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Concept
Multilayer switch
Summary
A multilayer switch (MLS) is a computer networking device that switches on OSI layer 2 like an ordinary network switch and provides extra functions on higher OSI layers. The MLS was invented by engineers at Digital Equipment Corporation.
Switching technologies are crucial to network design, as they allow traffic to be sent only where it is needed in most cases, using fast, hardware-based methods. Switching uses different kinds of network switches. A standard switch is known as a layer 2 switch and is commonly found in nearly any LAN. Layer 3 or layer 4 switches require advanced technology (see managed switch) and are more expensive and thus are usually only found in larger LANs or in special network environments.
Multi-layer switching combines layer 2, 3 and 4 switching technologies and provides high-speed scalability with low latency. Multi-layer switching can move traffic at wire speed and also provide layer 3 routing. There is no performance difference between forwarding at different layers because the routing and switching are all hardware-based - routing decisions are made by specialized application-specific integrated circuits (ASICs) with the help of content-addressable memory.
Multi-layer switching can make routing and switching decisions based on the following
MAC address in a data link frame
Protocol field in the data link frame
IP address in the network layer header
Protocol field in the network layer header
Port numbers in the transport layer header
MLSs implement QoS in hardware. A multilayer switch can prioritize packets by the 6 bit differentiated services code point (DSCP). These 6 bits were originally used for type of service. The following 4 mappings are normally available in an MLS:
From OSI layer 2, 3 or 4 to IP DSCP (for IP packets) or IEEE 802.1p
From IEEE 802.1p to IP DSCP
From IP DSCP to IEEE 802.1p
From VLAN IEEE 802.1p to port egress queue.
MLSs are also able to route IP traffic between VLANs like a common router. The routing is normally as quick as switching (at wire speed).
Official source
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