This is an old revision of this page, as edited by Akc9000 (talk | contribs) at 03:40, 29 June 2007 (added the fact to intro that some are wireless like the cisco 1800 series). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.
Revision as of 03:40, 29 June 2007 by Akc9000 (talk | contribs) (added the fact to intro that some are wireless like the cisco 1800 series)(diff) ← Previous revision | Latest revision (diff) | Newer revision → (diff)
In simple layman terms, router is a device that determines the proper path for data to travel between different networks. They connect networks together; a LAN to a WAN for example, to access the Internet. Some units, are available in both wired and wireless models.
Function
A more precise definition of a router is a computer networking device that interconnects separate logical subnets. Routers connect to two or more logical subnets, which do not necessarily map one-to-one to the physical interfaces of the router. The term switch or layer 3 switch or network switch often is synonymous with router, but switch is really a marketing term without a rigorous technical definition.
Routers operate in two different planes :
- Control Plane, in which the router learns the outgoing interface that is most appropriate for forwarding specific packets to specific destinations,
- Forwarding Plane, which is responsible for the actual process of sending a packet received on a logical interface to an outbound logical interface.
For the pure Internet Protocol (IP) forwarding function, router design tries to minimize the state information kept on individual packets. Routers do maintain state on routes, but not packets. Once a packet is forwarded, the router should retain no more than statistical information about it. It is the sending and receiving endpoint that keeps information on such things as errored or missing packets.
Forwarding decisions can involve decisions at layers other than the IP internetwork layer or OSI layer 3. Again, the marketing term switch can be applied to devices that have these capabilities. A function that forwards based on data link layer, or OSI layer 2, information, is properly called a bridge, or layer 2 switch. A physical device called a router may also have the capability to forward based on information at other layers.
Control Plane
Control Plane processing leads to the construction of what is variously called a routing table or routing information base (RIB). The RIB may be used by the Forwarding Plane to look up the outbound interface for a given packet, or, depending on the router implementation, the Control Plane may populate a separate Forwarding Information Base (FIB) with destination information. RIBs are optimized for efficient updating with control mechanisms such as routing protocols, while FIBs are optimized for the fastest possible lookup of the information needed to select the outbound interface.
The Control Plane constructs the routing table from knowledge of the up/down status of its local interfaces, from hard-coded static routes, and from exchanging routing protocol information with other routers. It is not compulsory for a router to use routing protocols to function, if for example it was configured solely with static routes. The routing table stores the best routes to certain network destinations, the "routing metrics" associated with those routes, and the path to the next hop router.
Routers do maintain state on the routes in the RIB/routing table, but this is quite distinct from not maintaining state on individual packets that have been forwarded.
Forwarding Plane
For the pure Internet Protocol (IP) forwarding function, router design tries to minimize the state information kept on individual packets. Once a packet is forwarded, the router should retain no more than statistical information about it. It is the sending and receiving endpoint that keeps information on such things as errored or missing packets.
Forwarding decisions can involve decisions at layers other than the IP internetwork layer or OSI layer 3. Again, the marketing term switch can be applied to devices that have these capabilities. A function that forwards based on data link layer, or OSI layer 2, information, is properly called a bridge, or layer 2 switch. A physical device called a router may also have the capability to forward based on information at other layers, if it has software that can make decisions at these other layers.
Types of routers
Routers may provide connectivity inside enterprises, between enterprises and the Internet, and inside Internet Service Providers (ISP). The largest routers (example: Cisco CRS-1, Juniper T1600) interconnect ISPs, are used inside ISPs, or may be used in very large enterprise networks. The smallest routers provide connectivity for small and home offices (example: Linksys befsr41).
Routers for Internet connectivity and internal use
Routers intended for ISP and major enterprise connectivity will almost invariably exchange routing information with the Border Gateway Protocol. RFC 4098 defines several types of BGP-speaking routers:
- Provider Edge Router: Placed at the edge of an ISP network, it speaks external BGP (eBGP) to a BGP speaker in another provider or large enterprise Autonomous System (AS).
- Subscriber Edge Router: Located at the edge of the subscriber's network, it speaks eBGP to its provider's AS(s). It belongs to an end user (enterprise) organization.
- Inter-provider Border Router: Interconnecting ISPs, this is a BGP speaking router that maintains BGP sessions with other BGP speaking routers in other providers' ASes.
- Core router: Internal to the provider's AS, such a router speaks internal BGP (iBGP) to that provider's edge routers, other intra-provider core routers, or the provider's inter-provider border routers.
Small and Home Office (SOHO) connectivity
Routers that are used in a homes usually connect to a broadband service such as IP over cable or DSL. A home router may allow connectivity to an enterprise via a secure Virtual Private Network.
Enterprise Routers
All sizes of routers may be found inside enterprises. While the most powerful routers tend to be found in ISPs, academic and research facilities, as well as large businesses, may need large routers.
History
The very first device that acted as a router does today was called an IMP, which stands for Interface Message Processor. The first functional IMP was placed at UCLA on August 30, 1969 and was developed at BBN by the IMP team as part of their contract to build out the original ARPANET. The IMP and the routers that followed are what make the Internet possible.
The first multiprotocol router was created at Stanford University by a staff researcher named William Yeager in January of 1980. As virtually all networking now uses IP at the network layer, multiprotocol routers are largely obsolete. Routers that handle both IPv4 and IPv6 arguably are multiprotocol, but in a far less variable sense than a router that processed AppleTalk, DECnet, IP, and Xerox protocols.
In the original era of routing (from the mid-1970s through the 1980s), general-purpose mini-computers served as routers. Although general-purpose computers can perform routing, modern high-speed routers are highly specialized computers, generally with extra hardware added to accelerate both common routing functions such as packet forwarding and specialised functions such as IPsec encryption.
Still, there is substantial use of LINUX and UNIX machines, running open source routing code, for routing research and selected other applications. While Cisco's operating system was independently designed, other major router operating systems, such as those from Juniper and Extreme, are extensively modified but still have UNIX ancestry.
Other changes also improve reliability, such as redundant control processors with stateful failover, and using storage having no moving parts for program loading. As much reliability comes from operational techniques for running critical routers as it does to the router design itself. It is the best common practice, for example, to use redundant uninterruptible power supplies for all critical network elements, with generator backup for the batteries or flywheels of those power supplies.
See also
- DSL router
- Wireless router
- Access point
- Flapping router
- History of the Internet
- Network address translation (NAT)
- Network bridge
- Network switch
- Hub
- TR-069
- Wi-Fi
External links
References
- How Routers Work?, Curt Franklin, Accessed June 22 2007.
- Requirements for IPv4 Routers,RFC 1812, F. Baker,June 1995
- Requirements for Separation of IP Control and Forwarding,RFC 3564, H. Khosravi & T. Anderson,November 2003
- Terminology for Benchmarking BGP Device Convergence in the Control Plane,RFC 4098, H. Berkowitz et al.,June 2005
- IMP -- Interface Message Processor, LivingInternet Accessed June 22 2007.
- Looking back at the ARPANET effort, 34 years later, Dave Walden, Accessed June 22 2007.
- A Technical History of the ARPANET - A Technical Tour, THINK Protocols team, Accessed June 22 2007.
- Router Man, NetworkWorld, Accessed June 22 2007.