Solved: C2950-24 Single VLAN issue

twhittle1 · ‎02-29-2008

Hi All

I am using a C2950-24 as a bit of practice for the CCNA exams, a snag I hit is that I can create multiple VLANs but when I enable one with the no shutdown command and previously enabled VLAN goes down.

Does anyone know if this is a limitation with the switch or maybe the version of IOS I'm using? "Version 12.1(6)EA2c, RELEASE SOFTWARE(fc1)" Btw

Any feedback would be appreciated.

Thanks

Jon Marshall · ‎02-29-2008

Vlans are layer 2. By assigning an interface to them they don't become layer 3, it just means that you can now route between vlans if you need to.

You can't have a L3 vlan interface without the layer 2 vlan existing but the reverse is not true. In most networks there is usually a direct correlation between a L2 vlan and a L3 vlan interface but you could quite easily just create a L2 vlan only. Any machines connected into this vlan would only be able to talk to each other within the vlan.

Hope that makes things clearer ?

Jon

View solution in original post

Jon Marshall · ‎02-29-2008

Hi

The 2950 is a layer 2 switch only. This means that you can have only 1 L3 vlan interface active at any one time. This L3 vlan interface is used for managment of the switch so that you can remotely telnet into it.

To create multiple vlans you would need a L3 switch. If you had a router that supported 802.1q trunking you could use that to do the inter-vlan routing.

Edit - just to clarify, you can have multiple vlans on this switch at layer 2. They are enabled and can be used as layer 2 vlans, it is just when you try and create multiple L3 vlan interfaces.

HTH

Jon

twhittle1 · ‎02-29-2008

Thanks for the prompt reply John, appreciate that.

Is this the same case with the 2960's as well?

What is the point in these switches supporting multiple VLANs if you can only have one active?

Thanks!

Jon Marshall · ‎02-29-2008

2960's are also layer 2 switches only. For layer 3 look at 3560/3750 switches.

Not all switches need to have L3 capabilities. For example you may have a L3 capable switch at the core of your network responsible for doing all the routing between vlans. Connected to this switch are a number of L2 only switches that you can connect your client PC's into. If one PC needs to communicate with another PC in a different vlan the packet will go via the L3 switch.

L2 switches are cheaper than L3 switches and if all you need is L2 then why pay more.

HTH

Jon

twhittle1 · ‎02-29-2008

Ah ok, I understand the difference between L2 and L3 switches, its just the L2 and L3 Vlans part which has thrown me.

I've just seen the edit in the first reply, thanks. How is it different for me to use Layer 2 and layer 3 VLANs? Is the command different? Or is it just that you can create the VLANs at layer 2 but when it comes to assigning them to an interface they become layer 3?

Thanks!

Jon Marshall · ‎02-29-2008

Vlans are layer 2. By assigning an interface to them they don't become layer 3, it just means that you can now route between vlans if you need to.

You can't have a L3 vlan interface without the layer 2 vlan existing but the reverse is not true. In most networks there is usually a direct correlation between a L2 vlan and a L3 vlan interface but you could quite easily just create a L2 vlan only. Any machines connected into this vlan would only be able to talk to each other within the vlan.

Hope that makes things clearer ?

Jon

twhittle1 · ‎02-29-2008

Ooooooooh I see. So just having the Switch port in a specific VLAN will issolate the PC connected to that port to a specific VLAN.

You only have to enable a VLAN if you want to have the ability to route between VLANs or something similar?

Thanks for all your help Jon