switchport trunk allowed vlan question

[JCMG-Network]

I have a general question about the "switchport trunk allowed vlan" command when configuring a trunk port on a switch. I understand that what it does is allow the specified VLANs to run across the trunk but if you have say a simple setup with three switches, one as a core and two as access switches. if the core has the routed interfaces for say VLAN 2 and 3 and you give access switch A to only allow VLAN 1 and 2 and access switch B to only allow VLAN 1 and 3 could a end device on switch A VLAN 2 communicate to a device on switch B VLAN 3?

I know this would be easy to test in a lab situation but I am in a position where I do not have any test equipment.

[Mark Malone]

No that wont work  switch b would still need to know about vlan 2 and be able to pass traffic for it on its trunk , before you hit layer 3 your utilizing layer 2 trunks , so it would have to be in place and allowed or  traffic will be blackholed and dropped , routers allow vlans to speak to each other but the trunk is allowing the vlan to get to the layer 3 device for this to happen

[Jon Marshall]

How are access switch A and B connected to the core switch ?

If they are each connected separately ie not chained off each other then yes it would work fine because the end device on switch A would send the packet to it's default gateway on the core switch which is the SVI for vlan 2.

The core switch would route it onto vlan 3 and then send it to switch B and the same logic in reverse.

Jon