Today's modern and complex networks require innovative solutions, and one of them is a Cisco Layer 3 switch. Also known as a multilayer switch, this is a network device that combines the functions of a traditional Layer 2 switch with the routing capabilities of a Layer 3 router. Due to this combined functionality, these switches operate at both Layer 2 and Layer 3 of the OSI model. Essentially, this means that the Cisco Layer 3 switch acts as both a switch and a router.
At Layer 2, this switch performs traditional functions like Ethernet frame forwarding, MAC address learning, and Virtual Local Area Network (VLAN) management. It uses the MAC addresses of devices connected to its ports to make forwarding decisions and enables devices within the same VLAN to communicate with each other.
At Layer 3, the Cisco switch does the routing functions, like routing IP packets between different IP subnets or VLANs. It also maintains a routing table and uses IP addresses to make routing decisions. This enables communication between devices on different subnets or VLANs within the network.
Why Use a Layer 3 Switch?
The most significant advantage of using a Cisco Layer 3 switch is that it can perform both routing and switching. It can even provide fast inter-VLAN routing that improves network performance. In some cases, using these switches eliminates the need for external routers. Additionally, Layer 3 switches also support advanced features like access control lists, Quality of Service (QoS) management, and even multicast routing.
Undoubtedly, the Cisco Layer 3 switches are a versatile networking solution. They are well-suited for enterprise networks and environments that require efficient inter-VLAN communication and IP routing.
Read on to learn how to configure a Cisco Layer 3 Switch for InterVLAN routing.
How to Configure a Cisco Layer 3 Switch for InterVLAN Routing?
InterVLAN routing is the process where hosts in one VLAN want to communicate with devices in another VLAN. In this section, we will look at a simple inter-VLAN routing, and this can be extended to a multi-switch environment as well.
Below are the steps to configure Cisco layer 3 switches for inter-VLAN routing.
Step 1
As a first step, check if routing is enabled on the switch. Though routing is enabled by default, still, it helps to check. Run the below command.
Switch(config)#ip routing
If the switch doesn't accept the command, you can know that this switch is not capable of routing. The workaround is to upgrade to the Cisco IOS software release 12.1 or later.
Step 2
Now that the switch is ready, move on to the VLANs. Make a note of the VLAN IDs between which you want to enable communication.
If you're unsure of the ID, type the show vlan command to check if the VLANs exist in the database. If they are not in the VLAN database, add them.
The command is:
Switch#vlan database
Switch(vlan)#vlan 200
In the above example, we're adding VLAN 200 to the database.
Step 3
The next step is to assign IP addresses to the VLAN interfaces on the switch. This is an important step because whenever a data packet reaches a VLAN, the switch looks into the routing table to decide where it should forward the packets. Accordingly, the packet is sent to the next VLAN interface, and eventually, the data packet reaches its destination.
Here's an example of how you can assign an IP address to a VLAN switch.
Switch(config)#interface Vlan200
Switch(config-if)#ip address 20.3.3.5 0.0.255.255
Follow this process for all the VLANs that you want to connect.
Step 4
Next, configure the interface to the default router of your network. Let's assume that you have a FastEthernet port.
Switch(config)#interface FastEthernet 0/1
Switch(config-if)#no switchport
Switch(config-if)#ip address 192.10.8.8 0.0.255.255
Note that the “no switchport” command enhances the capabilities of the layer 3 switch. Also, check if the IP address is in the subnet of the default router.
Step 5
In this step, configure the default router for the layer 3 switch. Here's the command for it.
Switch(config)#ip route 0.0.0.0 0.0.0.0 192.10.8.8
Let's break down this command a bit. In this case, the IP address of the default router is 192.10.8.8. When you execute this command, the switch checks the routing table. If the IP address is not in the table, it routes the packet to the default gateway.
With this, your configuration is done.
Step 6
In the last step, you can verify if your configuration is working the way it should. To check this, enter the command
show ip route
This command provides a list of the routing table entries.
Check if the routing table has an entry for every VLAN interface subnet. If there's an entry for all the VLANs that you want to communicate with, then you're all set to use the router for inter-VLAN communication. Also, with the default gateway configuration, you can ensure that packets not intended for any of the VLANs will be routed to the default gateway for further processing.
Lastly, you can check if the VLAN interfaces and ports are up and running. The command for this is:
show ip interface brief
This command will list a summary of your interface's status.
Thus, these are the steps for configuring a Cisco layer 3 switch for InterVLAN routing.
Bottom Line
Configuring a Cisco Layer 3 switch for InterVLAN routing can enhance network performance. In some cases, you can even do away with external routers. The process of configuring your switch for InterVLAN routing involves many steps.
First, check if routing is enabled on the switch and follow it up by identifying and assigning IP addresses to the VLANs for which you want to enable communication. After creating the VLANs, assigning IP addresses to the VLAN interfaces on the switch is crucial. This step enables the switch to make routing decisions based on the routing table. Next, to connect the switch to the network's default router, configure the interface connected to the router. Also, configure the default router for the layer 3 switch. This ensures that any packets not intended for the configured VLANs will be routed to the default gateway for further processing. Finally, verify the configuration and check if your VLAN interfaces are up and running.
The above steps can successfully configure a Cisco Layer 3 switch for your InterVLAN routing. Undoubtedly, the Cisco layer-3 switch offers a flexible and powerful networking solution that combines the capabilities of routing and switching in a single device.
