Layer 2 and Layer 3 Switching


Layer 2 and Layer 3 Switching
In this topic, you will review the concept of Layer 2 switching and learn about Layer 3 switching.
A Layer 2 LAN switch performs switching and filtering based only on the OSI data link layer (Layer 2) MAC address. A Layer 2 switch is completely transparent to network protocols and user applications. Recall that a Layer 2 switch builds a MAC address table that it uses to make forwarding decisions.
A Layer 3 switch, such as the Catalyst 3560, functions similarly to a Layer 2 switch, such as the Catalyst 2960, but instead of using only the Layer 2 MAC address information for forwarding decisions, a Layer 3 switch can also use IP address information. Instead of only learning which MAC addresses are associated with each of its ports, a Layer 3 switch can also learn which IP addresses are associated with its interfaces. This allows the Layer 3 switch to direct traffic throughout the network based on IP address information.
Layer 3 switches are also capable of performing Layer 3 routing functions, reducing the need for dedicated routers on a LAN. Because Layer 3 switches have specialized switching hardware, they can typically route data as quickly as they can switch.
Layer 3 information in an Ethernet packet to make forwarding decisions. Layer 3 switches can route packets between different LAN segments similarly to dedicated routers. However, Layer 3 switches do not completely replace the need for routers on a network.
Routers perform additional Layer 3 services that Layer 3 switches are not capable of performing. Routers are also capable of performing packet forwarding tasks not found on Layer 3 switches, such as establishing remote access connections to remote networks and devices. Dedicated routers are more flexible in their support of WAN interface cards (WIC), making them the preferred, and sometimes only, choice for connecting to a WAN. Layer 3 switches can provide basic routing functions in a LAN and reduce the need for dedicated routers.

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