Module 4

Module Title: Inter-VLAN Routing

Module Objectives

• Module Objective: Troubleshoot inter-VLAN routing on Layer 3 devices and understand the operational characteristics of inter-VLAN routing.

Topics Include:

• Inter-VLAN Routing Operation

• Router-on-a-Stick Inter-VLAN Routing

• Inter-VLAN Routing using Layer 3 Switches

• Troubleshoot Inter-VLAN Routing

4.1 Inter-VLAN Routing Operation

Overview

• Definition: VLANs (Virtual Local Area Networks) segment switched Layer 2 networks, providing fault isolation and improved performance by limiting broadcast domains.

• Purpose: Hosts in different VLANs must communicate through a router or Layer 3 switch because VLANs inherently prevent communication across different segments unless inter-VLAN routing is configured.

• Inter-VLAN Routing: It is the process that forwards data packets between VLANs, allowing hosts on separate VLANs to communicate seamlessly.

Inter-VLAN Routing Options

• Legacy Inter-VLAN Routing: This approach is non-scalable as it requires physical routers attached to multiple Ethernet interfaces, making it unsuitable for larger networks due to hardware limits.

• Router-on-a-Stick: Ideal for small to medium-sized networks, employing a single physical interface that is configured to support multiple VLANs through 802.1Q encapsulation.

• Layer 3 Switch (SVIs): This method is the most scalable option, allowing medium to large organizations to efficiently manage multiple VLANs using multi-layer switching capabilities via Switch Virtual Interfaces (SVIs).

4.2 Router-on-a-Stick Inter-VLAN Routing

Description

• Method: The router-on-a-stick method utilizes a single physical interface on a router, configured as an 802.1Q trunk link that enables VLAN tagging.

• Subinterfaces: Each VLAN is represented by a logical subinterface on the router, each assigned a unique IP address corresponding to its VLAN.

Configuration Steps

• Router Configuration: Create subinterfaces for each VLAN and assign corresponding IP addresses.

• Switch Configuration: Set up trunk links between switches and the router, ensuring all VLANs are appropriately configured on the switches and active.

Limitations

• Scalability: The router-on-a-stick method is limited in scalability; it does not efficiently serve more than 50 VLANs due to performance constraints of a single link.

4.3 Inter-VLAN Routing using Layer 3 Switches

Introduction

• Layer 3 Switches: Provide enhanced inter-VLAN routing capabilities through hardware optimization and faster processing, reducing latency in data transfer.

Capabilities

• Routing Between VLANs: Layer 3 switches route traffic between multiple VLANs with the use of SVIs, allowing faster processing compared to traditional routers.

• Layer 3 Routed Ports: Enable the conversion of specific Layer 2 switch ports into Layer 3 routed ports for direct inter-VLAN communication without relying on external routers.

Configuration Steps

• Create VLANs: Define necessary VLANs required for the network.

• Create SVIs: Configure SVIs for each VLAN, providing gateway IP addresses for the connected hosts in their respective VLANs.

• Enable IP Routing: Use the ip routing command to allow traffic routing between VLANs, making sure that inter-VLAN communications are properly established.

4.4 Troubleshoot Inter-VLAN Routing

Common Issues

• Physical Layer Issues: Verify the physical connectivity across devices, ensuring that all cables are connected to the appropriate ports and are functioning properly.

• Missing VLANs: Ensure that all required VLANs are created and active on the switches to eliminate any potential communication barriers.

• Trunk Port Issues: Confirm that the trunk ports are correctly enabled and configured for VLAN tagging and traffic flow.

• Access Port Issues: Check for proper VLAN assignment on access ports and that the host subnet configuration matches the assigned VLAN.

• Router Configuration Issues: Validate the IP address configurations on router subinterfaces to ensure they are correctly set for their respective VLANs.

Troubleshooting Steps

• Utilize diagnostic commands to identify and resolve issues:

  • Verify VLANs: Use the command show vlan brief to get an overview of VLAN configurations.

  • Check Trunk Status: Employ show interfaces trunk to ascertain the operational status of trunk links.

  • Router Subinterface Issues: Assess router configurations using show ip interface brief to confirm subinterface settings.

4.5 Module Practice and Quiz

Summary of Learning

• Inter-VLAN Routing: A key process essential for enabling communication between VLANs.

• Configuration Options: Understand the differences and applications for legacy routing, router-on-a-stick, and Layer 3 switch SVIs.

• Switch Configuration Steps: Focus on the creation of VLANs and the configuration of access/trunk ports.

• Router Configuration: Each router subinterface must have a unique IP address configured in accordance with its associated VLAN's subnet.

• Layer 3 Switch Benefits: Provide enhanced speed and scalability solutions for complex inter-VLAN routing scenarios.

New Terms and Commands

• Inter-VLAN Routing

• Router-on-a-Stick

• encapsulation dot1q X [native]

• no switchport

• router ospf

• ip routing