Glossary

access list

Rules applied to a router that will determine traffic patterns for data.

administrative distance

A value that ranges from 0 through 255 that determines the priority of a source's routing information.

advanced distance vector protocol

A routing protocol that combines the strengths of the distance vector and link state routing protocols. Cisco's Enhanced Interior Gateway Routing Protocol (EIGRP) is considered an advanced distance vector protocol.

Application layer

The highest layer of the OSI model (Layer 7). It is closest to the end user and selects appropriate network services to support end- user applications such as email and FTP.

area

See [autonomous system]

ARP (Address Resolution Protocol)

Used to map a known logical address to an unknown physical address. A device performs an ARP broadcast to identify the physical address of a destination device. This physical address is then stored in cache memory for later transmissions.

Asynchronous Transfer Mode ( ATM )

A dedicated-connection switching technology that organizes digital data into 53-byte cell units and transmits them over a physical medium using digital signal technology.

attenuation

A term that refers to the reduction in strength of a signal. Attenuation occurs with any type of signal, whether digital or analog. Sometimes referred to as signal loss .

autonomous system ( AS )

A group of networks under common administration that share a routing strategy. An autonomous system is sometimes referred to as a domain or area .

bandwidth

The available capacity of a network link over a physical medium.

BGP (Border Gateway Protocol)

An exterior routing protocol that exchanges route information between autonomous systems.

boot field

The lowest four binary digits of a configuration register. The value of the boot field determines the order in which a router searches for Cisco IOS software.

BRI (Basic Rate Interface)

An ISDN interface that contains two B channels and one D channel for circuit-switched communication for data, voice, and video.

bridge

A device used to segment a LAN into multiple physical segments. A bridge uses a forwarding table to determine which frames need to be forwarded to specific segments. Bridges isolate local traffic to the originating physical segment but forward all non-local and broadcast traffic.

Bridge Protocol Data Unit ( BDPU )

Data messages that are exchanged across the switches within an extended LAN that uses a spanning tree protocol topology.

broadcast

A data frame that every node on a local segment will be sent.

buffering

A method of flow control used by the Transport layer that involves the memory buffers on the receiving hosts . The Transport layer of the receiving system ensures that sufficient buffers are available and that data is not transmitted at a rate that exceeds the rate at which the receiving system can process it.

carrier detect signal

A signal received on a router interface that indicates whether the Physical layer connectivity is operating properly.

CDP (Cisco Discovery Protocol)

A Cisco proprietary protocol that operates at the Data Link layer. CDP enables network administrators to view a summary protocol and address information about other directly connected Cisco routers (and some Cisco switches).

channel

A single communications path on a system. In some situations, channels can be multiplexed over a single connection.

CHAP (Challenge Handshake Authentication Protocol)

An authentication protocol for the Point-to-Point Protocol (PPP) that uses a three-way, encrypted handshake to force a remote host to identify itself to a local host.

checksum

A field that performs calculations to ensure the integrity of data.

CIDR (Classless Interdomain Routing)

Implemented to resolve the rapid depletion of IP address space on the Internet and to minimize the number of routes on the Internet. CIDR provides a more efficient method of allocating IP address space by removing the concept of classes in IP addressing. CIDR enables routes to be summarized on powers-of-two boundaries, thus reducing multiple routes into a single prefix.

CIR (Committed Information Rate)

The rate at which a Frame Relay link transmits data, averaged over time. CIR is measured in bits per second.

classful addressing

Categorizes IP addresses into ranges that are used to create a hierarchy in the IP addressing scheme. The most common classes are A, B, and C, which can be identified by looking at the first three binary digits of an IP address.

classless addressing

Also commonly known as supernetting . The limited number of IP addresses was causing major concern and there was a huge waste of addressing as well, as routing became an issue. Because of a new hierarchical distribution of addresses, classless addressing resolved many concerns.

CO (central office)

The local telephone company office where all local loops in an area connect.

configuration register

A numeric value (typically displayed in hexadecimal form) used to specify certain actions on a router.

congestion

A situation that occurs during data transfer if one or more computers generate network traffic faster than it can be transmitted through the network.

connectionless network services

Connectionless network services involve using a permanently established link. Path selection and bandwidth allocation are done dynamically.

connection-oriented network services

Connection-oriented network services involve using a nonpermanent path for data transfer between systems. In order for two systems to communicate, they must establish a path that will be used for the duration of their connection.

console

A terminal attached directly to the router for configuring and monitoring the router.

convergence

The process by which all routers within an internetwork route information and eventually agree on optimal routes through the internetwork.

counting to infinity

A routing problem in which the distance metric for a destination network is continually increased because the internetwork has not fully converged .

CPE (customer premise equipment)

Terminating equipment such as telephones and modems supplied by the service provider, installed at the customer site, and connected to the network.

CRC (cyclic redundancy check)

An error-checking mechanism by which the receiving node calculates a value based on the data it receives and compares it with the value stored within the frame from the sending node.

CSMA/CD (Carrier Sense Multiple Access/Collision Detection)

A physical specification used by Ethernet to provide contention -based frame transmission. CSMA/ CD specifies that a sending device must share physical transmission media and listen to determine whether a collision occurs after transmitting. In simple terms, this means that an Ethernet card has a built-in capability to detect a potential packet collision on the internetwork.

cut-through switching

A method of forwarding frames based on the first 6 bytes contained in the frame. Cut-through switching provides higher throughput than store-and-forward switching because it requires only 6 bytes of data to make the forwarding decision. Cut-through switching does not provide error checking like its counterpart store-and-forward switching.

DARPA (Defense Advanced Research Projects Agency)

A government agency that develops advanced defense capabilities. It is now known as ARPA .

DCE (data communications equipment)

The device at the network end of a user-to-network connection that provides a physical connection to the network, forwards traffic, and provides a clocking signal used to synchronize data transmission between the DCE and DTE devices.

DDR (dial-on-demand routing)

The technique by which a router can initiate and terminate a circuit-switched connection over ISDN or telephone lines to meet network traffic demands.

de-encapsulation

The process by which a destination peer layer removes and reads the control information sent by the source peer layer in another network host.

default mask

A binary or decimal representation of the number of bits used to identify an IP network. The class of the IP address defines the default mask. A default mask is represented by four octets of binary digits. The mask can also be presented in dotted decimal notation.

default route

A network route (that usually points to another router) established to receive and attempt to process all packets for which no route appears in the route table.

delay

The amount of time necessary to move a packet through the internetwork from source to destination.

demarc

The point of demarcation is between the carrier's equipment and the customer premise equipment (CPE).

discard eligibility bit

A bit that can be set to indicate that a frame may be dropped if congestion occurs within the Frame Relay network.

distance vector protocol

An interior routing protocol that relies on distance and vector or direction to choose optimal paths. distance vector protocol requires each router to send all or a large part of its route table to its neighboring routers periodically.

DLCI (data link connection identifier)

A value that specifies a permanent virtual circuit (PVC) or switched virtual circuit (SVC) in a Frame Relay network.

DNS (Domain Name System)

A system used to translate fully qualified hostnames or computer names into IP addresses, and vice versa.

domain

See [autonomous system]

dotted decimal notation

A method of representing binary IP addresses in a decimal format. Dotted decimal notation represents the four octets of an IP address in four decimal values separated by decimal points.

DTE (data terminal equipment)

The device at the user end of the user-to-network connection that connects to a data network through a data communications equipment (DCE) device.

dynamic route

A network route that adjusts automatically to changes within the internetwork.

EGP (Exterior Gateway Protocol)

An exterior routing protocol that exchanges route information between autonomous systems. EGP has become obsolete and is being replaced by the Border Gateway Protocol (BGP).

EIGRP (Enhanced Interior Gateway Routing Protocol)

A Cisco proprietary routing protocol that includes features of both distance vector and link state routing protocols. EIGRP is considered an advanced distance vector protocol.

encapsulation

Generally speaking, encapsulation is the process of wrapping data in a particular protocol header. In the context of the OSI model, encapsulation is the process by which a source peer layer includes header and/or trailer control information with a Protocol Data Unit (PDU) destined for its peer layer in another network host. The information encapsulated instructs the destination peer layer how to process the information.

EXEC

The user interface for executing Cisco router commands.

exterior routing protocol

A routing protocol that conveys information between autonomous systems; it is widely used within the Internet. The Border Gateway Protocol (BGP) is an example of an exterior routing protocol.

FCS (frame check sequence)

Extra characters added to a frame for error control purposes. FCS is the result of a cyclic redundancy check (CRC).

Flash

Router memory that stores the Cisco IOS image and associated microcode . Flash is erasable, reprogrammable ROM that retains its content when the router is powered down or restarted.

flat routing protocol

A routing environment in which all routers are considered peers and can communicate with any other router in the network as directly as possible. A flat routing protocol functions well in simple and predictable network environments.

flow control

A mechanism that throttles back data transmission to ensure that a sending system does not overwhelm the receiving system with data.

Forward Explicit Congestion Notification ( FECN )

A Frame Relay message that notifies the receiving device that there is congestion in the network. An FECN bit is sent in the same direction in which the frame was traveling, toward its destination.

Frame Relay

A switched Data Link layer protocol that supports multiple virtual circuits using High-level Data Link Control (HDLC) encapsulation between connected devices.

frame tagging

A method of tagging a frame with a unique user-defined virtual local area network (VLAN). The process of tagging frames allows VLANs to span multiple switches.

FTP (File Transfer Protocol)

A protocol used to copy a file from one host to another host, regardless of the physical hardware or operating system of each device. FTP identifies a client and server during the file-transfer process. In addition, it provides a guaranteed transfer by using the services of the Transmission Control Protocol (TCP).

full-duplex

The physical transmission process on a network device by which one pair of wires transmits data while another pair of wires receives data. Full-duplex transmission is achieved by eliminating the possibility of collisions on an Ethernet segment, thereby eliminating the need for a device to sense collisions.

function

A term that refers to the different devices and the hardware tasks these devices perform within ISDN.

global configuration mode

A router mode that enables simple router configuration commands, such as router names, banners, and passwords, to be executed. Global configuration commands affect the whole router rather than a single interface or component.

GNS (Get Nearest Server)

A request sent by an Internetwork Packet Exchange (IPX) client to locate the closest active server of a particular service. Depending on where the service can be located, either a server or a router can respond to the request.

half-duplex

The physical transmission process whereby one pair of wires is used to transmit information and the other pair of wires is used to receive information or to sense collisions on the physical media. Half-duplex transmission is required on Ethernet segments with multiple devices.

handshake

The process of one system making a request to another system prior to a connection being established. Handshakes occur during the establishment of a connection between two systems, and they address matters such as synchronization and connection parameters.

HDLC (High-level Data Link Control)

A bit-oriented, synchronous Data Link layer protocol that specifies data encapsulation methods on serial links.

Header

Control information placed before the data during the encapsulation process.

hierarchical routing protocol

A routing environment that relies on several routers to compose a backbone. Most traffic from non-backbone routers traverses the backbone routers (or at least travels to the backbone) to reach another non-backbone router. This is accomplished by breaking a network into a hierarchy of networks, where each level is responsible for its own routing.

hold-down

The state into which a route is placed so that routers will not advertise or accept updates for that route until a timer expires .

hop count

The number of routers a packet passes through on its way to the destination network.

hostname

A logical name given to a router.

HSSI (High-Speed Serial Interface)

A physical standard designed for serial connections that require high data transmission rates. The HSSI standard allows for high-speed communication that runs at speeds up to 52Mbps.

ICMP (Internet Control Message Protocol)

A protocol that communicates error messages and controls messages between devices. Thirteen different types of ICMP messages are defined. ICMP allows devices to check the status of other devices, to query the current time, and to perform other functions such as ping and traceroute .

IEEE (Institute of Electrical and Electronics Engineers)

An organization that defines standards for network LANs, among other things. The IEEE is the industry standard used in today's computing world.

IGRP (Interior Gateway Routing Protocol)

A Cisco proprietary distance vector routing protocol that uses hop count as its metric.

initial configuration dialog

The dialog used to configure a router the first time it is booted or when no configuration file exists. The initial configuration dialog is an optional tool used to simplify the configuration process.

integrated routing

A technique in which a router that is routing multiple routed protocols shares resources. Rather than using several routing protocols to support multiple routed protocols, a network administrator can use a single routing protocol to support multiple routed protocols. The Enhanced Interior Gateway Routing Protocol (EIGRP) is an example of a routing protocol that supports integrated routing.

interdomain router

A router that uses an exterior routing protocol, such as the Border Gateway Protocol (BGP), to exchange route information between autonomous systems.

interfaces

Router components that provide the network connections where data packets move in and out of the router. Depending on the model of router, interfaces exist either on the motherboard or on separate, modular interface cards.

interior routing protocol

A routing protocol that exchanges information within an autonomous system. Routing Information Protocol (RIP), Interior Gateway Routing Protocol (IGRP), and Open Shortest Path First (OSPF) are examples of interior routing protocols.

intradomain router

A router that uses an interior routing protocol, such as the Interior Gateway Routing Protocol (IGRP), to convey route information within an autonomous system.

IP (Internet Protocol)

One of the many protocols maintained in the TCP/IP suite of protocols. IP is the transport mechanism for Transmission Control Protocol (TCP), User Datagram Protocol (UDP), and Internet Control Message Protocol (ICMP) data. It also provides the logical addressing necessary for complex routing activity.

IP extended access list

An access list that provides a way of filtering IP traffic on a router interface based on the source and destination IP address or port, IP precedence field, TOS field, ICMP-type, ICMP-code, ICMP-message, IGMP-type, and TCP-established connections.

IP standard access list

An access list that provides a way of filtering IP traffic on a router interface based on the source IP address or address range.

IPX (Internetwork Packet Exchange)

The Layer 3 protocol used within NetWare to transmit data between servers and workstations.

IPX extended access list

An access list that provides a way of filtering IPX traffic on a router interface based on the source and destination IPX address or address range, IPX protocol, and source and destination sockets.

IPX SAP filter

A method of filtering SAP traffic on a router interface. SAP filters are used to filter SAP traffic origination or traffic destined for specific IPX addresses or address ranges.

IPX standard access list

An access list that provides a way of filtering IPX traffic on a router interface based on the source IPX address or address range.

ISDN (Integrated Services Digital Network)

A communications protocol offered by telephone companies that permits telephone networks to carry data, voice, and other traffic.

ISL (interswitch link)

A protocol used to allow virtual local area networks (VLANs) to span multiple switches. ISL is used between switches to communicate common VLANs between devices.

keepalive frames

Protocol Data Units (PDUs) transmitted at the Data Link layer that indicate whether the proper frame type is configured.

LAN protocols

Protocols that identify Layer 2 protocols used for the transmission of data within a local area network (LAN). The three most popular LAN protocols used today are Ethernet, token ring, and Fiber Distributed Data Interface (FDDI).

LCP (Link Control Protocol)

A protocol that configures, tests, maintains, and terminates Point-to-Point Protocol (PPP) connections.

link state packet

A broadcast packet that contains the status of a router's links or network interfaces.

link state protocol

An interior routing protocol in which each router sends only the state of its own network links across the network, but sends this information to every router within its autonomous system or area. This process enables routers to learn and maintain full knowledge of the network's exact topology and how it is interconnected . Link state protocols use a "shortest path first" algorithm.

LLC (Logical Link Control) sublayer

A sublayer of the Data Link layer. The LLC sublayer provides the software functions of the Data Link layer.

LMI (Local Management Interface)

A set of enhancements to the Frame Relay protocol specifications used to manage complex networks. Some key Frame Relay LMI extensions include global addressing, virtual circuit status messages, and multicasting.

load

An indication of how busy a network resource is. CPU utilization and packets processed per second are two indicators of load.

local loop

The line from the customer's premises to the telephone company's central office (CO).

logical addressing

Network layer addressing is most commonly referred to as logical addressing (versus the physical addressing of the Data Link layer). A logical address consists of two parts : the network and the node. Routers use the network part of the logical address to determine the best path to the network of a remote device. The node part of the logical address is used to identify the specific node to forward the packet on the destination network.

logical ANDing

A process of comparing two sets of binary numbers to result in one value representing an IP address network. Logical ANDing is used to compare an IP address against its subnet mask to yield the IP subnet on which the IP address resides. ANDing is also used to determine whether a packet has a local or remote destination.

MAC address

A physical address used to define a device uniquely.

MAC (Media Access Control) layer

A sublayer of the Data Link layer that provides the hardware functions of the Data Link layer.

metric

The relative cost of sending packets to a destination network over a specific network route. Examples of metrics include bandwidth, delay, and reliability.

MIB (management information database)

A database that maintains statistics on certain data items. The Simple Network Management Protocol (SNMP) uses MIBs to query information about devices.

multicasting

A process of using one IP address to represent a group of IP addresses. Multicasting is used to send messages to a subset of IP addresses in a network or networks.

multipath routing protocol

A routing protocol that load- balances over multiple optimal paths to a destination network when the costs of the paths are equal.

multiplexing

A method of flow control used by the Transport layer in which application conversations are combined over a single channel by interleaving packets from different segments and trans mitting them.

NBMA (nonbroadcast multiaccess )

A multiaccess network that either does not support broadcasts or for which sending broadcasts is not feasible .

NCP (NetWare Core Protocol)

A collection of upper-layer server routines that satisfy requests from other applications.

NCP (network control protocol)

A collection of protocols that establishes and configures different Network layer protocols for use over a Point-to-Point Protocol (PPP) connection.

NetBIOS (Network Basic Input/Output System)

A common Session layer interface specification from IBM and Microsoft that enables applications to request lower-level network services.

NetWare

A popular LAN operating system developed by Novell Corporation that runs on a variety of different types of LANs.

NetWare shell

An upper-layer NetWare service that determines whether application calls require additional network services.

network discovery

When a router starts up, this is the process by which it learns of its internetwork environment and begins to communicate with other routers.

NIC (network interface card)

A board that provides network communication capabilities to and from a network host.

NLSP (NetWare Link State Protocol)

A link state routing protocol used for routing Internetwork Package Exchange (IPX).

NOS (network operating system)

A term used to describe distributed file systems that support file sharing, printing, database access, and other similar applications.

NVRAM (nonvolatile random access memory)

A memory area of the router that stores permanent information, such as the router's backup configuration file. The contents of NVRAM are retained when the router is powered down or restarted.

OSI (Open Systems Interconnection) model

A layered networking framework developed by the International Organization for Standardization. The OSI model describes seven layers that correspond to specific networking functions.

OSPF (Open Shortest Path First)

A hierarchical link state routing protocol that was developed as a successor to the Routing Information Protocol (RIP).

packet switching

A process by which a router moves a packet from one interface to another.

PAP (Password Authentication Protocol)

An authentication protocol for the Point-to-Point Protocol (PPP) that uses a two-way, unencrypted handshake to enable a remote host to identify itself to a local host.

parallelization

A method of flow control used by the Transport layer in which multiple channels are combined to increase the effective bandwidth for the upper layers; synonymous with multilink .

path length

The sum of the costs of each link traversed up to the destination network. Some routing protocols refer to path length as hop count .

PDU (Protocol Data Unit)

A unit of measure that refers to data that is transmitted between two peer layers within different network devices. Segments, packets, and frames are examples of PDUs.

peer-to-peer communication

A form of communication that occurs between the same layers of two different network hosts.

physical connection

A direct connection between two devices.

ping

A tool for testing IP connectivity between two devices. Ping is used to send multiple IP packets between a sending and a receiving device. The destination device responds with an Internet Control Message Protocol (ICMP) packet to notify the source device of its existence.

POP (point of presence)

A physical location where a carrier has installed equipment to interconnect with a local exchange carrier.

PPP (Point-to-Point Protocol)

A standard protocol that enables router-to-router and host-to-network connectivity over synchronous and asynchronous circuits such as telephone lines.

Presentation layer

Layer 6 of the OSI model. The Presentation layer is concerned with how data is represented to the Application layer.

PRI (Primary Rate Interface)

An ISDN interface that contains 23 B channels and 1 D channel for circuit-switched communication for data, voice, and video. In North America and Japan, a PRI contains 23 B channels and 1 D channel. In Europe, it contains 30 B channels and 1 D channel.

privileged mode

An extensive administrative and management mode on a Cisco router. This router mode permits testing, debugging, and commands to modify the router's configuration.

protocol

A formal description of a set of rules and conventions that defines how devices on a network must exchange information.

PSTN (public switched telephone network)

The circuit-switching facilities maintained for voice analog communication.

PVC (permanent virtual circuit)

A virtual circuit that is permanently established and ready for use.

RAM (random access memory)

A memory area of a router that serves as a working storage area. RAM contains data such as route tables, various types of caches and buffers, as well as input and output queues and the router's active configuration file. The contents of RAM are lost when the router is powered down or restarted.

RARP (Reverse Address Resolution Protocol)

This protocol provides mapping that is exactly opposite to the Address Resolution Protocol (ARP). RARP maps a known physical address to a logical address. Diskless machines that do not have a configured IP address when started typically use RARP. RARP requires the existence of a server that maintains physical-to-logical address mappings.

reference point

An identifier of the logical interfaces between functions within ISDN.

reliability

A metric that allows the network administrator to assign arbitrarily a numeric value to indicate a reliability factor for a link. The reliability metric is a method used to capture an administrator's experience with a given network link.

RIP (Routing Information Protocol)

A widely used distance vector routing protocol that uses hop count as its metric.

ROM (read-only memory)

An area of router memory that contains a version of the Cisco IOS image ”usually an older version with minimal functionality. ROM also stores the bootstrap program and power-on diagnostic programs.

ROM monitor

A mode on a Cisco router in which the executing software is maintained in ROM.

ROM monitor mode (RXBOOT)

A router-maintenance mode that enables router recovery functions when the IOS file in Flash has been erased or is corrupt.

route aggregation

The process of combining multiple IP address networks into one superset of IP address networks. Route aggregation is implemented to reduce the number of route table entries required to forward IP packets accurately in an internetwork.

route poisoning

A routing technique by which a router immediately marks a network as unreachable as soon as it detects that the network is down. The router broadcasts the update throughout the network and maintains this poisoned route in its route table for a specified period of time.

route table

An area of a router's memory that stores the network topology information used to determine optimal routes. Route tables contain information such as destination network, next hop, and associated metrics.

routed protocol

A protocol that provides the information required for the routing protocol to determine the topology of the internetwork and the best path to a destination. The routed protocol provides this information in the form of a logical address and other fields within a packet. The information contained in the packet allows the router to direct user traffic. The most common routed protocols include Internet Protocol (IP) and Internetwork Packet Exchange (IPX).

router modes

Modes that enable the execution of specific router commands and functions. User, privileged, and setup are examples of router modes that allow you to perform certain tasks.

routing algorithms

Well-defined rules that aid routers in the collection of route information and the determination of the optimal path.

routing loop

An event in which two or more routers have not yet converged and are propagating their inaccurate route tables. In addition, they are probably still switching packets based on their inaccurate route tables.

routing protocols

Routing protocols use algorithms to generate a list of paths to a particular destination and the cost associated with each path. Routers use routing protocols to communicate among each other the best route to use to reach a particular destination.

RS-232

A physical standard used to identify cabling types for serial data transmission for speeds of 19.2Kbps or less. RS-232 connects two devices communicating over a serial link with either a 25-pin (DB-25) or 9-pin (DB-9) serial interface. RS-232 is now known as EIA/TIA-232 .

running configuration file

The executing configuration file on a router.

SAP (Service Advertisement Protocol)

An Internetwork Package Exchange (IPX) protocol that serves as a means to inform network clients of available network resources and services.

SDLC (Synchronous Data Link Control)

Primarily used for terminal-to-mainframe communication, SDLC requires that one device be labeled as the primary station and all other devices be labeled as secondary stations . Communication can only occur between the primary station and a secondary station.

session

A dialogue between two or more different systems.

Session layer

As Layer 5 of the OSI model, the Session layer is concerned with establishing, managing, and terminating sessions between applications on different network devices.

setup mode

The router mode triggered on startup if no configuration file resides in nonvolatile random access memory (NVRAM).

shortest path first

See [link state protocol]

single-path routing protocol

A routing protocol that uses only one optimal path to a destination.

sliding windows

A method by which TCP dynamically sets the window size during a connection, allowing either device involved in the communication to slow down the sending data rate based on the other device's capacity.

SMTP (Simple Mail Transfer Protocol)

A protocol used to pass mail messages between devices, SMTP uses Transmission Control Protocol (TCP) connections to pass the email between hosts.

socket

The combination of the sending and destination Transmission Control Protocol (TCP) port numbers and the sending and destination Internet Protocol (IP) addresses defines a socket. Therefore, a socket can be used to define any User Datagram Protocol (UDP) or TCP connection uniquely.

Spanning Tree Protocol

A protocol used to eliminate all circular routes in a bridged or switched environment while maintaining redundancy. Circular routes are not desirable in Layer 2 networks because of the forwarding mechanism employed at this layer.

split horizon

A routing mechanism that prevents a router from sending information that it received about a network back to its neighbor that originally sent the information. This mechanism is very useful in preventing routing loops.

SPX (Sequenced Packet Exchange)

The Layer 4 protocol used within NetWare to ensure reliable, connection-oriented services.

startup configuration file

The backup configuration file on a router.

static route

A network route that is manually entered into the route table. Static routes function well in very simple and predictable network environments.

store-and-forward switching

A method of forwarding frames by copying an entire frame into the buffer of a switch and making a forwarding decision. Store-and-forward switching does not achieve the same throughput as its counterpart, cut-through switching, because it copies the entire frame into the buffer instead of copying only the first 6 bytes. Store-and-forward switching, however, provides error checking that is not provided by cut-through switching.

subinterface

One of possibly many virtual interfaces on a single physical interface.

subnetting

A process of splitting a classful range of IP addresses into multiple IP networks to allow more flexibility in IP addressing schemes. Subnetting overcomes the limitation of address classes and allows network administrators the flexibility to assign multiple networks with one class of IP addresses.

switch

A switch provides increased port density and forwarding capabilities as compared to bridges. The increased port densities of switches allow LANs to be microsegmented, thereby increasing the amount of bandwidth delivered to each device.

TCP (Transmission Control Protocol)

One of the many protocols maintained in the TCP/IP suite of protocols. TCP provides a connection-oriented and reliable service to the applications that use it.

TCP three-way handshake

A process by which TCP connections send acknowledgments between each other when setting up a TCP connection.

TCP windowing

A method of increasing or reducing the number of acknowledgments required between data transmissions. This allows devices to throttle the rate at which data is transmitted.

Telnet

A standard protocol that provides a virtual terminal. Telnet enables a network administrator to connect to a router remotely.

TFTP (Trivial File Transfer Protocol)

A protocol used to copy files from one device to another. TFTP is a stripped-down version of FTP.

tick

A measure of network delay time about 1/18th of a second. In RIP version 2, ticks serve as the primary value used to determine the best path.

traceroute

An IP service that allows a user to utilize the services of the User Datagram Protocol (UDP) and the Internet Control Message Protocol (ICMP) to identify the number of hops between sending and receiving devices and the paths taken from the sending to the receiving device. Traceroute also provides the IP address and DNS name of each hop. Typically, traceroute is used to troubleshoot IP connectivity between two devices.

trailer

Control information placed after the data during the encapsulation process.

See also [encapsulation]

Transport layer

As Layer 4 of the OSI model, it is concerned with segmenting upper-layer applications, establishing end-to-end connectivity through the network, sending segments from one host to another, and ensuring the reliable transport of data.

trunk

A switch port that connects to another switch to allow virtual local area networks (VLANs) to span multiple switches.

tunnel

A tunnel takes packets or frames from one protocol and places them inside frames from another network system.

See also [encapsulation]

UDP (User Datagram Protocol)

One of the many protocols maintained in the TCP/IP suite of protocols, UDP is a Layer 4, best-effort delivery protocol and, therefore, maintains connectionless network services.

user mode

A display-only mode on a Cisco router. Only limited information about the router can be viewed within this router mode; no configuration changes are permitted.

V.35

A physical standard used to identify cabling types for serial data transmission for speeds up to 4Mbps. The V.35 standard was created by the International Telecommunication Union-Telecommunication (ITU-T) standardization sector.

virtual connection

A logical connection between two devices created through the use of acknowledgments.

VLAN (virtual local area network)

A technique of assigning devices to specific LANs based on the port to which they attach on a switch rather than the physical location. VLANs extend the flexibility of LANs by allowing devices to be assigned to specific LANs on a port-by-port basis versus a device basis.

VLSM (variable-length subnet masking)

VLSM provides more flexibility in assigning IP address space. (A common problem with routing protocols is the necessity of all devices in a given routing protocol domain to use the same subnet mask.) Routing protocols that support VLSM allow administrators to assign IP networks with different subnet masks. This increased flexibility saves IP address space because administrators can assign IP networks based on the number of hosts on each network.

VTP

With VTP, an administrator can make configuration changes centrally on a single Catalyst series switch and have those changes automatically communicated to all the other switches in the network.

WANs (wide area networks)

WANs use data communications equipment (DCE) to connect multiple LANs. Examples of WAN protocols include but are not limited to Frame Relay, Point-to-Point Protocol (PPP), High-level Data Link Control (HDLC), and Integrated Services Digital Network (ISDN).

well-known ports

A set of ports between 1 and 1,023 that are reserved for specific TCP/IP protocols and services.