optional VLAN with 2650

[Joachim Hergeth]

In our school we are using 6 HP2650 in the LAN. They all have the default configuration with only minor adaptions (ID, IP adress, ...), no VLANs are configured.

I now would like to disconnect all 24 PCs in one training room from the servers to enable the students to install OS and network software without interruptions to the schools network. During other courses these PCs must remain in the school-LAN. The PCs are all connected to one HP2650. PCs from other rooms are also connected to the same switch and the switch itself is linked via Gigabit ethernet to a central switch.

My idea was to define a VLAN, put all PCs from my room plus one spare port inside this VLAN and use a patch cable to connect the VLAN to the schools network via the spare port. This setup would enable to switch the connection to the school network on/off via the patch cable which is located in the classroom.

But this does not work! The switch detects a loop via SPT when the patch cable is connected. (SPT ignores VLANs).
I disabled SPT on the switch, but then the GBit-switch one level up detects the loop and disables its downstream port.

Any suggestions to solve the problem?
The connection to the schools network should be hardware switchable, as this can be easily understood by teachers.....

Thanks in advance
J. Hergeth

[Jeff Brown_15]

You already have a connection for this switch to the core. That is way you are getting blocked.

Can you explain what purposes you have for the VLAN? I am a little lost. All you are doing is subnetting out your classroom's PCs. It will still interact with the schools network if you have that VLAN routed on an interface.

[Joachim Hergeth]

The idea is to disconnect all the PCs in the classroom from the schools LAN by unplugging only one patch cable.
The PCs have swappable hard disks, so with that I am able to let the students install operating systems without any conflict with the exsisting LAN, but still have a local classroom LAN for the students.

I forgot to mention, that I set the used ports to "forbid" within the default VLAN in the switch. So the PCs in the new VLAN are completely diconnected from the schools LAN and vice versa, unless I plug the patch cable to intentionally connect both networks (for other classes).

J. Hergeth

[Jeff Brown_15]

It sounds like your switch supports more than one classroom and disabling the gig port is not the anwser (you probably said this but I am slow).

The only way I can see this working is logical not physical. I would keep the classroom VLAN'd and at the router disable the VLAN when you don't want to interfere with the school network. With the VLAN down at the router your classroom will still route on the local subnet but will not cross over to the school subnet.

Then only other thing I can think of is to get a switch for each classroom (I am sure you have already thought of this).

If you route with DHCP make sure to setup a scope in DHCP for the subnet and add your IP helpers to the router.

[Joachim Hergeth]

Thanks Jeff,

my current impression also is that only two different configurations of the switch can solve the problem.
(should VLANs not divide big switches into seperated small switches, similar to having several small seperated devices?)

Anyway, does someone know a Windows compatible tool to change the switchs configuration by one click of a mouse? Could this be done using an SNMP command line tool in a "small" batch?

[Les Ligetfalvy]

PCM+ can be used to save and redeploy any of several VLAN configs with a few clicks of a mouse. The web agent can do it with a few more. You could script something in perl or VBS as well.

You do not say who would be doing the VLAN changes however, so I would have reservations if it is not a network admin. Best practice dictates that switches be in locked closets with only network admins having access.

[Joachim Hergeth]

Thats right.
I am the network admin, so I could do that. But I prefere not to do admin work like switch reconfiguration as part of my daily courses.

The "switch reconfiguration solution" is in any case only a work around. Switches should be configured only when changes in the network demand it.

[Jeff Brown_15]

I have searched this out a little. You can create a loop with multiple devices but one will be passive and the other active. I cannot find anything on single devices.

In your senario, if possible, with the information I found, you could only have one trunk active at a time. This could only be used for cascading switches.

PST is blocking the physical loop an is not aware of the VLANs.