Editing Multilayer switch (section)

== 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 [[Datagram|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).<ref name=switchoperation>{{cite web |url=http://www.ciscopress.com/articles/article.asp?p=101629&seqNum=4 |title=Switch Operation for the CCNP BCMSN Exam | last1=Hucaby |first1=David |date=Oct 24, 2003 |website=ciscopress.com |publisher=Cisco Press |accessdate=2015-02-05}}</ref><ref>{{cite web|url=http://www.cisco.com/en/US/tech/tk389/tk815/tk850/tsd_technology_support_sub-protocol_home.html|archiveurl=https://web.archive.org/web/20140401064954/http://www.cisco.com/c/en/us/tech/lan-switching/multi-layer-switching-mls/index.html|archivedate=April 1, 2014|title=Multi-Layer Switching|publisher=Cisco Systems|accessdate=2011-02-11|url-status=dead}}</ref> 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. [[VLAN]]s) 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 address]]ing
* check and recompute layer-3 header [[checksum]]s
* 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:
* fast hardware-based packet forwarding with low latency
* lower per-port cost compared to pure routers
* flow accounting
* [[Quality of service]] (QoS)

IEEE{{cn|reason=Couldn't verify this is an IEEE. [[IS-IS]], from OSI, uses similar terminology.|date=May 2020}} has developed hierarchical terminology that is useful in describing forwarding and switching processes. Network devices without the capability to forward packets between subnetworks are called end systems (ESs, singular ES), whereas network devices with these capabilities are called intermediate systems (ISs). ISs are further divided into those that communicate only within their routing domain (intradomain IS) and those that communicate both within and between routing domains (interdomains IS). A routing domain is generally considered as a portion of an internetwork under common administrative authority and is regulated by a particular set of administrative guidelines. Routing domains are also called autonomous systems.

A common layer-3 capability is an awareness of [[IP multicast]] through [[IGMP snooping]]. With this awareness, a layer-3 switch can increase efficiency by delivering the traffic of a multicast group only to ports where the attached device has signaled that it wants to listen to that group.

Layer-3 switches typically support [[IP routing]] between VLANs configured on the switch. Some layer-3 switches support the [[routing protocol]]s that routers use to exchange information about routes between networks.