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A layer-3 switch incorporates routing capability in addition to the layer-2 bridging found in a standard switch. The major difference between the packet forwarding operation of a router and that of a layer-3 switch is the actual implementation. In general-purpose routers, forwarding is usually implemented in software that runs on a microprocessor or a network processor, whereas a layer-3 switch performs the same operation using dedicated application-specific integrated circuit (ASIC) hardware.
Multilayer switch (MLS) - OSI layer 3 and/or 4
The following 4 mappings are normally available in an MLS:
- From OSI layer 2, 3 or 4 to IP DSCP (if IP packet) and/or VLAN IEEE 802.1p.
- From VLAN IEEE 802.1p to IP DSCP.
- From IP DSCP to VLAN IEEE 802.1p.
- From VLAN IEEE 802.1p to port queue.
Some MLSs are also able to route between VLANs and/or ports like a common router. The routing is normally as quick as switching (at wirespeed). According to Cisco, Layer 3 switches are basically routers that switch based on Layer 3 information, the basic difference being processing speed and/or the way they do the switching; Layer 3 switches use ASICs/hardware instead of the CPU/software that a router would.
Layer 4–7 switch, web switch, or content switch
Some switches can use up to OSI layer 7 packet information; these may be called layer 4–7 switches, content switches, content services switches, web switches or application switches.
Content switches are typically used for load balancing among groups of servers. Load balancing can be performed on HTTP, HTTPS, VPN, or any TCP/IP traffic using a specific port. Load balancing often involves destination network address translation so that the client of the load balanced service is not fully aware of which server is handling its requests. Some of the layer 4–7 switches can perform NAT at wirespeed. Also, content switches can often be used to perform standard operations such as SSL encryption/decryption to reduce the load on the servers receiving the traffic, or to centralise the management of digital certificates. Layer 7 switching is the base technology of a content delivery network.
Layer 4 load balancer
A typical network router simply sends incoming packets onto the appropriate IP address on its network. A layer 4 router, more correctly a NAT with port and transaction awareness, usually performs a form of port translation for sending incoming packets to one or more machines that are hidden behind a single IP address.
The "layer 4" refers to the layer 4 or transport layer of the OSI model. The router operates on the transport layer and makes decisions on where to send the packets. Modern load balancing routers can use different rules to make decisions on where to route traffic. This can be based on least load, or fastest response times, or simply balancing requests out. This is also a redundancy method, so if one machine is not up, the router will not send traffic to it.
- Application delivery controller
- Bridge router
- LAN switching
- Multiprotocol Label Switching (MPLS)
- Network switch
- Residential gateway