Direct link between two Vlans

[flokki123]

Hi all,

I have Vlan A spread across multiple switches and those switches are connected via trunk ports. If I now configure a second Vlan, Vlan B on one of those switches and configure two access ports, one in Vlan A and one in Vlan B and connect those ports via a single cable, would I run into any issues, or would I just extend the broadcast domain? All those switches run RSTP.

STP would still be in place on those access ports connecting the two Vlans, thus they would participate in STP. But as its the same STP instance, could that be a problem? Unfortunately, I cant lab it up at the moment.

Thanks for any help!

[Joseph W. Doherty]

How did you intended to configure the connecting port, access or trunk?  Are you using VTP?

[flokki123]

Hi Joseph,

those two ports connectiong the two Vlans are supposed to be access ports. I really just want to connect those specific two Vlans with each other. Thats why I dont want to use a trunk port.

BR

[Joseph W. Doherty]

Yes you can do that.

BTW, much later posting on STP/RSTP.  Both switches are Cisco?  Reason I ask, Cisco's STP/RSTP are per VLAN while other vendors often just a single common instance of STP/RSTP.

[flokki123]

Hi Joseph,

thanks for your reply! Well, it is actually HP Iam working with and its really just a single instance of STP. Thats why Iam a bit concernced as both Vlans will use the same STP instance. Do you think that could be an issue?

Would you configure any specific STP commands onto those two access ports?

Thanks for your help!

[Richard Burts]

It might change the discussion a bit knowing that this is an HP environment, but I suspect that both vendors work pretty close to the same. At the start I will say that while I believe that it would work that I would not advise connecting an access port in vlan A to an access port in vlan B. The result would be that what things appear to be (2 vlans) is not what they really are. 

I suggest that a starting point for the discussion is that an access port forwards a standard Ethernet frame, and in the standard Ethernet frame there is no identification of vlan. So if an access port in vlan A is connected to an access port in vlan B then we have just created a larger broadcast domain and functionaly it is a single vlan. In that sense we would expect the cross connected vlans to work.

But what if some of the Ethernet frames were vlan aware? What if a frame was a Spanning Tree frame that was aware of vlan identity, was aware that it had originated in vlan A but was no longer in vlan A? Could this cause Spanning Tree to not detect that a loop might exist in the network

[flokki123]

Forgot to mention, no VTP. I would just create the new Vlan on the switch manually and put a access port into Vlan A and one in Vlan b and plug in a cable.

[Martin L]

Yes you have extend the broadcast domain.  if you have 2 switches connected via trunk that allows vlans A and B and you have vlans A B on both switch databases,  you should have 2 vlan the broadcast domain: one for A, and 1 for B.

you can check this with show spanning tree vlan A or B and look for Root Switch (Bridge).  you should have 1 Root switch per vlan. if you have 2, then trunk link is missing or not allowing vlan to run on it.

Same STP instance is not a problem;it is good thing as STP is loop and B-cast storm prevention mechanism.

[flokki123]

Hi Martin,

well, I would not connect the two Vlans via a trunk but via access ports and cable them directly with each other. And the STP type is RSTP  not PVST, meaning I should have the same root for Vlan A and B respectively. But as I would connect two Vlans on the same switch with the same STP instance I was wondering if there is something to keep in mind when doing so.

BR

[paul driver]

Hello

Just to confirm do you mean connecting two switches together between two access ports in different vlans?

if so this will not work

[flokki123]

Hi Paul,

I would create a new Vlan on the same switch as the existing Vlan. I would have two Vlans on the same switch, configure an access port into each of them and plug in a cable. But not with PVST but RSTP.

BR