Layer 3 switch

Definition of Layer 3 switch in The Network Encyclopedia.

Layer 3 switch

An Ethernet switch that switches packets by looking at both their network address (for example, their IP or IPX address) and their physical address (for example, their MAC address). This type of switch operates at both the network layer (layer 3) and the data-link layer (layer 2) of the Open Systems Interconnection (OSI) reference model. A Layer 3 switch combines the speed of an Ethernet switch with some of the capabilities of a router for building advanced, high-speed Ethernet networks.

How It Works

Layer 3 switches perform on two levels: layer 2 and layer 3. Their layer 3 switching functionality can take one of two forms:

  • Packet-by-Packet Layer 3 (PPL3) switches:
    These switches look into every packet to determine its logical layer 3 destination address (such as its destination IP address). PPL3 switches essentially function as high-speed routers with the routing functionality built into its hardware instead of software. Like routers, besides forwarding a packet to its destination, these switches perform other functions that a standard router performs, such as using the packetís checksum to verify its integrity, updating the packetís Time to Live (TTL) information after each hop, and processing any option information in the packetís header. PPL3 switches typically communicate with each other using a routing protocol such as Routing Information Protocol (RIP) or Open Shortest Path First (OSPF) Protocol so that they become aware of the overall topology of the network.

     

  • Cut-through switches:
    These switches look into only the first packet of a series of packets to determine its logical layer 3 destination address (such as the destination IP address), and then switch the remainder of packets in the series using the layer 2 address (MAC address). This leads to higher data throughput rates.

     

In addition to performing layer 3 switching functions (routing functions), Layer 3 switches perform the functions of Layer 2 switches (bridging functions) at each switch interface. You can group switching interfaces in various ways to allocate bandwidth and contain broadcasts, which makes Layer 3 switches a powerful, scalable technology for building high-speed Ethernet backbone networks.

TIP

You can deploy a Layer 3 switch in your network anywhere a traditional router is used. The following table shows some of the differences between a traditional router and a Layer 3 switch.

Traditional Router vs. Layer 3 Switch

Feature Router Layer 3 Switch
Local area network (LAN) protocols supported
IP, IPX, AppleTalk
IP, IPX, AppleTalk
Packet-forwarding method
Software-based
Hardware-based
Throughput
Lower
Higher
Definition of subnet
Per port
Per layer 2 switching domain
Support for policy-based routing
Less
More
Relationship with bridges
Peer
Layered
Cost
Higher
Lower


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