2. Inter-VLAN Routing

Inter-VLAN Routing

The process of forwarding network traffic from one VLAN to another VLAN.

Purpose of Inter-VLAN Routing

Hosts in one VLAN cannot communicate with hosts in another VLAN unless a router or Layer 3 switch provides routing services.

Inter-VLAN Routing Options

Legacy Inter-VLAN routing, Router-on-a-Stick, and Layer 3 switch using Switched Virtual Interfaces (SVIs).

Legacy Inter-VLAN Routing

A method that uses a router with multiple physical Ethernet interfaces, with each interface connected to a different VLAN.

Legacy Inter-VLAN Routing Limitation

It does not scale well because it requires one physical router interface per VLAN.

Legacy Router Interfaces

The default gateways for the local hosts on each VLAN subnet.

Router-on-a-Stick

An Inter-VLAN routing method that uses one physical Ethernet interface with multiple router subinterfaces.

Router-on-a-Stick Advantage

It requires only one physical Ethernet interface to route traffic between multiple VLANs.

Router-on-a-Stick Physical Interface

The router Ethernet interface is configured as an 802.1Q trunk and connected to a trunk port on a Layer 2 switch.

Router Subinterface

A software-based virtual interface associated with a single physical Ethernet interface.

Router Subinterface Naming

Uses the physical interface followed by a period and a subinterface number, such as G0/0/1.10.

Subinterface Number

The VLAN number it normally matches.

Router Subinterface Commands

encapsulation dot1q vlan-id and ip address ip-address subnet-mask.

encapsulation dot1q vlan-id

Configures the subinterface to respond to 802.1Q encapsulated traffic from the specified VLAN.

Native Keyword in encapsulation dot1q

Sets the Native VLAN to something other than VLAN 1.

IP Address on Router Subinterface

Configures the IPv4 address of the subinterface, which typically serves as the default gateway for the VLAN.

Router Subinterface IP Address Requirement

Each subinterface must have an IP address on a unique subnet.

Enable Physical Router Interface

no shutdown

Disabled Physical Router Interface

All router subinterfaces are disabled.

Router-on-a-Stick Traffic Process

VLAN-tagged traffic enters the router, is forwarded to the appropriate subinterface, routed based on the destination IP network, and exits through the appropriate subinterface with the new VLAN tag.

Router-on-a-Stick Scalability

It does not scale beyond 50 VLANs.

Layer 3 Switch

A switch that operates at both Layer 2 and Layer 3 and can perform inter-VLAN routing.

SVI

Switched Virtual Interface; a virtual interface configured on a Layer 3 switch.

SVI Function

Provides Layer 3 processing for packets sent to or from all switch ports associated with that VLAN.

Modern Inter-VLAN Routing for Larger Networks

Using Layer 3 switches and SVIs.

Layer 3 Switch Speed Advantage

They use hardware-based switching and routing.

Layer 3 Switch External Router Advantage

They do not need external links from the switch to a router for routing.

Layer 3 Switch Bandwidth Advantage

Layer 2 EtherChannels can be used as trunk links between switches to increase bandwidth.

Layer 3 Switching Latency Advantage

Data does not need to leave the switch to be routed to a different network.

Layer 3 Switch Disadvantage

They are more expensive.

SVI Creation

interface vlan vlan-id

Routable VLAN Requirement

A Layer 3 SVI must be created for each routable VLAN.

Layer 3 Switch Inter-VLAN Configuration Steps

Create the VLANs, create the SVI VLAN interfaces, configure access ports, and enable IP routing.

Enable IPv4 Inter-VLAN Routing

ip routing

SVI IP Address

The default gateway for hosts in the respective VLAN.

Routed Port

A Layer 3 interface on a Layer 3 switch that is similar to a physical interface on a Cisco IOS router.

Convert Switchport to Routed Port

no switchport

Layer 3 Switch Routing Configuration

Configure the routed port, enable routing, configure a routing method, verify routing, and verify connectivity.

Routing Table Verification

show ip route

Connectivity Verification

ping

Router-on-a-Stick Verification

show ip route, show ip interface brief, show interfaces, and show interfaces trunk.

First Inter-VLAN Troubleshooting Check

Check the physical layer and make sure cables are connected to the correct ports.

Missing VLAN Causes

The VLAN was not created, was accidentally deleted, or is not allowed on the trunk link.

Deleted VLAN Effect

Ports assigned to that VLAN become inactive and remain associated with it until assigned to a new VLAN or the VLAN is recreated.

VLAN Membership Verification

show interface interface-id switchport

Common Router-on-a-Stick Problem

A misconfigured trunk port.

Router-to-Switch Trunk Verification

show interface trunk

Specific Switch Interface Configuration

show running-config interface X

Switch Access-Port Problem Indicator

The PC has the correct IP address, subnet mask, and default gateway but cannot ping its default gateway.

Switch Access-Port VLAN Verification

show vlan brief, show interface X switchport, and show running-config interface X

Router-on-a-Stick Configuration Issues

Incorrect router subinterface IPv4 address or incorrect VLAN assignment on the router subinterface.

Router Subinterface Status Verification

show ip interface brief

Router Subinterface VLAN Verification

Use the show interfaces command and examine the 802.1Q information.