Layer 3 Switches in Cisco

A layer 3 Switch is a special type of networking device which is able to perform/execute functions of 2 layers of the OSI Model i.e., the Data Link Layer (Layer 2) and the Network Layer (Layer 3). In simple words, a Layer 3 Switch is a networking device that can perform switching (functions of layer 2) as well as routing (functions of Layer 3). It is able to connect devices that are on the same subnet or the same VLAN (Virtual Local Area Network) to exchange information at a very fast speed which is the work of a traditional switch, and you can also connect devices across different subnets and enable some of the routing protocols which are the work of a router.

As Layer 3 Switches are able to perform the functions of 2 different Layers of the OSI model, they are also known as Multilayer Switches. These switches are capable of taking routing decisions, support routing protocols, and can even inspect the Network Layer for the data frames received, but these Layer 3 Switches don’t have WAN (Wide Area Network) ports which may be required while designing the network. These switches can also perform bridging functions at each interface, contain broadcasts, and they also allow us to group switching interfaces to increase bandwidth. All these features combined make Layer 3 Switches a powerful and scalable device that can be used while designing high-speed Ethernet Backbone Networks.

Working on Layer 3 Switches: 

Layer 3 Switching can be done by either of the two methods:

• Cut-through Switching: In this method, the Layer 3 switch will look into the first packet of the total series of packets to check the destination IP address of the packet and then switches all the remaining packets of the series using the MAC address. This method can result in higher rates of data throughput.
• Packet-by-Packet Layer 3 Switching (PPL3): In this method, the Layer 3 switch will look into every incoming packet to check the destination IP address of the packets. It basically functions as a high-speed router that can perform other functions aside from packet switching across networks, like verifying checksum, updating the packet’s TTL (time-to-live), and processing/adding any other options information in the packet.

Advantages of Layer 3 Switches: 

• You can perform routing functions.
• It reduces broadcasting traffic in the network.
• This makes the configuration and management of VLANs easier (as multiple routers are not required for different VLANs).
• It supports high-speed scalability and simplifies security management.
• It lowers the network latency as a packet doesn’t have to allow extra hops to go through the router.
• This makes troubleshooting the network easier compared to Layer 2-only switch troubleshooting.
• It is less expensive than routers.

Disadvantages of Layer 3 Switches: 

• The cost (in comparison to a traditional switch) is much higher and these switches require extra effort in configuration, administration, and management.
• Lack of WAN functionality is a major concern as if an organization needs to send its traffic outside the company it would need a separate router in order to overcome this situation.

Configuring Layer 3 Interfaces on Layer 3 Switches:

Logical Layer 3 VLAN interfaces: 

L3-Switch(config)#vlan <vlan-id>
L3-Switch(config-vlan)#exit
L3-Switch(config)#int vlan <vlan-id>
L3-Switch(config-if)#ip add <ip-address> <subnet-mask>
L3-Switch(config-if)#no shut

Physical Layer 3 VLAN interfaces: 

L3-Switch(config)#int <interface-id>
L3-Switch(config-if)#no switchport
L3-Switch(config-if)#ip add <ip-address> <subnet-mask>
L3-Switch(config-if)#no shut