To Tag or Not to Tag??

[Ben02]

Hi,

I was under the impression that "tagging" was necessary for different VLANs to communicate. However, when I tried "tagging" ports, I was unsuccessful at getting different VLANs across sites to communicate. So, I decided not to “tag” anything and…everything finally WORKED! – Now, I just want to know why??

Here's my Setup:

Site 1 - 2910al w/ Routing Enabled
10.140.0.0 network
One Access VLAN - "VLAN 20" - untagged 1-23
One Uplink VLAN - "VLAN 5" - untagged 24 - connects to Site 2.

Site 2 - 5412 w/ Routing Enabled
10.10.0.0 network (1 of many)
Multiple Access VLANs - "VLAN 10", "VLAN 30", etc. (note: no VLAN 20 at this site)
One Uplink VLAN - "VLAN 5" - untagged e24 - connects to Site 1.

As indicated earlier, everything works...PCs on "VLAN 20" @ Site 1 can communicate with PCs on "VLAN 10", "30", etc @ Site 2.

So, I just want to know what this setup works?

Is tagging unnecessary because I'm routing (Layer 3) instead of bridging (Layer 2)?

I would really appreciate any feedback on this.

Ben

[macrocozm · Accepted solution]

Hello Ben,

To get 2 vlans to communicate with each other you either need to bridge them together or route between them. A tag is simply adding info to the packet which vlan it belongs to, useful for switches and other devices that need to handle more than 1 vlan over a single interface.

In your case, it depends. If you wanna have more than one vlan spanning between two switches then yes, the uplinks need to have at least one vlan tagged. But from what I can see in your setup you've taken care of it with the routing.

[Ben02]

I really appreciate the info. I think I understand now. But just to clarify...

In my current setup, I have one common VLAN spanning across both switches - VLAN 5. However, if I created a new VLAN, i.e. VLAN 100, and had it span across the 2910al and the 5412, you're saying that I would need to tag the uplink port for one of those VLANs. Is that correct?

One more question...

If I keep my current setup, i.e. only one VLAN (VLAN 5) spanning two switches, and just added additional VLANS to the 2910al, I wouldn't need to tag anything because I still only have one VLAN spanning two switches, correct?

[macrocozm]

Q: I really appreciate the info. I think I understand now. But just to clarify...

In my current setup, I have one common VLAN spanning across both switches - VLAN 5. However, if I created a new VLAN, i.e. VLAN 100, and had it span across the 2910al and the 5412, you're saying that I would need to tag the uplink port for one of those VLANs. Is that correct?

A: Yes. With one vlan only, you can either have it T(agged) or U(ntagged) between two vlan aware switches, with N vlans at least N-1 must be T. Two vlans over the same port cannot be U both for obvious reasons =)

Q: One more question...

If I keep my current setup, i.e. only one VLAN (VLAN 5) spanning two switches, and just added additional VLANS to the 2910al, I wouldn't need to tag anything because I still only have one VLAN spanning two switches, correct?

A: Seems you got hold of this, the routing will take care of that yes.

[Ben02]

Just wanted to say Thanks Again! You were a big help!