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How Does VLAN Work?

Intra-VLAN Communication

Intra-VLAN communication refers to the communication of users in the same network segment and VLAN. Generally, this type of VLAN is applied into two scenarios: intra-VLAN communication through the same device and intra-VLAN communication through multiple devices. No matter what type, the whole transmission process mainly goes through the following two steps:

1. The ARP request sent from the source host: Before sending, the source host will compare its IP address with the designation’s. If the source host finds that they are in the same network segment, it will get destination host’s MAC address and fill the destination field MAC address of the frame with the obtained MAC address. On the contrary, the broadcast packet needs to be sent to the gateway. The MAC address of gateway will be used by the source host as its destination MAC address.

2. Adding and removing VLAN tags during the communication between devices: When frames processed in a switch, VLAN tags need to be carried.

Inter-VLAN Communication

Since broadcast packets are limited in the same VLAN, hosts in different VLANs are unable to directly communicate with each other in layer 2. Therefore, inter-VLAN routing that can forward network traffic from one VLAN to another is used to solve this problem. There are three options available in order to enable routing between different VLANs:

Inter-VLAN Routing with Separate Physical Interfaces

This inter-VLAN routing way is to connect an additional port from each VLAN with a router. Each VLAN needs one physical port on the router, which causes the great cost of routers. Therefore, this type of inter-VLAN routing has been rarely used due to its high cost and poor scalability.

Router-on-a-Stick Inter-VLAN Routing

This type of VLAN routing enables one single physical interface to achieve traffic forwarding between VLANs. After configuring the connection between the router and the switch as a trunk link, the router can receive frames with VLAN tags on the trunk interface from the connected switch, and forward the routed packets out to VLAN tagged destinations via the same interface.

Inter-VLAN Routing with Layer 3 Switch

The last method is to use layer 3 switches with routing function. Users need to create a SVI (Switch Virtual Interface) for each VLAN and configure an IP address for it. This IP address can be used for computers as their default gateway. In that way, the packets from one VLAN will be sent to the SVI to be routed to the other VLANs to realize the inter-VLAN communication.



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