Glossary D

C

cable modem

A cable modem is not actually an analog device, like an asynchronous modem, but rather a customer access device for linking to a broadband cable network. These devices are typically bridges that have a coaxial cable connection to link to the cable network and a 10BaseT Ethernet connection to link to the user's PC.

cable range

In an extended AppleTalk network, the range of numbers allotted for use by existing nodes on the network. The value of the cable range can be anywhere from a single number to a sequence of several touching network numbers. Node addresses are determined by their cable range value.

CAC

Connection Admission Control: The sequence of actions executed by every ATM switch while connection setup is performed in order to determine whether a request for connection is violating the guarantees of QoS for established connections. Also, CAC is used to route a connection request through an ATM network.

call admission control

A device for managing traffic in ATM networks, determining the possibility of a path containing adequate bandwidth for a requested VCC.

call priority

In circuit-switched systems, the defining priority given to each originating port; it specifies in which order calls will be reconnected. Additionally, call priority identifies which calls are allowed during a bandwidth reservation.

call setup time

The length of time necessary to effect a switched call between DTE devices.

CAM

Content Addressable Memory: A storage system with unique data storage attributes used by some Catalyst switches. Unlike traditional memory banks, CAM data is stored so as to ensure that the actual location of the data holds some information.

candidate packets

Packets identified by the MLS-SE as having the potential for establishing a flow cache. This determination is made based on the destination MAC (DMAC) address. The DMAC address must be a MAC addresses associated with a known MLS-RP. See also: MLS- SE and MLS-RP.

CatOS

Abbreviation of Catalyst Operating System; the native operating system for certain Catalyst switches, such as the 4000, 5000, and 6500 series.

CBR

Constant Bit Rate: An ATM Forum QoS class created for use in ATM networks. CBR is used for connections that rely on precision clocking to guarantee trustworthy delivery. Compare to: ABR and VBR.

CD

Carrier Detect: A signal indicating that an interface is active or that a connection generated by a modem has been established.

CDP

Cisco Discovery Protocol: Cisco's proprietary protocol that is used to tell a neighbor Cisco device about the type of hardware, software version, and active interfaces that the Cisco device is using. It uses a SNAP frame between devices and is not routable.

CDVT

Cell Delay Variation Tolerance: A QoS parameter for traffic management in ATM networks specified when a connection is established. The allowable fluctuation levels for data samples taken by the PCR in CBR transmissions are determined by the CDVT. See also: CBR and PCR.

CEF

Cisco Express Forwarding: A mechanism for forwarding packets using a replicated routing table and a forwarding database on the interface. Sometimes referred to by Cisco as one of their layer 3 switching mechanisms.

cell

In ATM networking, the basic unit of data for switching and multiplexing. Cells have a defined length of 53 bytes, including a 5-byte header that identifies the cell's data stream and 48 bytes of payload. See also: cell relay.

cell payload scrambling

The method by which an ATM switch maintains framing on some medium-speed edge and trunk interfaces (T3 or E3 circuits). Cell payload scrambling rearranges the data portion of a cell to maintain the line synchronization with certain common bit patterns.

cell relay

A technology that uses small packets of fixed size, known as cells. Their fixed length enables cells to be processed and switched in hardware at high speeds, making this technology the foundation for ATM and other high-speed network protocols. See also: cell.

Centrex

A local exchange carrier service, providing local switching that resembles that of an on-site PBX. Centrex has no on-site switching capability. Therefore, all customer connections return to the central office (CO). See also: CO.

CER

Cell Error Ratio: In ATM, the ratio of the number of transmitted cells having errors to the total number of cells sent in a transmission within a certain span of time.

CGMP

Cisco Group Management Protocol: A proprietary protocol developed by Cisco. The router uses CGMP to send multicast membership commands to Catalyst switches.

Challenge

Used to provide authentication in Challenge Handshake Authentication Protocol (CHAP) as part of the handshake process. This numerically unique query is sent to authenticate the user without sending the password unencrypted across the wire. See also: CHAP.

channelized E1

Operating at 2.048Mbps, an access link that is sectioned into 29 B channels and one D channel, supporting DDR, Frame Relay, and X.25. Compare to: channelized T1.

channelized T1

Operating at 1.544Mbps, an access link that is sectioned into 23 B channels and 1 D channel of 64Kbps each, where individual channels or groups of channels connect to various destinations, supporting DDR, Frame Relay, and X.25. Compare to: channelized E1.

CHAP

Challenge Handshake Authentication Protocol: Supported on lines using PPP encapsulation, it is a security feature that identifies the remote end, helping keep out unauthorized users. After CHAP is performed, the router or access server determines whether a given user is permitted access. It is a newer, more secure protocol than PAP. Compare to: PAP.

character mode connections

Character mode connections are typically terminated at the access server and include Telnet and console connections.

checksum

A test for ensuring the integrity of sent data. It is a number calculated from a series of values taken through a sequence of mathematical functions, typically placed at the end of the data from which it is calculated, and then recalculated at the receiving end for verification. Compare to: CRC.

choke packet

When congestion exists, it is a packet sent to inform a transmitter that it should decrease its sending rate.

CIDR

Classless Interdomain Routing: An IP addressing method designed to allow Class C addresses to be issued in blocks. Used by routing protocols such as BGP, OSPF and ISIS. See also: BGP4.

CIP

Channel Interface Processor: A channel attachment interface for use in Cisco 7000 series routers that connects a host mainframe to a control unit. This device eliminates the need for a Front-End Processor (FEP) to attach channels.

CIR

Committed Information Rate: Averaged over a minimum span of time and measured in bits per second (bps), a Frame Relay network's agreed-upon minimum rate of transferring information.

circuit switching

Used with dial-up networks such as PPP and ISDN. Passes data, but needs to set up the connection first-just like making a phone call.

Cisco Express Forwarding

See: CEF.

Cisco FRAD

Cisco Frame-Relay Access Device: A Cisco product that supports Cisco IPS Frame Relay SNA services, connecting SDLC devices to Frame Relay without requiring an existing LAN. Can be upgraded to a fully functioning multiprotocol router. Can activate conversion from SDLC to Ethernet and Token Ring, but does not support attached LANs. See also: FRAD.

Cisco hierarchical model

A Cisco model for building switched networks. The model consists of access layer devices, distribution layer devices, and core layer devices.

Cisco IOS software

Cisco Internetworking Operating System software. The kernel of the Cisco line of routers and switches that supplies shared functionality, scalability, and security for all products under its CiscoFusion architecture. See also: CiscoFusion.

CiscoFusion

Cisco's name for the internetworking architecture under which its Cisco IOS operates. It is designed to 'fuse' together the capabilities of its disparate collection of acquired routers and switches.

CiscoView

GUI-based management software for Cisco networking devices, enabling dynamic status, statistics, and comprehensive configuration information. Displays a physical view of the Cisco device chassis and provides device-monitoring functions and fundamental troubleshooting capabilities. Can be integrated with a number of SNMP-based network management platforms.

Class A network

Part of the Internet Protocol hierarchical addressing scheme. Class A networks have only 8 bits for defining networks and 24 bits for defining hosts on each network. Compare to: Class B network and Class C network.

Class B network

Part of the Internet Protocol hierarchical addressing scheme. Class B networks have 16 bits for defining networks and 16 bits for defining hosts on each network. Compare to: Class A network and Class C network.

Class C network

Part of the Internet Protocol hierarchical addressing scheme. Class C networks have 24 bits for defining networks and only 8 bits for defining hosts on each network. Compare to: Class A network and Class B network.

classical IP over ATM

Defined in RFC 1577, the specification for running IP over ATM that maximizes ATM features. Also known as 'CIA.'

classification

Classification is the term applied to setting specific parameters inside the Type of Service and DiffServe fields in the IP header, thus requesting special handling by all routers in the traffic path. See also: policing.

classless routing

Routing that allows use of Variable-Length Subnet Masks (VLSMs) and supernetting. Routing protocols that support classless routing are RIP version 2, EIGRP, and OSPF.

CLI

command-line interface: Enables you to configure Cisco routers and switches with maximum flexibility.

clocking

Used in synchronous connections to provide a marker for the start and end of data frames.

CLP

Cell Loss Priority: The area in the ATM cell header that determines the likelihood of a cell being dropped during network congestion. Cells with CLP = 0 are considered insured traffic and are not apt to be dropped. Cells with CLP = 1 are considered best-effort traffic that might be dropped during congested episodes, delivering more resources to handle insured traffic.

CLR

Cell Loss Ratio: The ratio of discarded cells to successfully delivered cells in ATM. CLR can be designated a QoS parameter when establishing a connection.

CO

central office: The local telephone company office where all loops in a certain area connect and where circuit switching of subscriber lines occurs.

collapsed backbone

A nondistributed backbone where all network segments are connected to each other through an internetworking device. A collapsed backbone can be a virtual network segment at work in a device such as a router, hub, or switch.

collapsed core

One switch performing both core and distribution layer functions. Typically found in a small network, the functions of the core and distribution layer are still distinct.

collision

The effect of two nodes sending transmissions simultaneously in Ethernet. When they meet on the physical media, the frames from each node collide and are damaged. See also: collision domain.

collision domain

The network area in Ethernet over which frames that have collided will spread. Collisions are propagated by hubs and repeaters, but not by LAN switches, routers, or bridges. See also: collision.

Common Spanning Tree (CST)

See: CST.

composite metric

Used with routing protocols, such as IGRP and EIGRP, that use more than one metric to find the best path to a remote network. IGRP and EIGRP both use bandwidth and delay of the line by default. However, Maximum Transmission Unit (MTU), load, and reliability of a link can be used as well.

compression

A technique to send more data across a link than would be normally permitted by representing repetitious strings of data with a single marker.

configuration register

A 16-bit configurable value stored in hardware or software that determines how Cisco routers function during initialization. In hardware, the bit position is set by using a jumper. In software, it is set by specifying specific bit patterns used to set startup options, configured by using a hexadecimal value with configuration commands.

congestion

Traffic that exceeds the network's capability to handle it.

congestion avoidance

To minimize delays, the method an ATM network uses to control traffic entering the system. Lower-priority traffic is discarded at the edge of the network when indicators signal it cannot be delivered, thus using resources efficiently.

congestion collapse

The situation that results from the retransmission of packets in ATM networks where little or no traffic successfully arrives at destination points. It usually happens in networks made of switches with ineffective or inadequate buffering capabilities combined with poor packet discard or ABR congestion feedback mechanisms.

connection ID

Identifications given to each Telnet session into a router. The show sessions command will give you the connections a local router will have to a remote router. The show users command will show the connection IDs of users telnetted into your local router.

connectionless

Data transfer that occurs without the creation of a virtual circuit. It has no overhead, uses best-effort delivery, is not reliable. Contrast with: connection-oriented. See also: virtual circuit.

connection-oriented

Data transfer method that sets up a virtual circuit before any data is transferred. Uses acknowledgments and flow control for reliable data transfer. Contrast with: connectionless. See also: virtual circuit.

console port

Typically an RJ-45 port on a Cisco router and switch that allows command-line interface capability.

Content Addressable Memory

See: CAM.

contention media

Media access method that is a baseband media-that is, first come, first served. Ethernet is an example of a contention media access.

contiguous buffers

Contiguous buffers are formed from a single physical memory block.

control direct VCC

One of three control connections defined by Phase I LAN emulation; a bi- directional virtual channel connection (VCC) established in ATM by an LEC to an LES. See also: control distribute VCC and data direct VCC.

control distribute VCC

One of three control connections defined by Phase 1 LAN emulation; a unidirectional virtual channel connection (VCC) set up in ATM from an LES to an LEC. Usually, the VCC is a point-to-multipoint connection. See also: control direct VCC and data direct VCC.

convergence

The process required for all routers in an internetwork to update their routing tables and create a consistent view of the network, using the best possible paths. No user data is passed during a convergence time.

core block

If you have two or more switch blocks, the Cisco rule of thumb states that you need a core block. No routing is performed at the core, only transferring of data. It is a pass- through for the switch block, the server block, and the Internet. The core is responsible for transferring data to and from the switch blocks as quickly as possible. You can build a fast core with a frame, packet, or cell (ATM) network technology.

core layer

Top layer in the Cisco three-layer hierarchical model, which helps you design, build, and maintain Cisco hierarchical networks. The core layer passes packets quickly to distribution-layer devices only. No packet filtering should take place at this layer.

cost

An arbitrary value, based on hop count, bandwidth, or other calculation, that is typically assigned by a network administrator and used by the routing protocol to compare different routes through an internetwork. Routing protocols use cost values to select the best path to a certain destination: the lowest cost identifies the best path. Also known as 'path cost.' See also: routing metric.

count to infinity

A problem occurring in routing algorithms that are slow to converge where routers keep increasing the hop count to particular networks. To avoid this problem, various solutions have been implemented into each of the different routing protocols. Some of those solutions include defining a maximum hop count (defining infinity), route poisoning, poison reverse, and split horizon.

CPCS

Common Part Convergence Sublayer: One of two AAL sublayers that is service-dependent, it is further segmented into the CS and SAR sublayers. The CPCS prepares data for transmission across the ATM network; it creates the 48-byte payload cells that are sent to the ATM layer. See also: AAL and ATM layer.

CPE

Customer Premises Equipment: Items such as telephones, modems, and terminals installed at customer locations and connected to the telephone company network.

crankback

In ATM, a correction technique used when a node somewhere on a chosen path cannot accept a connection setup request, blocking the request. The path is rolled back to an intermediate node, which then uses GCAC to attempt to find an alternate path to the final destination.

CRC

cyclic redundancy check: A methodology that detects errors, whereby the frame recipient makes a calculation by dividing frame contents with a prime binary divisor and compares the remainder to a value stored in the frame by the sending node. Compare to: checksum.

crossbar

A switch-fabric comprising a matrix of totally connected cross-points. Sometimes referred to as a matrix.

CSMA/CD

Carrier Sense Multiple Access with Collision Detection: A technology defined by the Ethernet IEEE 802.3 committee. Each device senses the cable for a digital signal before transmitting. Also, CSMA/CD allows all devices on the network to share the same cable, but one at a time. If two devices transmit at the same time, a frame collision will occur and a jamming pattern will be sent; the devices will stop transmitting, wait a predetermined amount of time, and then try to transmit again.

CST

Common Spanning Tree: One spanning tree instance encompassing every VLAN in the switched network.

CSU

channel service unit: A digital mechanism that connects end-user equipment to the local digital telephone loop. Frequently referred to along with the data service unit as 'CSU/DSU.' See also: DSU.

CTD

Cell Transfer Delay: For a given connection in ATM, the time period between a cell exit event at the source user-network interface (UNI) and the corresponding cell entry event at the destination. The CTD between these points is the sum of the total inter-ATM transmission delay and the total ATM processing delay.

custom queuing

Used by Cisco router IOS to provide a queuing method to slower serial links. Custom queuing enables an administrator to configure the type of traffic that will have priority over the link.

cut-through

See: cut-through frame switching.

cut-through frame switching

A frame-switching technique that flows data through a switch so that the leading edge exits the switch at the output port before the packet finishes entering the input port. Frames will be read, processed, and forwarded by devices that use cut- through switching as soon as the destination address of the frame is confirmed and the outgoing port is identified.