CCNA Blog

# 01 ldgotz

I am presently studing to my ccna exame..

One of the videos that I watched was and example of setting up a trunk link

the discussion talked the definition of a trunk being..

 A link that passes all vlan trafic …
then they continued with the tutorial of setting up this trunk link..
and it ended with them assigning a Vlan to the trunk link…

If the trunk passes all vlans.. why would you assign a vlan to it? after a day of searching the web, with no real luck for an answer

other then   ==> best practice..
I came here to ask the question….

Is there a real reason to assign a vlan to a trunk link?

# 02 Chris Buscemi

Because it is a trunk link and is designed to pass all VLAN traffic to the next switch you don’t (actually I believe you can’t) assign that port to a VLAN. The only time you assign any VLAN configuration on a trunk link (aside from manual pruning) is when configuring router-on-a-stick. And the VLAN configuration goes on the router side of the trunk under the subinterface, not on the switch port.

# 03 Conwyn

Normally trunks carry all the vlans  but in voice network you often have two trunks one carry vlan traffic for data and the other carrying the voice vlan traffic. You can also prune so you do not send data for vlan x down a trunk when you know there are no vlan x ports beyond the trunk.

# 04 ldgotz

I understand this concept…
BUT why would you assign a vlan to a trunk…..
yes it can be done.. i did it on the lab equipment,
and in the simulator program….

I Just do not understand the reason for doing it..  if there is a reason.

# 05 Chris Buscemi

From what I’ve seen on my lab equipment, you’re right, you can assign a VLAN to a trunk port. However, as long as the port is in Trunking mode, the VLAN assignment is listed as “inactive” under the show interface (int#) switchport output. Once you code the interface to be in access mode, the access VLAN comes up.

I don’t know of any circumstance where you would even bother assigning the trunk port to an VLAN, given what the trunks are designed to do. Perhaps one of the resident experts here know of some reason why you would. However it is best practice to know what interfaces on your switches are trunks, and to therefore hard code them to be trunks, thereby disabling the VLAN assignment you’ve given them.

Now, in reference to Conwyn’s comments, I can definitely see why you would want a seperate trunk for your VoIP traffic, but that would actually be a seperate physical trunk link on the switch that you only allow the Voice traffic on. And while you can control which VLANs are allowed on the trunk, that is not the same as assigning the trunk port to a VLAN.

So in short, I too don’t see the reason why you would bother, and more over why they would have shown you the process to do that.

# 06 Matt Kerry

Hello ldgotz, Can you post the config that was demonstrated in the video you watched?

# 07  Conwyn

Sorry I mis-understood. You assign a vlan to trunk port because if the other end is not defined as a trunk then this end becomes an access port in that vlan.

# 08 Travis Newshott

Conwyn is correct, assuming the port is configured with DTP to allow it to negotiate the state.  If you hardcode a port to be trunk and disable DTP, the purpose of assigning a VLAN is generally through the native vlan command to tell the trunk which VLAN should be untagged.  This allows you to specify the untagged VLAN to comply with network design standards, security procedures, etc.

When DTP is configured for negotiation, Conwyn is absolutely correct – you set a VLAN to ensure if it falls back to access mode, the correct VLAN will be utilized.

Robin Lee :: Jun.07.2009 :: CCNA Q&As :: 1 Comment »

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