InterVLAN connectivity refers to the process of enabling communication between devices that belong to different Virtual Local Area Networks (VLANs). By default, VLANs are isolated from each other at Layer 2, meaning devices in one VLAN cannot communicate with devices in another VLAN. To allow traff…InterVLAN connectivity refers to the process of enabling communication between devices that belong to different Virtual Local Area Networks (VLANs). By default, VLANs are isolated from each other at Layer 2, meaning devices in one VLAN cannot communicate with devices in another VLAN. To allow traffic to flow between VLANs, Layer 3 routing is required.
There are three primary methods to achieve InterVLAN connectivity:
1. **Traditional InterVLAN Routing**: This method uses a physical router with multiple interfaces, where each interface connects to a separate VLAN on the switch. Each router interface serves as the default gateway for its respective VLAN. While simple to understand, this approach requires multiple physical connections and router interfaces.
2. **Router-on-a-Stick (ROAS)**: This popular method uses a single physical router interface configured with multiple subinterfaces. The router connects to the switch via a trunk link carrying traffic from all VLANs. Each subinterface is assigned to a specific VLAN using 802.1Q encapsulation and acts as the default gateway for that VLAN. This approach is cost-effective but can create bandwidth bottlenecks.
3. **Layer 3 Switching (SVIs)**: Modern multilayer switches can perform routing functions using Switched Virtual Interfaces (SVIs). An SVI is a virtual interface created for each VLAN that requires routing. This method provides the fastest InterVLAN routing because switching and routing occur within the same device, eliminating external router dependencies.
For InterVLAN routing to function properly, you must configure the following: enable IP routing on the device, create VLANs and assign ports, configure the routing interfaces or SVIs with appropriate IP addresses, and ensure hosts have correct default gateway settings. Understanding InterVLAN connectivity is essential for CCNA candidates as it forms the foundation for enterprise network design and segmentation strategies.
InterVLAN Connectivity - Complete CCNA Guide
Why InterVLAN Connectivity is Important
VLANs segment network traffic for security and performance, but hosts on different VLANs cannot communicate by default. InterVLAN connectivity, also known as InterVLAN routing, is essential because it allows devices on separate VLANs to exchange data while maintaining logical segmentation. This is fundamental in enterprise networks where departments need isolation but still require access to shared resources.
What is InterVLAN Connectivity?
InterVLAN connectivity refers to the process of routing traffic between different VLANs. Since VLANs operate at Layer 2 and create separate broadcast domains, a Layer 3 device (router or Layer 3 switch) is required to forward packets between them. Each VLAN requires its own IP subnet, and the Layer 3 device acts as the default gateway for hosts in each VLAN.
How InterVLAN Routing Works
There are three primary methods:
1. Legacy InterVLAN Routing (Router with Separate Interfaces) - Each VLAN connects to a dedicated physical router interface - Each interface is configured with an IP address serving as the default gateway - Requires multiple physical connections and ports - Not scalable for many VLANs
2. Router-on-a-Stick (ROAS) - Uses a single physical router interface divided into subinterfaces - The switch port connecting to the router is configured as a trunk - Each subinterface is assigned to a VLAN using 802.1Q encapsulation - Configuration example: interface G0/0.10 encapsulation dot1q 10 ip address 192.168.10.1 255.255.255.0
3. Layer 3 Switch with SVIs (Switched Virtual Interfaces) - Most efficient and common method in modern networks - Create SVIs for each VLAN that needs routing - Enable IP routing on the switch - Configuration example: ip routing interface vlan 10 ip address 192.168.10.1 255.255.255.0 no shutdown
Key Configuration Requirements
- Trunk links must be properly configured between switches - VLANs must exist in the VLAN database - IP addresses must be configured on routing interfaces or SVIs - Hosts must have correct default gateway settings - For Layer 3 switches, ip routing must be enabled
Exam Tips: Answering Questions on InterVLAN Connectivity
Troubleshooting Scenarios: - Verify the trunk link is operational and allows the required VLANs - Check that subinterfaces have the correct VLAN encapsulation - Confirm SVIs are in the up/up state - Ensure hosts have the correct default gateway configured - Verify ip routing is enabled on Layer 3 switches
Common Exam Question Types: - Identify missing configuration commands - Select the correct encapsulation type (dot1q is standard) - Determine why hosts on different VLANs cannot communicate - Choose the appropriate InterVLAN routing method for a scenario
Key Points to Remember: - Native VLAN subinterfaces may or may not require encapsulation specification - The subinterface number does not need to match the VLAN ID, but it is best practice - Layer 3 switches provide better performance than router-on-a-stick - All participating VLANs must be allowed on trunk links - SVI interfaces require the VLAN to exist and have at least one active port in that VLAN
Watch for Trick Questions: - Missing no shutdown on interfaces or SVIs - Trunk mode set incorrectly (access instead of trunk) - VLAN not created on the switch - Wrong subnet masks causing addressing issues - Mismatched native VLAN configurations