Editing Multilayer switch (section)

== Layer-4 switching ==
Layer-4 switching means hardware-based layer-3 switching technology that can also consider the type of network traffic (for example, distinguishing between [[User Datagram Protocol|UDP]] and [[Transmission Control Protocol|TCP]]). Layer-4 switching provides additional datagram inspection by reading the [[port number]]s found in the transport layer header to make routing decisions (i.e. ports used by [[HTTP]], [[FTP]] and [[VoIP]]). These port numbers are found in RFC 1700 and reference the upper-layer protocol, program, or application.

Using layer-4 switching, the network administrator can configure a layer-4 switch to prioritize data traffic by application. Layer-4 information can also be used to help make routing decisions. For example, extended access lists can filter packets based on layer-4 port numbers. Another example is accounting information gathered by open standards using [[sFlow]].

A layer-4 switch can use information in the transport-layer protocols to make forwarding decisions. Principally this refers to an ability to use source and destination port numbers in TCP and UDP communications to allow, block and prioritize communications.<ref>{{cite book |title=CCNP : building CISCO multilayer switched networks : study guide |last=Jack |first=Terry |isbn=9780585496849 |date=2004 |publisher=Sybex |page=15}}</ref>

===Layer 4–7 switch, web switch, or content switch===
Some switches can use packet information up to OSI layer 7; these may be called layer 4–7 switches, '''{{vanchor|content switches|content switch}}''', '''{{vanchor|content services switches|content services switch}}''', web switches or application switches.

Content switches are typically used for [[Load balancer|load balancing]] among groups of servers.  Load balancing can be performed on [[HTTP]], [[HTTPS]], [[VPN]], or any TCP/IP traffic using a specific port.  Load balancing often involves [[destination network address translation]] so that the client of the load-balanced service is not fully aware of which server is handling its requests.  Some layer 4–7 switches can perform [[Network address translation]] (NAT) at wire speed.  Content switches can often be used to perform standard operations such as [[Secure Sockets Layer|SSL]] encryption and decryption to reduce the load on the servers receiving the traffic, or to centralize the management of [[digital certificate]]s. Layer-7 switching is a technology used in a [[content delivery network]] (CDN).

Some applications require that repeated requests from a client are directed at the same application server.  Since the client isn't generally aware of which server it spoke to earlier, content switches define a notion of stickiness. For example, requests from the same source IP address are directed to the same application server each time. Stickiness can also be based on SSL IDs, and some content switches can use [[HTTP cookie|cookies]] to provide this functionality.

===Layer-4 load balancer===
The router operates on the [[transport layer]] and makes decisions on where to send the packets.  Modern [[Load balancing (computing)|load balancing]] routers can use different rules to make decisions on where to route traffic.  This can be based on least load, or fastest [[Response time (technology)|response time]]s, or simply balancing requests out to multiple destinations providing the same services.  This is also a [[redundancy (engineering)|redundancy]] method, so if one machine is not up, the router will not send traffic to it.

The router may also have NAT capability with port and transaction awareness and performs a form of [[port translation]] for sending incoming packets to one or more machines that are hidden behind a single IP address.

===Layer 7===
Layer-7 switches may distribute the load based on [[uniform resource locator]]s (URLs), or by using some installation-specific technique to recognize application-level transactions. A layer-7 switch may include a [[web cache]] and participate in a ''[[content-distribution network]]'' (''CDN'').<ref>{{cite web |url=http://www.irbs.net/internet/nanog/0110/0618.html |title=How worried is too worried? Plus, a Global Crossing Story. |archive-url=https://web.archive.org/web/20170103033926/http://www.irbs.net/internet/nanog/0110/0618.html |archive-date=2017-01-03 |website=NANOG mailing list archives |author=S. Gibbard |date=October 2001}}{{rs|date=December 2020}}</ref>{{failed verification|date=April 2018}}