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New Member

Two vlan communication in a switch


Here's the problem:


Equipment in VLAN 26 from the trunk link must send a request to the interface 39 of the switch at the top to reach the interface 1 of the switch at the bottom. This request must exit through the interface 12 of the switch at the bottom to reach the interface 43 of the switch at the top. The application must exit through the trunk link to reach a device located in VLAN 1.

The bottom switch is a Catalyst 2950. It has only one VLAN 1 interface that contains the IP address that is in the same subnet as the two devices that need to communicate with each other. Packets that arrive at the switch at the bottom are not tagged and therefore the bottom switch is unaware of VLAN provided by the switch at the top. Interfaces 1 and 12 of the switch at the bottom are switchport access mode and no shutdown.

If anyone can help me.




Two vlan communication in a switch

Is this an exam/learning question or a genuine issue?

Where are the two devices in the topology?

Why would the traffic take this path if neither device is plugged into either switch?

New Member

Re: Two vlan communication in a switch

I am a little confused as well!

You mention a trunk link, so the packets are tagged, the only way they would be sent untagged by either device, is if they are from the native VLAN.

It's impossible to send traffic inbetween VLANS without routing it (something the bottom switch can not do)

Martin - CCNA

Re: Two vlan communication in a switch


It is very confusing what you're trying to do, but from what I can figure it doesn't sound like it's going to be possible. You stated that vlan 26 needs to send through interface 39 down to interface 1 on the bottom switch, but the bottom switch only has vlan 1. That won't work because once it arrives at the bottom switch, the switch won't know what to do with the vlan 26 tag and will drop traffic. In order to use vlans, you'll need to have the same vlans on both switches and that vlan will need to be allowed over the trunk. You can still change the native vlan over the trunk in order to send a different untagged vlan, but I don't believe what you're wanting to do is going to work at all. What application is needing this? It almost sounds like you're trying to sniff traffic, use a proxy server, or something like that....


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HTH, John *** Please rate all useful posts ***