CCNA Glossary Study Guide

ATM

Asynchronous Transfer Mode: The international standard, identified by fixed-length 53-byte cells, for transmitting cells in multiple service systems, such as voice, video, or data. Transit delays are reduced because the fixed-length cells permit processing to occur in the hardware. ATM is designed to maximize the benefits of high-speed transmission media, such as SONET, E3, and T3.

AST

Automatic Spanning Tree: A function that supplies one path for spanning explorer frames traveling from one node in the network to another, supporting the automatic resolution of spanning trees in SRB networks. AST is based on the IEEE 802.1d standard.

ASBR

Autonomous System Boundary Router: An Area Border Router placed between an OSPF autonomous system and a non-OSPF network that operates both OSPF and an additional routing protocol, such as RIP. ASBRs must be located in a non-stub OSPF area.

BGP4

BGP version 4: Version 4 of the interdomain routing protocol most commonly used on the Internet. BGP4 supports CIDR and uses route-counting mechanisms to decrease the size of routing tables.

BECN

Backward Explicit Congestion Notification: BECN is the bit set by a Frame Relay network in frames moving away from frames headed into a congested path. A DTE that receives frames with the BECN may ask higher-level protocols to take necessary flow control measures.

address mask

A bit combination descriptor identifying which portion of an address refers to the network or subnet and which part refers to the host. Sometimes simply called the mask.

autonomous confederation

A collection of self-governed systems that depend more on their own network accessibility and routing information than on information received from other systems or groups.

ATM user-user connection

A connection made by the ATM layer to supply communication between at least two ATM service users, such as ATMM processes. These communications can be uni- or bidirectional, using one or two VCs, respectively.

AURP tunnel

A connection made in an AURP WAN that acts as a single, virtual link between AppleTalk internetworks separated physically by a foreign network such as a TCP/IP network.

ATM ARP server

A device that supplies logical subnets running classical IP over ATM with address-resolution services.

baseband

A feature of a network technology that uses only one carrier frequency. Ethernet is an example. Also named "narrowband." Compare with: broadband.

access link

A link used with switches that is part of only one virtual LAN (VLAN). Trunk links carry information from multiple VLANs.

area

A logical, rather than physical, set of segments (based on CLNS, DECnet, or OSPF) along with their attached devices. Areas are commonly connected to others using routers to create a single autonomous system.

back end

A node or software program supplying services to a front end.

administrative distance (AD)

A number between 0 and 255 that expresses the level of trustworthiness of a routing information source. The lower the number, the higher the integrity rating.

authority zone

A portion of the domain-name tree associated with DNS for which one name server is the authority.

autoreconfiguration

A procedure executed by nodes within the failure domain of a token ring wherein nodes automatically perform diagnostics, trying to reconfigure the network around failed areas.

algorithm

A set of rules or processes used to solve a problem. In networking, algorithms are typically used for finding the best route for traffic from a source to its destination.

access list

A set of test conditions kept by routers that determines "interesting traffic" to and from the router for various services on the network.

ATM layer

A sublayer of the Data Link layer in an ATM network that is service independent. To create standard 53-byte ATM cells, the ATM layer receives 48-byte segments from the AAL and attaches a 5-byte header to each. These cells are then sent to the physical layer for transmission across the physical medium.

AAL2

ATM Adaptation Layer 2: One of four AALs recommended by the ITU-T, it is used for connection-oriented services that support a variable bit rate, such as compressed voice traffic.

AAL3/4

ATM Adaptation Layer 3/4: One of four AALs (a product of two initially distinct layers) recommended by the ITU-T, supporting both connectionless and connection-oriented links. Its primary use is in sending SMDS packets over ATM networks.

ASN.1

Abstract Syntax Notation One: An OSI language used to describe types of data that are independent of computer structures and depicting methods. Described by ISO International Standard 8824.

ARP

Address Resolution Protocol: Defined in RFC 826, the protocol that traces IP addresses to MAC addresses.

ATG

Address Translation Gateway: The mechanism within Cisco DECnet routing software that enables routers to route multiple, independent DECnet networks and to establish a userdesignated address translation for chosen nodes between networks.

ACR

Allowed cell rate: A designation defined by the ATM Forum for managing ATM traffic. Dynamically controlled using congestion control measures, the ACR varies between the minimum cell rate (MCR) and the peak cell rate (PCR).

AMI

Alternate Mark Inversion: A line-code type on T1 and E1 circuits that shows zeros as 01 during each bit cell and ones as 11 or 00, alternately, during each bit cell. The sending device must maintain ones density in AMI but not independently of the data stream. Also known as binary-coded, alternate mark inversion.

AM

Amplitude modulation: A modulation method that represents information by varying the amplitude of the carrier signal.

AARP

AppleTalk Address Resolution Protocol: The protocol in an AppleTalk stack that maps data-link addresses to network addresses.

ABR

Area Border Router: An OSPF router that is located on the border of one or more OSPF areas. ABRs are used to connect OSPF areas to the OSPF backbone area.

AAA

Authentication, Authorization, and Accounting: A system developed by Cisco to provide network security.

AFI

Authority and Format Identifier: The part of an NSAP ATM address that delineates the type and format of the IDI section of an ATM address.

BDR

Backup designated router: This is used in an OSPF network to back up the designated router in case of failure.

100BaseT

Based on the IEEE 802.3u standard, 100BaseT is the Fast Ethernet specification of 100Mbps baseband that uses UTP wiring. 100BaseT sends link pulses (containing more information than those used in 10BaseT) over the network when no traffic is present.

100BaseTX

Based on the IEEE 802.3u standard, 100BaseTX is the 100Mbps baseband Fast Ethernet specification that uses two pairs of UTP or STP wiring. The first pair of wires receives data; the second pair sends data. To ensure correct signal timing, a 100BaseTX segment cannot be longer than 100 meters.

baseline

Baseline information includes historical data about the network and routine utilization information. This information can be used to determine whether there were recent changes made to the network that may contribute to the problem at hand.

B channel

Bearer channel: A full-duplex, 64Kbps channel in ISDN that transmits user data.

AppleTalk

Currently in two versions, the group of communication protocols designed by Apple Computer for use in Macintosh environments. The earlier Phase 1 protocols support one physical network with only one network number that resides in one zone. The later Phase 2 protocols support more than one logical network on a single physical network, allowing networks to exist in more than one zone.

attenuation

In communication, weakening or loss of signal energy, typically caused by distance.

active state

In regard to an EIGRP routing table, a route will be in active state when a router is undergoing a route convergence.

analog transmission

Signal messaging whereby information is represented by various combinations of signal amplitude, frequency, and phase.

baud

Synonymous with bits per second (bps), if each signal element represents 1 bit. It is a unit of signaling speed equivalent to the number of separate signal elements transmitted per second.

autonomous switching

The ability of Cisco routers to process packets more quickly by using the ciscoBus to switch packets independently of the system processor.

authorization

The act of permitting access to a resource based on authentication information in the AAA model.

backbone

The basic portion of the network that provides the primary path for traffic sent to and initiated from other networks.

address resolution

The process used for resolving differences between computer addressing schemes. Address resolution typically defines a method for tracing Network layer (layer 3) addresses to Data Link layer (layer 2) addresses.

auto-detect mechanism

Used in Ethernet switch, hub, and interface cards to determine the duplex and speed that can be used.

A&B bit signaling

Used in T1 transmission facilities and sometimes called 24th channel signaling. Each of the 24 T1 subchannels in this procedure uses one bit of every sixth frame to send supervisory signaling information.

Basic Management Setup

Used with Cisco routers when in setup mode. Only provides enough management and configuration to get the router working so someone can telnet into the router and configure it.

AS

autonomous system: A group of networks under mutual administration that share the same routing methodology. Autonomous systems are subdivided by areas and must be assigned an individual 16-bit number by the IANA.

B8ZS

Binary 8-Zero Substitution: A line-code type, interpreted at the remote end of the connection, that uses a special code substitution whenever eight consecutive zeros are transmitted over the link on T1 and E1 circuits. This technique assures ones density independent of the data stream. Also known as bipolar 8-zero substitution. Contrast with: AMI. See also: ones density.

address mapping

By translating network addresses from one format to another, this methodology permits different protocols to operate interchangeably.

Application layer

Layer 7 of the OSI reference network model, supplying services to application procedures (such as electronic mail and file transfer) that are outside the OSI model. This layer chooses and determines the availability of communicating partners along with the resources necessary to make the connection, coordinates partnering applications, and forms a consensus on procedures for controlling data integrity and error recovery.

access layer

One of the layers in Cisco's three-layer hierarchical model. The access layer provides users with access to the internetwork.

accounting

One of the three components in AAA. Accounting provides auditing and logging functionalities to the security model.

AARP probe packets

Packets sent by the AARP to determine whether a given node ID is being used by another node in a nonextended AppleTalk network. If the node ID is not in use, the sending node appropriates that node's ID. If the node ID is in use, the sending node will select a different ID and then send out more AARP probe packets.

10BaseT

Part of the original IEEE 802.3 standard, 10BaseT is the Ethernet specification of 10Mbps baseband that uses two pairs of twisted-pair, Category 3, 4, or 5 cabling—using one pair to send data and the other to receive. 10BaseT has a distance limit of about 100 meters per segment.

auxiliary port

The console port on the back of Cisco routers that allows you to connect a modem and dial the router and make console configuration settings.

authentication

The first component in the AAA model. Users are typically authenticated via a username and password, which are used to uniquely identify them.

bandwidth

The gap between the highest and lowest frequencies employed by network signals. More commonly, it refers to the rated throughput capacity of a network protocol or medium.

bandwidth on demand (BoD)

This function allows an additional B channel to be used to increase the amount of bandwidth available for a particular connection.

address learning

Used with transparent bridges to learn the hardware addresses of all devices on a network. The switch then filters the network with the known hardware (MAC) addresses.

acknowledgment

Verification sent from one network device to another signifying that an event has occurred. May be abbreviated as ACK.

AAL1

ATM Adaptation Layer 1: One of four AALs recommended by the ITU-T, it is used for connection-oriented, time-sensitive services that need constant bit rates, such as isochronous traffic and uncompressed video.

AAL5

ATM Adaptation Layer 5: One of four AALs recommended by the ITU-T, it is used to support connection-oriented VBR services primarily to transfer classical IP over ATM and LANE traffic. This least complex of the AAL recommendations uses SEAL, offering lower bandwidth costs and simpler processing requirements but also providing reduced bandwidth and error-recovery capacities.

access rate

Defines the bandwidth rate of the circuit. For example, the access rate of a T1 circuit is 1.544Mbps. In Frame Relay and other technologies, there may be a fractional T1 connection— 256Kbps, for example—however, the access rate and clock rate are still 1.544Mbps.

ATDM

Asynchronous Time-Division Multiplexing: A technique for sending information, it differs from normal TDM in that the time slots are assigned when necessary rather than preassigned to certain transmitters.

automatic call reconnect

A function that enables automatic call rerouting away from a failed trunk line.

auto duplex

A setting on layer 1 and layer 2 devices that sets the duplex of a switch or hub port automatically.

administrative weight

A value designated by a network administrator to rate the preference given to a network link. It is one of four link metrics exchanged by PTSPs to test ATM network resource availability.

AAL

ATM Adaptation Layer: A service-dependent sublayer of the Data Link layer, which accepts data from other applications and brings it to the ATM layer in 48-byte ATM payload segments. CS and SAR are the two sublayers that form AALs. Currently, the four types of AAL recommended by the ITU-T are AAL1, AAL2, AAL3/4, and AAL5. AALs are differentiated by the source-destination timing they use, whether they are CBR or VBR, and whether they are used for connection-oriented or connectionless mode data transmission.

ADSU

ATM Data Service Unit: The terminal adapter used to connect to an ATM network through an HSSI-compatible mechanism.

AIP

ATM Interface Processor: Supporting AAL3/4 and AAL5, this interface for Cisco 7000 series routers minimizes performance bottlenecks at the UNI.

ATMM

ATM Management: A procedure that runs on ATM switches, managing rate enforcement and VCI translation.

access server

Also known as a "network access server," it is a communications process connecting asynchronous devices to a LAN or WAN through network and terminal emulation software, providing synchronous or asynchronous routing of supported protocols.

ANSI

American National Standards Institute: The organization of corporate, government, and volunteer members that coordinates standards-related activities, approves U.S. national standards, and develops U.S. positions in international standards organizations. ANSI assists in the creation of international and U.S. standards in disciplines such as communications, networking, and a variety of technical fields. It publishes over 13,000 standards for engineered products and technologies ranging from screw threads to networking protocols. ANSI is a member of the International Electrotechnical Commission (IEC) and International Organization for Standardization (ISO).

ASCII

American Standard Code for Information Interchange: An 8-bit code for representing characters, consisting of 7 data bits plus 1 parity bit.

anycast

An ATM address that can be shared by more than one end system, allowing requests to be routed to a node that provides a particular service.

beacon

An FDDI frame or Token Ring frame that points to a serious problem with the ring, such as a broken cable. The beacon frame carries the address of the station thought to be down.

amplitude

An analog or digital waveform's highest value.

alignment error

An error occurring in Ethernet networks, in which a received frame has extra bits—that is, a number not divisible by eight. Alignment errors are generally the result of frame damage caused by collisions.

all-routes explorer packet

An explorer packet that can move across an entire SRB network, tracing all possible paths to a given destination. Also known as an all-rings explorer packet.

ATCP

AppleTalk Control Program: The protocol for establishing and configuring AppleTalk over PPP, defined in RFC 1378.

AEP

AppleTalk Echo Protocol: A test for connectivity between two AppleTalk nodes where one node sends a packet to another and receives an echo, or copy, in response.

AFP

AppleTalk Filing Protocol: A Presentation layer protocol, supporting AppleShare and Mac OS File Sharing, that permits users to share files and applications on a server.

ARA

AppleTalk Remote Access: A protocol for Macintosh users establishing their access to resources and data from a remote AppleTalk location.

ASP

AppleTalk Session Protocol: A protocol employing ATP to establish, maintain, and tear down sessions as well as sequence requests.

ATP

AppleTalk Transaction Protocol: A transport-level protocol that enables reliable transactions between two sockets; one requests the other to perform a given task and to report the results. ATP fastens the request and response together, assuring a loss-free exchange of request-response pairs.

AURP

AppleTalk Update-based Routing Protocol: A technique for encapsulating AppleTalk traffic in the header of a foreign protocol that allows the connection of at least two noncontiguous AppleTalk internetworks through a foreign network (such as TCP/IP) to create an AppleTalk WAN. The connection made is called an AURP tunnel. By exchanging routing information between exterior routers, the AURP maintains routing tables for the complete AppleTalk WAN.

ASICs

Application-specific integrated circuits: Used in layer 2 switches to make filtering decisions. The ASIC looks in the filter table of MAC addresses and determines which port the destination hardware address of a received hardware address is destined for. The frame will be allowed to traverse only that one segment. If the hardware address is unknown, the frame is forwarded out all ports.

ABM

Asynchronous Balanced Mode: When two stations can initiate a transmission, ABM is an HDLC (or one of its derived protocols) communication technology that supports peeroriented, point-to-point communications between both stations.

ARM

Asynchronous Response Mode: An HDLC communication mode using one primary station and at least one additional station, in which transmission can be initiated from either the primary or one of the secondary units.

asynchronous transmission

Digital signals sent without precise timing, usually with different frequencies and phase relationships. Asynchronous transmissions generally enclose individual characters in control bits (called start and stop bits) that show the beginning and end of each character.

ATM endpoint

The initiating or terminating connection in an ATM network. ATM endpoints include servers, workstations, ATM-to-LAN switches, and ATM routers.

ATM Forum

The international organization founded jointly by Northern Telecom, Sprint, Cisco Systems, and NET/ADAPTIVE in 1991 to develop and promote standards-based implementation agreements for ATM technology. The ATM Forum broadens official standards developed by ANSI and ITU-T and creates implementation agreements before official standards are published.

access method

The manner in which network devices approach gaining access to the network itself.

active monitor

The mechanism used to manage a token ring. The network node with the highest MAC address on the ring becomes the active monitor and is responsible for management tasks such as preventing loops and ensuring that tokens are not lost.

advertising

The process whereby routing or service updates are transmitted at given intervals, allowing other routers on the network to maintain a record of viable routes.

adjacency

The relationship made to exchange routing information between defined neighboring routers and end nodes using a common media segment.

AS path prepending

The use of route maps in BGP to lengthen the autonomous system path by adding false ASNs.