ROUTER

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a 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 are now available in many types, though all are fundamentally doing the same job. A modern router is effectively a small network computer with an operating system (e.g. Cisco 's IOS ), RAM , NVRAM , flash memory , and a small processor which does all of the work in a router.

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 used interchangably with router, but switch is really a marketing term without a rigorous technical definition (though a switch is commonly understood as a network hub with switched ports, which might or might not also perform additional routing functions).

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.

To understand the role of a router, understand that it does not, in a network of any real complexity, take you directly to the destination. Instead, your information will pass through a series of routers and intermediate subnets, each getting you one "hop" closer to the destination, until you reach the router that connects to the subnet that contains your final destination.

As a simple analogy, assume that you want to travel from Washington DC to New York City. Getting on a highway, you see an exit marked "US Capitol". That does not get you closer to your destination, so you continue. Eventually, you see a sign reading "Baltimore and New York". You take that exit, which leads you to another freeway, where you pass a number of exits for destinations in suburban Maryland.

Eventually, you see an exit marked "Philadelphia and New York", and take that to another highway. You repeat this process until you get into New York City, and then take a local exit to your destination. In like manner, routers receive packets, look up their destination addresses in routing tables that have entries that tell you the interface that is one hop closer to the destination, and sends the packet out the destination. This is characteristic of the Network Layer , which deals with hop-by-hop communications as opposed to the end-to-end communications of the Transport Layer .

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.

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 (example: Cisco 7600 Series). 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: A router that resides within the middle or backbone of the network rather than at its periphery.

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.

A three-layer model is in common use, not all of which need be present in smaller networks.

Access

Access routers, including SOHO, are located at customer sites such as branch offices that do not need hierarchical routing of their own. Typically, they are optimized for low cost.

Distribution

Distribution routers aggregate traffic from multiple access routers, either at the same site, or to collect the data streams from multiple sites to a major enterprise location. Distribution routers often are responsible for enforcing quality of service across a WAN, so they may have considerable memory, multiple WAN interfaces, and substantial processing intelligence.

They may also provide connectivity to groups of servers. Either with a separate firewall or with an integrated firewall function, they may provide Internet or virtual private network VPN connectivity for an enterprise.

When an enterprise is primarily on one campus, there may not be a distinct distribution tier, other than perhaps off-campus access. In such cases, the access routers, connected to LANs, interconnect via core routers.

Core

In enterprises, core routers may provide a "collapsed backbone" interconnecting the distribution tier routers from multiple buildings of a campus, or large enterprise locations. They tend to be optimized for high bandwidth.

When an enterprise is widely distributed with no central location(s), the function of core routing may be subsumed by the WAN service to which the enterprise subscribes, and the distribution routers become the highest tier.