A network device that forwards traffic based on layer 3 information at very high speeds. Traditionally, routers, which inspect layer 3, were considerably slower than layer 2 switches. In order to increase routing speeds, many "cut-through" techniques were used, which perform an "inspect the first packet at layer 3 and send the rest at layer 2" type of processing. Ipsilon's IP Switch and Cabletron's SecureFast switches were pioneers in cut-through switching.
From Software to Hardware
As more routing lookup functions were moved from software into the ASIC chips, layer 3 switches could inspect each packet just like a router at high speed without using proprietary cut-through methods. If a layer 3 switch supports packet-by-packet inspection and supports routing protocols, it is called a "routing switch" or "switch router," which simply means "fast router." For example, Cisco calls its high-end routers Gigabit Switch Routers.
More Inspection Takes Time
The more deeply a packet is examined, the more forwarding decisions can be made based upon type of traffic, quality of service (QoS) and so on. To obtain this information requires digging into the packet's headers to ferret out the data, which takes processing time. To understand how packets are formed, see TCP/IP abc's. The following shows which data are examined at each layer. See layer 3, layer 2 switch, Web switch and virtual LAN.
TCP/IP SWITCHING/ROUTING DECISIONS
Layer and Forwarding
Protocol Decision
Inspected Based on
2 - Ethernet MAC address
3 - IP Network address
3 - IP Service quality
4 - TCP/UDP Traffic type
socket (HTTP, FTP, etc.)
7 - HTTP HTTP request type
The BlackDiamond 6800 provided a non-blocking backplane for switching 48 million packets per second. It provided wire-speed IP routing at layer 3 and wire-speed switching at layer 2. (Image courtesy of Extreme Networks, www.extremenetworks.com)
In 1886, this 50-line magneto switchboard, made by Bell Telephone of Canada, was used to switch voice conversations in small localities. These instruments forged a world of switches and routers that forward billions of calls and data packets every day. (Image courtesy of Nortel Networks.)
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