Multilayer switch

The term Layer 3 switch is a bit misleading, because there are multi -function devices, which are a combination of router and switch. The term Layer 3 switch is usually for advanced high-performance switches that have been added to the IP routing functionality (and vice versa). Locations for Layer 3 switches are areas where especially high performance ( high throughput of the router) is required. The emphasis on the switch functionality has its roots in marketing: You did not want to be associated with (according to marketing beliefs ) old-fashioned, complicated and cumbersome routers (and their manufacturers ) in conjunction.

Function

When routing, the forwarding decision is based on an OSI Layer 3 information about an IP address taken. When switching on the other hand, a hardware-assisted packet processing, the data packets based on MAC addresses ( OSI Layer 2 ) are forwarded. When switching is more or less a colloquial term without clear distinction to ( multi-port ) Bridges. A Layer 3 switch can do both, he can assign individual ports to different domains ( subnets ) and work within those domains as a switch, but in addition he can also do the routing between these domains ( together with the necessary to routing protocols ).

Brouter

Devices that combine the functions of bridges and routers, also called brouter. However, a Layer 3 switch is not exactly the same ( see main article brouter ).

Construction

Because routers and (Store & Forward ) switches do something very similar ( they receive and store packages, make decisions and then forward the packets ), it was obvious that the manufacturers would develop combined devices. From the lineage of manufacturers are two basic types of bodywork result, either it is a by use of specific hardware ( ASICs) accelerated router or extended in its functionality switch. Some manufacturers switches can be optionally extended by special plug- in expansion port modules for routing functions (eg 3Com SuperStack 3300). For other systems, optional firmware upgrades are available.

Particularly clear (in a positive and negative sense ) is the concept of Layer 3 switches in the old models of the modular Catalyst series of the manufacturer Cisco: Originally as a pure Layer 2 switches with its own operating system ( " Cat - OS" ) and designed modularly scalable port density, these devices could then be offset by subsequent installation of a so-called routing module in a position on specific ports ( or port groups) to route directly. This meant to put it simply, that certain ports of the switch had no longer the previous " end switch ", but a " nonrouting " property, that is, the device could speak directly with the Internet Protocol connected to this other routed port devices. It was thus on each other routed port (or port group ) is now reachable with its own IP - address within the IP networks are connected to these ports and mediated only via the IP Layer 3 between these and other other routed ports on the device.

In practice, this scenario looked so from that suddenly two separate devices were (more or less) taken in a chassis in operation with the use of the router module in the switch: The actual pure Layer 2 switch ( still with CatOS operating system), but now have 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 and 2, IOS for routing at Layer 3 Logically speaking communicated both subsystems through a kind of virtual LAN connections to one another, but physically they used for communication initially, so that the overall performance ( at the beginning ) could not keep up with the "pure" routers only the backplane of the common chassis '. This concept of the interaction between Cat - OS and IOS is the Cisco marketing world known as " hybrid mode ", but precisely in this concept, the term " layer 3 switch " particularly clear: In essence, such a device continues to be a switch, the router setup was ultimately "superstructure " is added only as a kind.

This process was greatly enhanced by the manufacturer over time and modified - today's modern equipment of the modular Catalyst series run solely with the IOS operating system (ie in the "native -mode" ), as are the two components were largely fused together, so that modern devices of this series now can route or switchen over multiple 10 - Gigabit ports with no problems. Depending on the purpose and stage of development can be such a device, therefore now easily order both as a pure router or as a pure switch as well as a mixed form and operate, resulting in still occasionally confusion for potential customers.

Manufacturer of the competition thus refer to like the " grown" structures of these devices while Cisco itself the flexibility and performance of its Catalyst series priced.

Special

Cut -Through and Store- and -Forward

In addition to the additional switching functionality a Layer 3 switch can begin with the forwarding of a routed packet in contrast to the classical router before this was completely read, according to a switch in cut-through mode of operation. However, devices of this design have on the market no meaning, almost all devices operate in the proven Store & Forward mode.

Route caching

Layer 3 switches in the upper performance segment dominate depending on the manufacturer and model an additional performance-optimized routing hybrid mode, which is also referred to as route caching or Cisco as multilayer switching ( MLS). Here, the router of the Layer 3 switches makes the decision where a packet should be forwarded, only the first packet of a packet sequence, the determined route (~ destination MAC address of the next router ) then, quite similar to the switching, in a stored table and used for all subsequent packets with the same destination. Subsequent packets received so immediately by the switching engine of the Layer 3 switches have the same target MAC address as the first packet and geswitcht directly to the corresponding output port. Only after the end of the session, but after a timeout or a change in routing tables, etc., a regular route decision is made again. By this method achieve Layer 3 switches, especially with large routing tables extremely high throughput rates, as they are otherwise typical only for Layer 2 switches.