InterVLAN Routing

Overview

VLANs control broadcast domain size and keep local traffic local.

Key components of InterVLAN Routing

• Requirements for InterVLAN Routing

• VLAN capable switch• Router: External or integrated

within a switch• Router to VLAN connection

VL

AN

30

VL

AN

20

VL

AN

10

InterVLAN Routing Using a Route Processor

InternalRoute Processor

Layer 3 Switch

VL

AN

30

VL

AN

20

VL

AN

10

InterVLAN Routing Using a single link

VLAN 30VLAN 20VLAN 10

InterVLAN Routing Using one link per VLAN

External Routers

• Router-on-a-Stick• Trunk Link Advantages:

– Saves router and switch ports…saves money and reduces complexity.

– Scales to larger number of VLANs than one-link-per-VLAN design.

• Trunk Link Disadvantages:– Additional router overhead– Older IOS versions support

limited sets of features on trunked interfaces and may only support ISL trunking encapsulation.

Physical interfaceFastEthernet 0/0

Logical interfaceFastEthernet 0/0.10ip address 192.168.1.1

Logical interfaceFastEthernet 0/0.20ip address 192.168.2.1

Logical interfaceFastEthernet 0/0.30ip address 192.168.3.1

VLAN 30192.168.3.0

VLAN 20192.168.2.0

VLAN 10192.168.1.0

InterVLAN Routing using a single link

Trunk

Cisco Layer 3 Feature Cards

Catalyst 6000 Family Multilayer Switch Feature Card (MSFC2) Router-on-a-stick most

appropriate when other options not available.

Route switch processors generally adequate unless a high proportion of traffic crosses VLAN boundaries or a large number of gigabit or greater interfaces are present.

Switch Fabric Module adds switching capacity.

Configuring InterVLANRouting (1)

• First task to turn on switch routing functionality.– Switch(config)# ip routing

• Switch virtual interface (most common).– Switch#interface vlan 40– Switch(config-if)# ip address 10.0.40.1 255.255.255.0

• Routed port (turns an Ethernet switch into an Ethernet Router by turning off a switch port functionality).– Switch(config)# interface fa 0/1– Switch(config-if)# no switchport– Switch(config-if)# ip address 10.0.1.1 255.255.255.0

• The switch port can now be used as a physical router port for connection to an external device such as a router.

Configuring InterVLANRouting (2)

SiSi

Fa 0/1 Fa 0/0

WAN

Ext RouterS 0/0

Routed port

Connecting an external router to aroute switch processor

• RSP(config)#ip routing

• RSP(config)# interface fa 0/1

• RSP(config-if)# no switchport

• RSP(config)-if# ip address 10.0.1.1 255.255.255.0

• Ext(config)# interface fa0/0

• Ext(config-if)# ip address 10.0.1.2 255.255.255.0

InterVLAN routing most likely achieved through a virtual interface. As an example, VLAN 10 and VLAN 20 configured on the multi-layer switch.

RSP(config)# interface vlan 10

RSP(config-if)# ip address 10.0.10.1 255.255.255.0

RSP(config)# interface vlan 20

RSP(config-if)# ip address 10.0.20.1 255.255.255.0

Don’t forget to enable a routing protocol on both the RSP and External router.

Configuring InterVLAN Routing (3)

Physical interfaceFastEthernet 0/0

Logical interfaceFastEthernet 0/0.10ip address 192.168.1.1

Logical interfaceFastEthernet 0/0.20ip address 192.168.2.1

Logical interfaceFastEthernet 0/0.30ip address 192.168.3.1

VLAN 30192.168.3.0

VLAN 20192.168.2.0

VLAN 10192.168.1.0

Router-on-a-stick configuration example

Trunk

Verifying InterVLAN Routing Configurations

SiSi

Fa 0/1 Fa 0/0

WAN

Ext RouterS 0/0

Routed port

Verifying interVLAN routing

10.0.10.0 / 24VLAN 10

10.0.20.0 / 24VLAN 20

• Switch# show ip route• Switch# show ip interface brief• Switch# show ip interface

fastethernet module/port– Can show if the port is

configured for routing• Switch# show ip interface

fastethernet module/port switchport– Either switchport information

will be displayed or “Switchport: Disabled” will indicate the port is likely configured for routing.