant

If you want to find out more about how trunk ports and access ports are configured on a switch, take a look at this lesson:

There you will find the various options for dynamic desirable and dynamic auto. These configurations are part of what is know as Dynamic Trunking Protocol (DTP) which is the protocol that operates this negotiation feature.

Some more useful links for you include:

If you need any further help on more specific scenarios, please let us know!

I hope this has been helpful!

Laz

109

Hello Laz!

Some questions I had: Towards the end of this lesson, on SW1, we configured int Fa0/14 as a static access port, but the lesson provides the following output after that:

SW1#show interfaces f0/14 switchport
Name: Fa0/14
Switchport: Enabled
Administrative Mode: static access
Operational Mode: trunk

Why does the operational mode say trunk if we configured it as a static access port using the switchport mode access command? It should be in access mode correct?

Second question, if we configure a link connecting two switches as manual trunk ports using the switchport mode trunk command, but dont apply the nonegotiate command to turn of DTP, would this affect the switchports at all? I believe they will remain as trunk ports even if both sides are still sending DTP frames since we manually made them trunks, is this correct?

Finally, the lesson mentions that “Dynamic auto will prefer to become an access port, but if the other interface has been configured as a trunk we will end up with a trunk”, however wouldnt this scenario only apply if the trunk port in question is sending DTP frames? Because if we turn off auto negotiation, and no DTP frames are hitting the switchport configured as dynamic auto, then it does indeed run as an access port right?

Thank you Laz!

110

Hello Paul

The administrative mode is what we have manually configured on the port itself. The operational mode is the actual mode that the port is in as a result of negotiation or additional factors. More about these differences between the two can be found in this NetworkLessons note.

Now having said that, there is no situation in which you would have an administrative mode of “static access” and an operation mode of “trunk”. If you statically configure the port to be an access port, then the operational mode should be access. So you are indeed correct, and this is likely a typo, that I will let Rene know about. For comparison, an example of a possible combination could be an administrative mode of “dynamic auto”, and an operation mode of “access”.

Yes, you are correct. To find out how to actually disable DTP, take a look at this NetworkLessons note.

Yes, that is correct as well. If DTP is disabled, then this will not happen. Possibly a better wording would be:

Dynamic auto will prefer to become an access port, but if the other interface has been configured as a trunk (and DTP has not been disabled), we will end up with a trunk.

I hope this has been helpful!

Laz

111

Hello -
Following along using Packet Tracer with two 2950-24’s, I never see the Ports listed like in your “show vlan” screenshot. What am I doing wrong?

CPT Show vlan
CPT Show vlan610×526 9.49 KB

112

Hello Robert

I tried recreating your scenario in my Packet Tracer, but I don’t get the same results as you:

image

I get the same results as the lesson.

Remember, only access ports will appear in the output of the show vlan command.
If Fa0/1 doesn’t appear, then that means that the port has been configured as a trunk. I suspect that’s what happened, but take a look to confirm this. I also notice that your Fa0/2 doesn’t appear in the output, have you configured both of those interfaces as trunks? Check it out and let us know.

If you have further problems, let us know and we’ll do our best to help you out.

I hope this has been helpful!

Laz

113

As i tested this in LAB with two PC behing switches and not able to ping each other.

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