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Community Member

Vlan communication

I have a scenario.


Two major companies have their Two switches connected together (3750X-24P) via a patch cable. The link between the two switches is an access port. I want Vlan 12 to communicate with users of Vlan 15 on other side. I know on trunk ports we have tags on headers to differentiate between vlans. But here will the two communicate. On my side of network - part of vlan 12 -  i have only created a L2 interface. On the other side I have a L3 interface and L3 routing for this vlan.



Will the two users part of different vlan communicate? if yes how is traffic carried out on port and how it knows the destination on other side of company.


Also, if I use the same network and default gateway for users part of vlan 12  - then I guess it should communicate with users on vlan 15

VIP Super Bronze

Sine you have a layer-3

Sine you have a layer-3 interface for vlan 12 on the other side and if the same device also has a layer-3 interface for vlan 15, then the router or the layer-3 device routes between the 2 vlans locally.


Hall of Fame Super Gold

I find your question

I find your question confusing. Why would two major companies connect their networks using a layer 2 VLAN? But aside from that let me answer your question in this way - if two networks are connected using switch ports that are configured as access ports then the two networks will communicate. An access port transmits Ethernet frames that do not have tags so the networks will not recognize that one is vlan 12 and the other is vlan 15. There is a potential issue with spanning tree when there is a vlan mismatch such as you describe. But it will not prevent communication.


I also believe that the question confuses what is involved in vlan communication at layer 2 with IP communication at layer 3. A switch access port does layer 2 forwarding based on MAC address. And if these are layer 2 access ports then traffic will go back and forth and MAC addresses will be learned and there will be successful layer 2 communication.


However layer 3 communication is different. If you are saying that one network does have an SVI for the VLAN but that the other company does not have an SVI then this does impact layer 3 communication. And if one company does have its own subnet, with its own range of addresses and its own default gateway and the other company has its own subnet, with its own range of addresses and its own default gateway then layer 3 communication will be problematic.





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