Real quick trunking/vlan question

ldelossa · 2014-01-20T16:17:47.000Z

Hey Guys,

Quick question here, if I have a switch that is tagging packets on say for example vlan 10, then I set a trunk port to forward packets with a native vlan of 10, do packets that go up the trunk tagged as Vlan 10 still get dumped onto the appropriate vlan? Or does the trunk with a native vlan of 10 ONLY place UNTAGGED packets on vlan 10 when it enters the switch.

This is not a permanent setup and is an intermediate phase between trunking out esx environment.

jeremyspencer · 2014-01-20T16:30:45.000Z

Typically, packets on the native VLAN will leave the switch untagged. Then the switch on the other side, will associate untagged packets with the configured native VLAN.

ldelossa · 2014-01-20T16:37:39.000Z

Okay so let me describe this a little more, since I think I did a horrible job, but I’m at work right now handling a million things at once.

I have a distribution switch, which is receiving untagged packets from our esxi host, and currently doing the tagging at the switch port itself. I want to go ahead and take each switch port connected to out host and trunk them, with a native vlan of, for example, 10. Now this vlan 10 will exist on our 2 core switches also, so where I have a dilemma is whether I NEED to set a native vlan for the trunk going up to the core switch.

My first impression is that:

Untagged packet enters distribution switch, gets tagged 8, the packet then goes up to the trunk into the core STILL tagged as vlan 8 → thus putting it on the correct vlan through the core switches.

However what could see also is that the packet leaving the distribution switch will not leave the switch tagged at 8 and go up the trunk, if the packet leaves the distribution with the vlan 8 tag, it will wind up on the wrong lan and we’ll have connectivity issues.

Thanks.

cybersharktech · 2014-01-20T16:39:24.000Z

I honestly don’t know, but have you tried testing it in Packet Tracer?

ldelossa · 2014-01-20T16:41:26.000Z

Great idea!

cybersharktech · 2014-01-20T16:50:31.000Z

I actually try to keep a Packet Tracer or GNS3 with my current configs. This way when I want to make a change. I can change the simulator first and get a better grasp on what is going on. Hope this helps =)

ldelossa · 2014-01-20T16:53:38.000Z

Just simmed it, looks like you need the native vlan all the way, through your upstream switches.

Good advice man, that saved me some sweat.

jeremyspencer · 2014-01-20T16:54:36.000Z

As long as your native VLANs match on each side of the trunk at all points, you should be fine.

So, assuming VLAN 10 is the native VLAN for the ESX trunk, when the switch receives an untagged frame from ESX, it will associate that frame with VLAN 10. Now, let’s say the trunk between the core and distribution switch is using VLAN 8 as it’s native VLAN. That same previously untagged frame will now be tagged with VLAN 10 when leaving the trunk port to the core switch since the native VLAN is 8.

Now let’s say a frame tagged with VLAN 8 leaves the ESX host, that same frame would then be sent untagged over the trunk to the core switch.

This of course is assuming you’re using dot1q. ISL does not have native VLANs and everything is tagged. You can also set dot1q trunks to tag everything with the “vlan dot1q tag native” command.

ldelossa · 2014-01-20T16:56:38.000Z

Jeremy, that’s a great explanation. Thanks man. Explains exactly what I just saw in PacketTracers