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.
Multilayer switch 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).
Layer-2 switching Layer-2 switching uses the MAC addresses of the hosts’ network interface controllers (NICs) to decide where to forward frames. Layer-2 switching is hardware-based, which means switches use ASICs to build and maintain the forwarding information base and to perform packet forwarding at wire speed. One way to think of a layer-2 switch is as a multiport bridge. Layer-2 switching is highly efficient because there is no modification to the frame required. Encapsulation of the packet changes only when the data packet passes through dissimilar media (such as from Ethernet to FDDI). Layer-2 switching is used for workgroup connectivity and network segmentation (breaking up collision domains). This allows a flatter network design with more network segments than conventional networks joined by repeater hubs and routers. Layer-2 switches have the same limitations as bridges. Bridges break up collision domains, but the network remains one large broadcast domain which can cause performance issues and limits the size of a network. Broadcast and multicasts, along with the slow convergence of spanning tree, can cause major problems as the network grows. Because of these problems, layer-2 switches cannot completely replace routers. Bridges are good if a network is designed by the 80/20 rule: users spend 80 percent of their time on their local segment.
Layer-3 switching A layer-3 switch can perform some or all of the functions normally performed by a router. Most network switches, however, are limited to supporting a single type of physical network, typically Ethernet, whereas a router may support different kinds of physical networks on different ports. Layer-3 switching is solely based on the (destination) IP address stored in the header of an IP datagram (layer-4 switching may use other information in the header). The difference between a layer-3 switch and a router is the way the device is making the routing decision. Conventionally, routers use microprocessors to make forwarding decisions in software, while the switch performs only hardware-based packet switching (by specialized ASICs with the help of content-addressable memory). However, many routers now also have advanced hardware functions to assist with forwarding. The main advantage of layer-3 switches is the potential for lower network latency as a packet can be routed without making extra network hops to a router. For example, connecting two distinct segments (e.g. VLANs) with a router to a standard layer-2 switch requires passing the frame to the switch (first L2 hop), then to the router (second L2 hop) where the packet inside the frame is routed (L3 hop) and then passed back to the switch (third L2 hop). A layer-3 switch accomplishes the same task without the need for a router (and therefore additional hops) by making the routing decision itself, i.e. the packet is routed to another subnet and switched to the destination network port simultaneously. Because many layer-3 switches offer the same functionality as conventional routers they can be used as cheaper, lower latency replacements in some networks. Layer-3 switches can perform the following actions that can also be performed by routers:
determine paths based on logical addressing check and recompute layer-3 header checksums examine and update time to live (TTL) field process and respond to any option information update Simple Network Management Protocol (SNMP) managers with Management Information Base (MIB) information The benefits of layer-3 switching include the following:
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