F

failure domain

The region in which a failure has occurred in a Token Ring. When a station gains information that a serious problem, such as a cable break, has occurred with the network, it sends a beacon frame that includes the station reporting the failure, its NAUN, and everything between. This defines the failure domain. Beaconing then initiates the procedure known as autoreconfiguration. See also: autoreconfiguration and beacon.

fallback

In ATM networks, this mechanism is used for scouting a path if it’s not possible to locate one using customary methods. The device relaxes requirements for certain characteristics, such as delay, in an attempt to find a path that meets a certain set of the most important requirements.

Fast Ethernet

Any Ethernet specification with a speed of 100Mbps. Fast Ethernet is 10 times faster than 10BaseT, while retaining qualities like MAC mechanisms, MTU, and frame format. These similarities make it possible for existing 10BaseT applications and management tools to be used on Fast Ethernet networks. Fast Ethernet is based on an extension of IEEE 802.3 specification. Compare with: Ethernet. See also: 100BaseT, 100BaseTX, and IEEE.

fast switching

A Cisco feature that uses a route cache to speed packet switching through a router. Contrast with: process switching.

fault tolerance

A design method that ensures continued system operation in the event of individual failures by providing redundant elements.

At the component level, the design includes redundant chips and circuits and the capability to bypass faults automatically. At the computer-system level, any elements that are likely to fail, such as processors and large disk drives, are replicated.

Fault-tolerant operations often require backup or UPS (uninterruptible power supply) systems in the event of a main power failure. In some cases, the entire computer system is duplicated in a remote location to protect against vandalism, acts of war, or natural disaster.

FDM

Frequency-Division Multiplexing: A technique that permits information from several channels to be assigned bandwidth on one wire based on frequency. See also: TDM, ATDM, and statistical multiplexing.

FDDI

Fiber Distributed Data Interface: An LAN standard, defined by ANSI X3T9.5, that can run at speeds up to 200Mbps and uses token-passing media access on fiber-optic cable. For redundancy, FDDI can use a dual- ring architecture.

FECN

Forward Explicit Congestion Notification: A bit set by a Frame Relay network that informs the DTE receptor that congestion was encountered along the path from source to destination. A device receiving frames with the FECN bit set can ask higher-priority protocols to take flow-control action as needed. See also: BECN.

FEIP

Fast Ethernet Interface Processor: An interface processor employed on Cisco 7000 series routers, supporting up to two 100Mbps 100BaseT ports.

FIFO

(First In First Out) Often used with queuing, FIFO uses a first in first out method.

flat network

A term for describing a Layer 2 network that is one large broadcast domain.

filtering

One of the five states of a network switch is filtering. The others are blocking, listening, learning, and forwarding.

firewall

A barrier purposefully erected between any connected public networks and a private network, made up of a router or access server or several routers or access servers, that uses access lists and other methods to ensure the security of the private network.

flash memory

Developed by Intel and licensed to other semiconductor manufacturers, it is nonvolatile storage that can be erased electronically and reprogrammed. Flash memory permits software images to be stored, booted, and rewritten as needed. Cisco routers and switches use flash memory to hold the IOS by default.

flooding

When traffic is received on an interface, it is then transmitted to every interface connected to that device with exception of the interface from which the traffic originated. This technique can be used for traffic transfer by bridges and switches throughout the network.

flow control

A methodology used to ensure that receiving units are not overwhelmed with data from sending devices. Pacing, as it is called in IBM networks, means that when buffers at a receiving unit are full, a message is transmitted to the sending unit to temporarily halt transmissions until all the data in the receiving buffer has been processed and the buffer is again ready for action.

forwarding

One of the five states of a network switch is forwarding. The others are blocking, listening, learning, and filtering.

FRAD

Frame Relay Access Device: Any device affording a connection between a LAN and a Frame Relay WAN. See also: Cisco FRAD, FRAS.

fragment

Any portion of a larger packet that has been segmented into smaller pieces.

FragmentFree

A LAN switch type that reads into the data field before forwarding to make sure that no fragmentation has taken place.

fragmentation

The process of segmenting a packet into smaller pieces when sending data over a network medium that cannot support the larger packet size.

frame

A logical unit of information sent by the Data Link layer over a transmission medium. The term often refers to the header and trailer, employed for synchronization and error control, that surround the data contained in the unit.

frame filtering

A LAN switch function that reads the destination hardware address and only forwards the frame out the correct port. This process filters the frame from going out unneeded ports.

Frame Relay

A more efficient replacement of the X.25 protocol, Frame Relay is the industry-standard, switched Data Link layer protocol that services multiple virtual circuits using HDLC encapsulation between connected mechanisms.

Frame Relay bridging

Defined in RFC 1490, this bridging method uses the identical spanning-tree algorithm as other bridging operations but permits packets to be encapsulated for transmission across a Frame Relay network.

FRAS

Frame Relay Access Support: A feature of Cisco IOS software that enables SDLC, Ethernet, Token Ring, and Frame Relay–attached IBM devices to be linked with other IBM mechanisms on a Frame Relay network. See also: FRAD.

frequency

The number of cycles of an alternating current signal per time unit, measured in hertz (cycles per second).

FRTS

(Frame Relay Traffic Shaping) A combination of techniques used to efficiently shape network traffic on a Frame Relay circuit.

FSIP

Fast Serial Interface Processor: The Cisco 7000 routers’ default serial interface processor, it provides four or eight high-speed serial ports.

FTP

File Transfer Protocol: The TCP/IP protocol used for transmitting files between network nodes, it supports a broad range of file types and is defined in RFC 959. See also: TFTP.

full duplex

The capacity to transmit information between a sending station and a receiving unit at the same time. See also: half duplex.

full mesh

A type of network topology where every node has either a physical or a virtual circuit linking it to every other network node. A full mesh supplies a great deal of redundancy but is typically reserved for network backbones because of its expense. See also: partial mesh.