Will a trunk carry vlans if that vlan is not configured?

adam-b

I could use your help here guys.... pretty basic question but i dont have a lab right now to test.

Lets say i have three switches: a, b, and c
a and b are connected via a trunk link just as b and c are. these trunk links are configured to carry all vlans.
the following vlans have been configured on the switches
a: 1 and 2
b: 1 (default native)
c: 1 and 2
all three switches are in transparent vtp mode.


[ switch a ]

---

trunk

---

[ switch b ]

---

trunk

---

[ switch c ]
vlan 1 and 2 vlan 1 vlan 1 and 2

if a host in vlan 2 off of switch a sends a frame to a host in vlan 2 off of switch c will this frame be carried over the trunk link between b and c? In other words, will a trunk carry a vlan over one of its trunk ports if that vlan is not configured on the switch? I have been debating this one in my head but have no way to verify my conclusions so im turning to the board. Thanks alot guys!

GT-Rob

By default, unless you have configured it otherwise, trunks will carry vlans 1-4096 regardless.

If you do a show interface trunk, you will see the allowed vlans.

dtlokee

The frames for VLAN 2 will be dropped by switch B in your example. Switch A and C will send frames for VLAN 2 over the trunk to switch B, but switch B will drop them when they are received.

kryolla

when you create a vlan a spanning tree instance is also created so you need to create the vlan for all transit swtiches. HTH

adam-b

So if the vlan must be configured on the switch in order for a trunk connected to that switch to carry packets tagged with that vlan then what is the point of a transparent mode switch allowing vtp advertisements to pass through? If the transparent mode switch does not take on those vlans then the down stream vtp swtich would be taking on a vlan that it would not be able to receive packets from on the trunk connected to the transparent mode switch.....

correct me if i am wrong or need to further explain. thanks again for all your replies!

EdTheLad

adam-b wrote:

So if the vlan must be configured on the switch in order for a trunk connected to that switch to carry packets tagged with that vlan then what is the point of a transparent mode switch allowing vtp advertisements to pass through? If the transparent mode switch does not take on those vlans then the down stream vtp swtich would be taking on a vlan that it would not be able to receive packets from on the trunk connected to the transparent mode switch.....

correct me if i am wrong or need to further explain. thanks again for all your replies!

Just throwing this scenario out there.
You work for a compnay which has many departments, each manages there own set of switches.You have two offices with 10 switches in each, these offices are interconnected via another departments switch, they will not allow you admin access.Neither department wants to communicate via vtp but you dont want to have to statically configure vlans everytime you add a new one to one office.
Solution, ask them to create a vlan range 100-200 and enable vtp transparent mode.Now next time you need to add a vlan all you have to do is ensure its within you allocated range.Vtp will advertise between you two offices, the switch in the middle is manually configured and just relays vtp info.

mattrgee

The VLAN trunks will carry traffic from all vlans unless: the vlan has been manually removed from the 'allowed list', or the vlan traffic has been pruned with vlan pruning. If a switch receives traffic for a vlan that none of its ports are members of, then the traffic is simply discarded.

VTP sends vlan configuration changes to other switches in the same VTP domain. VTP changes are made on the VTP server and VTP clients receive the changes. Switches in VTP transparent mode simply ignore the vlan configuration changes and forward the information on to the next switch.