Routers

In computer networking and telecommunications, route flapping occurs when a router alternately advertises a destination network via one route then another (or as unavailable, and then available again) in quick sequence.

Route flapping is caused by pathological conditions (hardware errors, software errors, configuration errors, unreliable connections, etc.) within the network which cause certain reachability information to be repeatedly advertised and withdrawn. The most common causes of route flapping are configuration errors and intermittent errors in communications links. Route flapping often forces a router to recalculate a new or preferred route to a particular network, while traffic destined for that network is in transit through the router.

While the exact meaning of the term Layer-4 switch is vendor dependent, it almost always starts with a capability for network address translation, but then adds some type of load distribution based on TCP sessions.The device may include a stateful firewall, a VPN concentrator, or be an IPSec security gateway.

Router is a marketing term for a Layer 3 switch, typically a router optimized for Ethernet interfaces. Like other switches, it connects devices to single ports for microsegmentation. The ports normally operate in full duplex.Switches, even primarily Layer 2 switches, can be aware of Layer 3 multicast and 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. In a switch not aware of multicasting and broadcasting, frames are also forwarded on all ports of each broadcast domain, but in the case of IP multicast this causes inefficient use of bandwidth. To work around this problem some switches implement IGMP snooping.

This method uses a forwarding database to send frames across network segments. The forwarding database is initially empty and entries in the database are built as the bridge receives frames. If an address entry is not found in the forwarding database, the frame is rebroadcast to all ports of the bridge, forwarding the frame to all segments except the source address. By means of these broadcast frames, the destination network will respond and a route will be created. Along with recording the network segment to which a particular frame is to be sent, bridges may also record a bandwidth metric to avoid looping when multiple paths are available. Devices that have this transparent bridging functionality are also known as adaptive bridges.

In computer networking and telecommunications, route flapping occurs when a router alternately advertises a destination network via one route then another (or as unavailable, and then available again) in quick sequence.Route flapping is caused by pathological conditions (hardware errors, software errors, configuration errors, unreliable connections, etc.) within the network which cause certain reachability information to be repeatedly advertised and withdrawn. The most common causes of route flapping are configuration errors and intermittent errors in communications links. Route flapping often forces a router to recalculate a new or preferred route to a particular network, while traffic destined for that network is in transit through the router.

One IEEE 802.11 WAP can typically communicate with 30 client systems located within a radius of 100 m. However, the actual range of communication can vary significantly, depending on such variables as indoor or outdoor placement, height above ground, nearby obstructions, other electronic devices that might actively interfere with the signal by broadcasting on the same frequency, type of antenna, the current weather, operating radio frequency, and the power output of devices. Network designers can extend the range of WAPs through the use of repeaters and reflectors, which can bounce or amplify radio signals that ordinarily would go un-received. In experimental conditions, wireless networking has operated over distances of several kilometers.

Most jurisdictions have only a limited number of frequencies legally available for use by wireless networks. Usually, adjacent WAPs will use different frequencies to communicate with their clients in order to avoid interference between the two nearby systems. But wireless devices can “listen” for data traffic on other frequencies, and can rapidly switch from one frequency to another to achieve better reception on a different WAP. However, the limited number of frequencies becomes problematic in crowded downtown areas with tall buildings housing multiple WAPs, when overlap causes interference.

Wireless networking lags behind wired networking in terms of increasing bandwidth and throughput. While (as of 2004) typical wireless devices for the consumer market can reach speeds of 11 Mbit/s (megabits per second) (IEEE 802.11b) or 54 Mbit/s (IEEE 802.11a, IEEE 802.11g), wired hardware of similar cost reaches 1000 Mbit/s (Gigabit Ethernet). One impediment to increasing the speed of wireless communications comes from Wi-Fi’s use of a shared communications medium, so a WAP is only able to use somewhat less than half the actual over-the-air rate for data throughput. Thus a typical 54 MBit/s wireless connection actually carries TCP/IP data at 20 to 25 Mbit/s. Users of legacy wired networks expect the faster speeds, and people using wireless connections keenly want to see the wireless networks catch up.

As of 2006 a new standard for wireless, 802.11n is awaiting final certification from IEEE. This new standard operates at speeds up to 540 Mbit/s and at longer distances (~50 m) than 802.11g. Use of legacy wired networks (especially in consumer applications) is expected to decline sharply as the common 100 Mbit/s speed is surpassed and users no longer need to worry about running wires to attain high bandwidth.

Interference can commonly cause problems with wireless networking reception, as many devices operate using the 2.4 GHz ISM band. A nearby wireless phone or anything with greater transmission power within close proximity can markedly reduce the perceived signal strength of a wireless access point. Microwave ovens are also known to interfere with wireless networks.

In computer networking, a wireless access point (WAP or AP) is a device that connects wireless communication devices together to form a wireless network. The WAP usually connects to a wired network, and can relay data between wireless devices and wired devices. Several WAPs can link together to form a larger network that allows “roaming”. (In contrast, a network where the client devices manage themselves - without the need for any access points - becomes an ad-hoc network.) WAPs have IP addresses for configuration.Low-cost and easily-installed Wi-Fi WAPs grew rapidly in popularity in the early 2000s. These devices offered a way to avoid the tangled messes of category 5 cable associated with typical Ethernet networks of the day. Whereas wiring a business, home, or school often requires stringing many cables through walls and ceilings, wireless networking allows connecting with few or no new cables. Wireless networks also allow greater mobility, freeing users from the restrictions of using a computer cabled to the wall. In the industrial and commercial contexts, wireless networking has had a big impact on operations: employees in these areas now often carry portable data terminals integrating barcode scanners and wireless links, allowing them to update work in progress and inventory in real-time. At home with a residential gateway, any convenient chair or lawn becomes a desk for the laptop.

A typical corporate use involves attaching several WAPs to a wired network and then providing wireless access to the office LAN. Within the range of the WAPs, the wireless end user has a full network connection with the benefit of mobility. In this instance, the WAP functions as a gateway for clients to access the wired network. Another use involves bridging two wired networks in conditions inappropriate for cable: for example, a manufacturer can wirelessly connect a remote warehouse’s wired network with a separate (though within line of sight) office’s wired network.

Another wireless topology, a lily-pad network, consists of a series of access points spread over a large area, each connected to a different network. This provides hot spots where wireless clients can connect to the Internet without regard for the particular networks to which they have attached for the moment. The concept can become organic in large cities, where a combination of coffeehouses, libraries, other public spaces offering wireless access, as well as privately owned open access points, allow clients to roam over a large area (like hopping from lily pad to lily pad), staying more or less continuously connected.

Home wireless networks, the majority, generally have only one WAP to connect all the computers in a home. Most are wireless routers, meaning converged devices that include a WAP, Ethernet router, and often a switch in the same package. Many also converge a broadband modem. Most owners leave their encryption settings at default, hence neighbors can use them. In places where most homes have their own WAP within range of the neighbors’ WAP, it’s possible for technically savvy people to turn off their encryption and set up a wireless community network, creating an intra-city communication network without the need of wired networks.

A WAP may also act as the network’s arbitrator, negotiating when each nearby client device can transmit. However, the vast majority of currently installed IEEE 802.11 networks do not implement this, using a distributed pseudo-random algorithm instead.

A router is a computer whose software and hardware are usually tailored to the tasks of routing and forwarding, generally containing a specialized operating system (e.g. Cisco’s IOS or Juniper Networks JUNOS and JUNOSe or Extreme Networks XOS), RAM, NVRAM, flash memory, and one or more processors. High-end routers contain many processors and specialized Application-specific integrated circuits (ASIC) and do a great deal of parallel processing. Chassis based systems like the Nortel MERS-8600 or ERS-8600 routing switch, (pictured right) have multiple ASICs on every module and allow for a wide variety of LAN, MAN, METRO, and WAN port technologies or other connections that are customizable. However, with the proper software (such as XORP or Quagga), even commodity PCs can act as routers.

Routers connect two or more logical subnets, which do not necessarily map one-to-one to the physical interfaces of the router. The term layer 3 switch often is used interchangeably with router, but switch is really a marketing term without a rigorous technical definition. In marketing usage, it is generally optimized for Ethernet LAN interfaces and may not have other physical interface types.

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.