Unanswered Question
Jan 3rd, 2008

trying to get a better understanding of vlans other than the book resource. I dont understand why I can do a show vlan and I see a vlan listed but not when I do sh int vlan. I check my distibution & access and no vlan interface. My switches are in Server mode, does that make a difference and if not when do I know if I should be server/transparent/client. Is there a rule of thumb to follow.

I have this problem too.
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Collin Clark Thu, 01/03/2008 - 13:51

The SHOW VLAN command shows the Layer 2 VLANS configured on the switch. The SHOW INTERFACE VLAN shows the Layer 3 configured for the VLAN. Typically the Layer 3 interfaces for VLANs are on distribution layers, but are sometimes on the core or access layer. It really depends on the design. On Layer 2 only switches, the only layer 3 interface you should see is the management interface.

HTH and please rate

cpubob Thu, 01/03/2008 - 13:54

The only vlan that gets a interface created by default is vlan1. Other vlans can have a layer 3 interface but you have to create it.

To do so, go to config mode and simply enter "interface vlan xx". From there you can create a l3 interface for a vlan.

"Show vlan" shows you which switchports, meaning layer 2 ports, are assigned to which vlan.

Jon Marshall Thu, 01/03/2008 - 14:02


You are referring to 2 different things. When you do a "sh vlan" you are looking at the vlans that exist on the switch at Layer 2.

When you look for interface vlans your are looking for L3 interfaces.

Put simply you have a single switch with 2 vlans that you see when you do a "sh vlan"

vlan 10

vlan 20

There are no interface vlans on the switch for these vlans.

So if you allocate 10 ports to vlan 10 and 10 ports to vlan 20 then attach PC's to these ports the 10 PC's attached to the vlan 10 ports can communicate with each other.

The 10 pc's attached to the vlan 20 ports can talk to each other.

BUT the 10 pc's in vlan 10 cannot talk to the 10 pc's in vlan 20 and vice-versa.

To allow them to talk to each other you have to enable inter-vlan routing. This is what the interface vlans are. So assuming your switch is a lyer 3 switch you can

int vlan 10

ip address

int vlan 20

ip address

If you now address the pc's in vlan 10 using addresses from the 192.168.5.x/24 subnet and set the default-gateway to and for pc's in vlan 20 use 192.168.6.x/24 addressing with the default-gateway of the machines can now communicate.

Bear in mind that the interface vlans for your vlans might not be on the switches you are looking at, they may be on other switches, it depends on your topology.

Whether your switches are in VTP server/client or transparent mode does not affect the L2/L3 discussion above.


Juan Carlos Ari... Thu, 01/03/2008 - 17:06

I recommend you to define only 1 switch as server mode, so when you need to create, modify or delete a VLAN you can do it on this switch. All others change it to client with the VTP domain and password if is the case that it belongs to.

Hope this help.


Juan Carlos Arias


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