Layer 3 switch
The term layer 3 switch is a multifunctional device that is a combination of router and switch . Layer 3 switches mostly stand for advanced high-performance switches that have been expanded to include IP routing functionality (and vice versa). Locations for this are areas where high performance (high data throughput of the router) is required. The emphasis on switch functionality has its roots in marketing: You don't want to be associated with (according to marketing beliefs) old-fashioned, complicated and clumsy routers (and their manufacturers).
function
With routing, the forwarding decision is made on the basis of OSI layer 3 information, such as an IP address. With switching, however, a hardware-supported packet processing, the data packets are forwarded on the basis of MAC addresses (OSI layer 2). Switching is more or less a slang term without a clear distinction from (multiport) bridges . A layer 3 switch can do both, it can assign individual ports to different domains (subnets) and work as a switch within these domains, but it also masters the routing between these domains (including the necessary routing protocols).
BRouter
Devices that combine the function of bridges and routers are also called BRouters . However, a layer 3 switch is not the same as it usually does not implement the multi-protocol functionality of a bridge, for example.
construction
Since routers and ( store & forward ) switches do something very similar (they receive and store packets, make decisions, and then forward the packets), it was obvious that manufacturers would develop combined devices. The origins of the manufacturer result in two basic types of structure, either a router accelerated through the use of specific hardware (ASICs) or a switch with expanded functionality. With some manufacturers, switches can optionally be expanded to include routing functions using special modules that can be plugged into expansion ports (e.g. 3Com SuperStack 3300). Optional firmware upgrades are available for other systems .
The concept of the layer 3 switch becomes particularly clear (in a positive as well as in a negative sense) in old models of the modular Catalyst series from the manufacturer Cisco: Originally as a pure Layer 2 switch with its own operating system ("Cat OS") and Designed with a modular scalable port density, these devices could then be enabled to route directly to certain ports (or port groups) by adding a so-called routing module. Put simply, this meant that certain ports of the switch no longer had the previous "switch end", but rather a "routing" property; that is, the device could talk directly to the Internet protocol with devices connected to this routing port. It could now be reached on each routing port (or port group) with its own IP address within the IP networks connected to these ports and only switched between these and other routing ports on the device via IP layer 3.
In practice, this scenario was such that when the router module was used in the switch, two (more or less) separate devices were suddenly put into operation in one chassis: The actual, pure Layer 2 switch (still with CatOS Operating system), but now an additional layer 3 routing module with its own CPU and operating system ("Internetwork Operating System", IOS). The division of tasks between the two subsystems was originally strictly separated: Cat-OS took care of the management of layers 1 to 2, IOS took care of the routing on layer 3. From a logical point of view, both subsystems communicated with each other via a kind of virtual LAN connection, but physically they used them originally only the backplane of the shared chassis was used for communication, so that the overall performance (at the beginning) could not keep up with that of "pure" routers. This concept of the interaction of Cat OS and IOS became known in the Cisco marketing world as “hybrid mode”, but it is precisely with this concept that the term “layer 3 switch” becomes particularly clear: at its core is such a device still a switch, the router structure was ultimately only added as a kind of "superstructure".
This process has been greatly developed and modified by the manufacturer over the course of time - today's modern devices in the modular Catalyst series run solely with the IOS operating system (ie in "native mode"), and the two components have largely been merged, so that modern devices of this series can now route or switch over several 10 gigabit ports without any problems. Depending on the purpose and level of expansion, such a device can therefore be ordered and operated nowadays both as a pure router and as a pure switch as well as a mixed form, which still leads to confusion among potential customers now and then.
Competitors manufacturers like to refer to the "grown" structures of these devices, while Cisco itself praises the high flexibility and performance of its Catalyst series.
particularities
Cut-Through and Store - & - Forward
In addition to the additional switching functionality, a layer 3 switch, in contrast to the classic router, can already start forwarding a routed packet before it has been fully read, corresponding to a switch in cut-through operating mode. Devices of this design are of no importance on the market, almost all devices work in the tried and tested store & forward mode.
Route caching
Layer 3 switches in the upper performance segment also have a performance-optimized routing hybrid mode, depending on the manufacturer and model, which is also known as route caching or, at Cisco, as multilayer switching (MLS). Here, the router of the layer 3 switch makes the decision as to where a packet should be forwarded to only with the first packet in a packet sequence. The route determined (~ destination MAC address of the next router) is then stored in a table, similar to switching, and used for all subsequent packets with the same destination. Subsequent packets are immediately given the same destination MAC address as the first packet by the switching engine of the layer 3 switch and are switched directly to the associated output port. A regular route decision is only made again after the session has ended, but also after a timeout or when the routing tables are changed, etc. With this method, layer 3 switches achieve enormously high throughput rates, particularly with large routing tables, which are otherwise only typical for layer 2 switches.