**Principle of Communication Between Different VLANs**
After VLANs are configured, direct Layer 2 communication between different VLANs is not possible. To enable inter-VLAN communication, there are typically three common approaches.
1. **Layer 3 VLANIF Interface Solution**
This method uses a Layer 3 interface (VLANIF) to allow communication between VLANs. Each VLAN can be assigned a logical Layer 3 VLANIF interface, which acts as the default gateway for hosts in that VLAN. The Layer 3 switch then performs routing between different VLANs on the same device. If the VLANs are on different switches, routing between their respective network segments must be enabled.
This solution is supported by all Huawei S series switches except the S1700 series.
For example, in a network with two VLANs—VLAN 2 and VLAN 3—the following steps are taken:
- Create two VLANIF interfaces and assign IP addresses that belong to different subnets.
- Configure the default gateway on each host to the corresponding VLANIF interface’s IP address.
When Host A in VLAN 2 communicates with Host C in VLAN 3:
- Host A checks if Host C is in the same subnet. If not, it sends an ARP request to find the gateway's MAC address.
- The VLANIF2 interface responds, providing its MAC address.
- Host A then sends the data frame to the gateway (VLANIF2), which performs Layer 3 forwarding.
- The packet is routed through VLANIF3, which forwards it to Host C after resolving its MAC address via ARP if needed.
This process enables seamless Layer 3 communication between VLANs.
2. **Layer 3 Ethernet Sub-interface Solution**
A Layer 3 Ethernet sub-interface functions as both a Layer 2 and Layer 3 interface. It supports VLAN tagging and allows routing between VLANs. This method is commonly referred to as “single-arm routing†and is used in Layer 3 switches and routers.
It is supported on Huawei models such as the 5700HI, 5710EI, S7700, S9300, and S9700 series.
In this configuration:
- Two sub-interfaces (e.g., Port1.1 and Port1.2) are created on a Layer 3 port, each associated with a specific VLAN.
- IP addresses are assigned to these sub-interfaces.
- The connecting Layer 2 switch must be configured as trunk or hybrid to allow VLAN traffic.
Host A in VLAN 2 sends a packet to Host C in VLAN 3:
- Host A broadcasts an ARP request to find the sub-interface’s MAC address.
- The sub-interface replies with its MAC address.
- Host A then sends the packet to the sub-interface, which routes it to the correct VLAN using Layer 3 routing.
This method effectively enables inter-VLAN communication without requiring a full Layer 3 switch.
3. **VLAN Switch Solution**
The VLAN Switch approach involves forwarding packets based on VLAN tags. It requires static forwarding paths to be set up across the network. Once a packet matches a forwarding rule, it is directly sent to the appropriate interface based on the VLAN switching table.
This method improves efficiency and security by eliminating the need to update MAC address tables. It also helps prevent MAC address spoofing and broadcast storms.
This feature is available only on high-end Huawei switches like the S7700, S9300, S9300E, and S9700.
Key features include:
- **Stack-VLAN Function**: Adds an outer VLAN tag, similar to VLAN stacking.
- **Switch-VLAN Function**: Converts VLAN tags between interfaces, similar to VLAN mapping.
These functions help enable efficient and secure inter-VLAN communication.
By choosing one of these methods, network administrators can ensure smooth communication between different VLANs, enhancing network performance and management.
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