What are the disadvantages of using the router-on-a-stick inter-VLAN routing method in a large network?

As a network administrator, tasks may include having to segment a single switched Local Area Network (LAN) logically without having to run new cables and changing the existing network. The concept of “VLAN” and its configurations will help one handle the task at hand with ease.

In this article, we will learn about what inter-VLAN routing is, various methods for VLAN routing, and also build a network configuration to understand how it works.

Table of contents

Prerequisite

As a prerequisite, the reader must have a good understanding of basic networking concepts, and how to work with Cisco packet tracer.

To download and install Cisco packet tracer on your computer, visit this link.

VLAN routing terminologies

• VLAN is a network segment on a switched LAN which groups together the hosts on the network, logically, regardless of their physical locations on the network.
• A Local Area Network (LAN) is a physical interconnection of network devices in a small geographical area.
• Layer 3 switch/multilayer switch is a special type of switch which performs the function of both switches at layer 2 and routers at layer 3 of the OSI model i.e., it can forward both the frames and packets.
• Switched Virtual Interface (SVI) refers to a virtual interface which connects VLANs on the network devices to their respective router engines.
• Default Gateway is a device that forwards IP packets and provides an access point to the other networks. For instance, a router.
• Access Point is a network device that connects to a router or switch, creating a wireless local area network and acting as a link between the router and the network users.
• Switch port refers to a physical opening on the switch or router where ethernet cables can be plugged.
• Router Interface refers to a path that enables the router to connect to the network.

What is Inter-VLAN routing?

Virtual LANs (VLANS) are networks segments on a switched LAN. Inter-VLAN routing refers to the movement of packets across the network between hosts in different network segments.

VLANs make it easier for one to segment a network, which in turn improves the performance of the network and makes it more flexible, since they are logical connections.

VLANs act as separate subnet on the network. To move packets from one VLAN to another and enable communications among hosts, we have to configure the VLAN network.

Inter-VLAN routing methods

Legacy inter-VLAN routing

In this method, multiple router interfaces are used, each connecting to a switch port in different VLANs. These interfaces are served as default gateways, which requires additional cabling when the network has to be expanded.

Hence, adding additional network cables and improving infrastructure is more expensive.

Router-on-a-stick

In this method, unlike the legacy routing, one physical interface port is used for routing the traffic between the network segments. The network administrator doesn’t need to create separate VLAN interfaces like fa0/1 to fa0/10.

Instead, all the interfaces from 1 to 10 are created with a single interface. This method is simple to implement and used for small to medium-sized networks.

Layer 3 switch using Switched Virtual Interface (SVI)

Currently, this method of inter-VLAN routing that uses layer 3/multilayer switch and Switched Virtual Interfaces (SVI) is the most preferred.

SVIs are created for VLANs exists on the switch which performs the same function for the VLANs as that of a router.

Layer 3 switches are expensive, which are primarily suitable for large organization networks.

Network configurations for Inter-VLAN communication using Router-On-Stick method

In this article, we will learn how to configure inter-VLAN routing using the router-on-a-stick method.

Consider a LAN with 4 PCs, 1 switch, and a router connected as shown in the image:

Now, we have to configure two VLANs 10 and 20, with PC0 and PC1 on VLAN10, and PC2 and PC3 are on VLAN20.

• IP Address of PC0 - 192.168.1.10
• IP Address of PC1 - 192.168.1.20
• IP Address of PC2 - 192.168.2.10
• IP Address of PC3 - 192.168.2.20
• The default gateway for VLAN10 - 192.168.1.1
• The default gateway for VLAN20 - 192.168.2.1

Step 1

For us to subdivide the network into two subnets, we have to create two VLANS on the switch, VLAN10 and VLAN20. Give them custom names like VLAN 10 - student and VLAN 20 - staff.

To create two VLANs, we enter the configuration mode using the config terminal command, and then we enter the VLAN number like vlan 10 along with the name.

Switch>enable !moving from user exec mode to priviledge mode Switch#config terminal !moving from priviledge mode to global configuration mode Switch(config)#vlan 10 !assigning vlan number Switch(config-vlan)#name student !assigns the vlan 10 the name student Switch(config-vlan)#exit Switch(config)#vlan 20 Switch(config-vlan)#name staff !assigns vlan 20 the name staff Switch(config-vlan)#exit

Step 2

Assign switch ports to the VLANS. Ports fa0/1 and fa0/2 acting as access ports for VLAN10, while ports fa0/3 andfa0/4 for VLAN20.

We shall use fa0/5 port for the trunk port for carrying the traffic between the two VLANS via the router.

NOTE: fa refers to fast ethernet ports used for connecting the network hosts to the switch or router.

Configurations for access ports fa0/1 and fa0/2

Switch>enable !moving from user exec mode to the priviledge mode Switch#config terminal !moving from priviledge mode to the global configuration mode Switch(config)#int fa 0/1 !entering the interface port Switch(config-if)#switchport mode access !making the interface fa0/1 an access port Switch(config-if)#switchport access vlan 10 !making interface access port for vlan 10 Switch(config-if)#exit !exiting from the interface Switch(config)#int fa 0/2 !entering the interface port Switch(config-if)#switchport mode access !making the interface fa0/2 an access port Switch(config-if)#switchport access vlan 10 !making interface access port for vlan 10 Switch(config-if)#exit !exiting from the interface

In the configuration above, fa0/1 and fa0/2 are configured as access ports using the command switchport mode access.

Since they belong to vlan10, the switchport access vlan 10 command is used to configure them as access ports within vlan10.

Configurations for access ports fa0/3 and fa0/4

Switch(config)#int fa 0/3 !entering the interface port Switch(config-if)#switchport mode access !making the interface fa0/3 an access port Switch(config-if)#switchport access vlan 20 !making interface access port for vlan 10 Switch(config-if)#exit !exiting from the interface Switch(config)#int fa 0/4 !entering the interface port Switch(config-if)#switchport mode access !making the interface fa0/4 an access port Switch(config-if)#switchport access vlan 20 !making interface access port for vlan 10 Switch(config-if)#exit

In the configuration above, fa0/3 and fa0/4 are configured as access ports using the command switchport mode access.

Since they belong to vlan20, the switchport access vlan 20 command is used to configure them as access ports within vlan20.

Configurations for trunk port fa0/5

Switch(config)#int fa 0/5 !entering the interface port Switch(config-if)#switchport mode trunk !making interface a trunk port. Switch(config-if)#do write !saving the running configurations to start-up file

From the above code interface, fa0/5 is serving as our trunk port. To configure it to serve as a trunk port and not an access port, we use the command switchport mode trunk in the global interface mode.

Step 3

Using static IP addressing, set the IP addresses to static on each PC on the network.

Step 4

Configure the router to enable the traffic to move from VLAN10 to VLAN20. For the PCs to communicate, we subdivide the single interface into many sub-interfaces, where each sub-interface will act as the default gateways for each of the VLANs. This will allow two sub networks to communicate using the single interface.

Router>enable !moving from user exec mode to the priviledge exec mode Router#config terminal !moving from priviledge exec mode to the global configuration mode Router(config)#int g0/0 !entering on our physical router interface gigabitEthernet 0/0 Router(config-if)#no shutdown !activating the interface Router(config-if)#int g0/0.10 !first sub interface for vlan 10 on g0/0 Router(config-subif)#encapsulation dot1q 10 !configuring the sub interface to respond to traffic from vlan 10 Router(config-subif)#ip add 192.168.1.1 255.255.255.0 !configuring the IP address of the sub interface g0/0.10 Router(config-subif)#exit !exiting from the sub interface Router(config)#int g0/0 !entering our physical router interface Router(config-if)#no shutdown !activating our physical interface Router(config-if)#int g0/0.20 !second sub interface for vlan 20 on g0/0 Router(config-subif)#encapsulation dot1q 20 !configuring the sub interface to respond to traffic from vlan 20 Router(config-subif)#ip add 192.168.2.1 255.255.255.0 !configuring the IP address of the sub interface g0/0.20 Router(config-subif)#exit Router(config)#do write !saving our running configurations into the start-up configuration file Router(config)#exit

From the configurations above, the interface g0/0 is subdivided into two sub interfaces: g0/0.10 for VLAN10 and g0/0.20 for VLAN20.

Then, the two sub-interfaces are assigned IP addresses and serve as the trunk ports for carrying the traffic.

Step 5

Test the inter-VLAN connectivity by trying to ping the different PCs.

For instance, if we ping PC2 in VLAN20 from PC0 in VLAN10, it should be successful as shown below:

Conclusion

Inter-VLAN routing is an essential concept for anyone who has the passion for networking. It is the convenient and better way to subdivide a large LAN and to enable communication between the network hosts.

You can find more information about inter vlan routing here.

To summarize:

• We learned what VLAN routing is.
• We explored different methods to route.
• We configured the route using route-with-stick method.

Happy coding.

Peer Review Contributions by: Srishilesh P S