In modern enterprise networks, VLANs are essential for segmenting broadcast domains, enhancing security, and improving network performance. However, devices in different VLANs cannot communicate natively. This is where InterVLAN Routing becomes a critical CCNA (200-301) objective.

As a Cisco NetAcad Expert with over 20 years of experience and a 98% student pass rate, I have trained thousands of professionals on this exact topic. In this guide, we will break down the three primary methods of InterVLAN routing, complete with real-world Cisco IOS configurations you can practice in Packet Tracer or Cisco Modeling Labs (CML).

1. Traditional InterVLAN Routing (Per VLAN, Per Router Interface)

This is the legacy method of InterVLAN routing. It requires a router with multiple physical interfaces, where each interface is connected to a separate switch port assigned to a specific VLAN.

How It Works:

• Each router interface is configured with an IP address that serves as the default gateway for its respective VLAN.
• The switch ports connected to the router are configured as access ports.

Sample Configuration:

Pros & Cons:

✅ Pros: Simple to configure and troubleshoot; no router CPU overhead for 802.1Q tagging.

❌ Cons: Highly unscalable. Requires a physical router interface for every single VLAN, which is expensive and wastes switch ports.

2. Router-on-a-Stick (ROAS)

Router-on-a-Stick is the classic CCNA solution to the scalability problem of the legacy method. It uses a single physical router interface divided into multiple logical subinterfaces, each representing a different VLAN.

How It Works:

• The switch port connected to the router is configured as an 802.1Q Trunk.
• Each router subinterface is assigned an 802.1Q encapsulation ID matching the VLAN number.
• Each subinterface is assigned an IP address to act as the default gateway for that VLAN.

Sample Configuration:

Pros & Cons:

✅ Pros: Highly scalable compared to legacy; saves physical router ports; cost-effective for small to medium networks.

❌ Cons: The single physical link can become a bandwidth bottleneck. All InterVLAN traffic must traverse this one link.

3. Layer 3 Switch (Switched Virtual Interface – SVI)

This is the industry standard and the most highly recommended method for enterprise networks. A Layer 3 (Multilayer) switch performs routing at hardware speed (ASICs), eliminating the bottleneck of a traditional router.

How It Works:

• IP routing is globally enabled on the switch (ip routing).
• A Switched Virtual Interface (SVI) is created for each VLAN (interface vlan X).
• The SVI is assigned an IP address, serving as the default gateway for devices in that VLAN.

Sample Configuration:

Pros & Cons:

✅ Pros: Wire-speed routing, highly scalable, low latency, and consolidates switching and routing into a single device.

❌ Cons: Layer 3 switches are more expensive than Layer 2 switches (though cheaper than high-end routers).

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