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I created a vlan in Juniper qfx3600 switch by specifying just VLAN name. When I tried to commit the configuration I got an error saying "need L2domain configured". I asked with my collegue, he said you must provide vlan id, when I specified VLAN-id to the created vlan the commit was successful. But the documentations I read says VLAN automatically will be assosciated with VLAN ID if one is not provided by the user.

I got confused by all of these, can anyone please tell me which is right and why the commit was successfull after I provided VLAN ID but not before that.

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The documentation for the Juniper EX series switches indeed mentions the possibility of creating a VLAN without specifying a VLAN id (1):

When you assign just a VLAN name, an ID is generated by Junos OS.

and

2 Configure the VLAN tag ID or VLAN ID range for the VLAN. (If you assigned a VLAN name, you do not have to do this, because a VLAN ID is assigned automatically, thereby associating the name of the VLAN to an ID number. However, if you want to control the ID numbers, you can assign both a name and an ID.)

However, you are using a QFX3500, and I cannot find a similar statement in the QFX range documentation. Here, Juniper states (2):

Configure the VLAN tag ID or VLAN ID range for the VLAN

without the remark about automatic VLAN id assignnment.

I only have EX devices so I cannot verify this.

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But the documentations I read says VLAN automatically will be assosciated with VLAN ID if one is not provided by the user.

Juniper would never be able to automatically associate a VLAN name to a VLAN tag by default. If you just specify a VLAN by name, there isn't any way to figure out what VLAN that traffic belongs to.

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  • Why wouldn't they be able to just assign an unused vlan-id? As long as you always use the name to refer to the vlan, you would not need to know the assigned id. I'm not saying it is a good idea, but it could be done (locally). When a vlan has to span multiple switches, this becomes rather tricky.
    – Gerben
    Feb 26, 2014 at 10:06
  • @Gerben You're right on the EX platforms, you can certainly do that. I can only assume it is because it's on the QFX platform; which is really designed to be at the heart of a datacenter (i.e. not a standalone). I would imagine they'll eventually move to the same way EX's do business with regard to auto-VLAN id assignment.
    – Ryan Foley
    Feb 26, 2014 at 11:15
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802.1Q tracks VLANs based on ID (number) not name, so this would be an awkward way to program a switch. Example:

Customer A is plugged into VLAN 10 named "Clients" on Switch A.

Switch A and Switch B are connected together and share all VLAN traffic.

Customer B is plugged into VLAN 20 named "Clients" on Switch B.

Customer C is plugged into VLAN 10 named "Soap" on Switch B.

Because the 802.1Q tag is number based, Customer A and Customer C would be able to communicate while Customer B would not be able to talk to either other customer.

I'm not sure about the Juniper method of assigning VLAN tags but I hope this helps clear up why you shouldn't rely on autoconfiguration for that.

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