Layer 3 Forwarding Using a Layer 3 Switch

Last Updated on Fri, 30 Jun 2023 | Data Link

The term Layer 3 switch (L3 switch) refers to a switch that also has routing features. So, instead of requiring a router external to the switch, as in Figure 10-8, the router internal to the switch performs the same routing function.

The only difference between routing using a router, as in Figure 10-8, and using a Layer 3 switch lies in the internal processing. Outwardly, nothing is different. For instance, Figure 10-9 shows routing and L3 switching between two interfaces in two different VLANs. If you were to put a LAN analysis tool at the points shown in each of the two topologies and compare the packets being forwarded between the two, you would see no difference.

Figure 10-9 Analysis Points Showing No Difference Between L3 Switching and Routing

Figure 10-9 Analysis Points Showing No Difference Between L3 Switching and Routing

Diagram Using Layer Switches
L3 Switch

VLAN 1 VLAN 2

By tracing the two similar networks at the points shown, you can confirm that there are no differences to the effect of the external router versus the L3 switch. The L3 switch runs routing protocols and builds an IP routing table, and the switch makes the forwarding decision based on destination IP address. The L3 switch even discards the only Ethernet data link header and builds a new one, as described in Chapter 5, "Fundamentals of IP."

The differences between the two options relates to what happens inside the L3 switch. L3 switches used specialized hardware to make the forwarding process run very fast. The switch ASICs (Application Specific Integrated Circuits) on an L3 switch have been built so that the normal, very fast L2 forwarding path can also be caused to perform the forwarding for Layer 3. In other words, the actual receipt, changing of headers, and forwarding of the packets uses the same high-speed internal processing of the L2 switch. The L3 switch also includes the software used to run other processes, such as routing protocols, which are used to populate the tables used by the specialized forwarding hardware. You do not need to know the specifics of how any one single L3 switch works internally; just know that the difference between L3 switching and routing is based on what happens inside the L3 switch.

Continue reading here: Layer 4 Switching

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  • Layer 3 Forwarding Using a Layer 3 Switch - Data Link 2
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Readers' Questions

  • giordano
    What does an l3 device use to make decisions on where to forward a packet?
    1 month ago
    • An L3 (Layer 3) device, such as a router or a layer 3 switch, uses the destination IP address in the packet's header to make decisions on where to forward the packet. It analyzes the destination IP address and consults its routing table to determine the next hop or outgoing interface for the packet. The routing table contains information about network destinations and their corresponding next hops or interfaces. By matching the destination IP address with the entries in the routing table, the L3 device determines the best path or interface to forward the packet to its destination.
    • mentha puddifoot
      What is layer3 forwarding(1)?
      1 month ago
      • Layer 3 forwarding refers to the process of routing network traffic at the network layer (Layer 3) of the OSI model. It involves making decisions based on the IP addresses of the source and destination devices in order to determine the best path for the data packets to travel through a network. Layer 3 forwarding is typically performed by routers, which examine the IP packet headers and use routing tables to make forwarding decisions. This process enables the packets to be directed to the correct destination across multiple interconnected networks within an internetwork.
      • hubert
        What is the difference between l2 and l3 switch?
        8 months ago
        • L2 switches are layer 2 devices that process Ethernet frames and provide basic switching functions. They are capable of forwarding traffic based on layer 2 information such as MAC addresses. L3 switches, or multi-layer switches, are layer 3 devices that perform the same switching functions as L2 switches but are also capable of performing routing functions. They can forward traffic based on layer 3 information such as IP addresses.
        • rio cunningham
          What is the difference between routing and forwarding?
          8 months ago
          • Routing is the process of selecting paths in a network along which to send network traffic. Routing is performed by specialized devices called routers, which use routing protocols to determine the best path for forwarding the packets. Forwarding, on the other hand, is the process of actually sending the packets along the chosen path. This is typically done by switches that examine the destination address of each packet and forward it to the appropriate outgoing interface.
          • billie aitken
            What is the function of asics in a multilayer switch?
            8 months ago
            • ASICs (Application Specific Integrated Circuits) are specialized chips used in multilayer switches to provide higher switching speeds while reducing the overall power consumption of the switch. They are used to analyze and process data in high-speed Ethernet switches, provide high-speed data routing, switching, and forwarding capabilities, and help with data traffic management tasks such as rate limiting and error management.