Glossary

ABR (area border router)

An OSPF routing device that is situated at the border of the backbone area and one or more other OSPF areas.

access list

A set or grouping of conditions that are maintained by routers to determine which traffic qualifies for filtering, rules, and other network services.

access rate

The designated bandwidth of a particular network circuit, such as 1.544Mbps for a T1 link.

acknowledgement

A notification packet (ACK) sent from one node to another node to verify that a message has been received or a certain event has transpired.

AD (administrative distance)

A numerical value between 0 and 255, with a higher value being preferred, that specifies the degree of trustworthiness or integrity for the source of routing information.

address resolution

A mechanism for determining and resolving the disparities between computer system addressing schemes such as Layer 3 (Network) addresses and Layer 2 (Data Link) MAC addresses.

adjacency

A bond formed between end nodes and select peer routers on a common media for the purpose of exchanging routing information.

administrative domain ( AD )

A group of hosts , network devices, and network segments that function as a single organizational unit.

AFI (Authority and Format Identifier)

The 1-byte portion of the OSI NSAP address that defines the format and authority that assigned the address. It also distinguishes the type and format of the IDI part of an ATM address.

aggregation

See [route summarization]

algorithm

A collection of rules or mechanisms for solving a particular problem. In networking, algorithms are typically used for optimal path determination in routing traffic from a source to a destination device.

area

A logical collection of network segments and their devices, which are usually linked to other areas to form a single autonomous system.

AS (autonomous system)

A collection of networks that are under common administrative control and share identical routing information. An AS is designated by a 16-bit numerical identifier assigned by the Internet Assigned Numbers Authority (IANA).

ASBR (autonomous system boundary router)

An area border router that resides on the border of an OSPF autonomous system and a non-OSPF network running another routing protocol in addition to OSPF.

ASN (autonomous system number)

A 16-bit number used to identify an autonomous system. Public ASNs are allocated by the Internet Assigned Numbers Authority (IANA).

ATM (Asynchronous Transfer Mode)

An international networking standard that implements a fixed-length 53-byte cell for transmitting multiple data types such as voice, video, and regular data packets. ATM leverages high-speed transmission media such as E3, T3, SONET, and OC carriers .

attribute

An attribute is a characteristic. BGP update messages have information about BGP metrics, which are called path attributes.

backbone

The primary bus or wire that connects networking devices. The term is often used to describe the core network connections of the Internet. In an internetwork, the backbone is the main path for traffic sent to and received from other networks.

bandwidth

A range in a band of frequencies or wavelengths . In networking, bandwidth is a measurement of the amount of data that can be transmitted in a finite amount of time and is typically expressed in bits per second (bps) or bytes per second (Bps).

BBR (backbone router)

The routers that are part of the main network connections that compose an internetwork. In OSPF, a backbone router is a part of Area 0.

BDR (backup designated router)

The backup designated router (BDR) accepts all the same information as the designated router (DR), but does not conduct any synchronization or forwarding functions unless the DR fails.

See also [DR (designated router)]

BGP (Border Gateway Protocol)

An interdomain routing protocol that replaces the obsolete EGP. BGP shares reachability information with other BGP domains. BGP version 4 is the principal interdomain routing protocol implemented on the Internet.

BGP peers

Also known as BGP neighbors.

BGP speakers

Routers that run the BGP protocol and exchange routing information.

BGP-4 (Border Gateway Protocol version 4)

The dominant interdomain routing protocol used on the Internet.

black hole

In routing, a destination that shows up in routing tables, but when traffic is sent to that destination, the traffic is discarded.

border router

Within OSPF, a router that links one area to backbone Area 0.

broadcast

The process of simultaneously sending the same message to multiple destination nodes. In networking, a distinction is made between broadcasting and multicasting. Broadcasting floods a message to each node on the network, whereas multicasting transmits a message to a select group of destination nodes.

buffer

A temporary data storage area, typically in dynamic RAM (DRAM) memory. Most buffers serve as a holding area while the CPU manipulates data packets before sending them to a network device.

CIDR (Classless Inter-Domain Routing)

Also called supernetting, an IP addressing scheme that replaces an older system based on classes A, B, and C. A single IP address is used to designate many unique IP addresses. A CIDR IP address has the appearance of a typical IP address, except that it ends with a slash followed by a number known as the IP prefix, such as 172.16.0.0/16.

CIR (committed information rate)

A designated amount of guaranteed bandwidth (measured in bits per second) on a frame relay service.

circuit

A communication pathway between two or more endpoints.

Cisco IOS

The Internetwork Operating System (IOS) is the software module that represents the kernel of the Cisco router and switch operating system.

Class A network

The portion of the IP hierarchical addressing scheme where the first 8 bits define the network and the remaining 24 bits represent the hosts on the network.

Class B network

The portion of the IP hierarchical addressing scheme where the first 16 bits define the network and the remaining 16 bits represent the hosts on the network.

Class C network

The portion of the IP hierarchical addressing scheme where the first 24 bits define the network and the remaining 8 bits represent the hosts on the network.

classful routing

A routing process used by protocols such as RIP and IGRP, which do not send any information about the prefix length.

classless routing

A routing process used by protocols such as RIPv2, EIGRP, and OSPF, which send subnet mask information in the routing updates.

CLI (command-line interface)

The interface of a Cisco router or switch that enables you to configure them from a console session.

client

A network host, node, or program that requests services from a server.

CLNP (Connectionless Network Protocol)

The OSI equivalent of IP.

CLNS (Connectionless Network Service)

One of two types of OSI Network layer services available to OSI Layer 4 (Transport). CLNS does not demand that a circuit be created before transmitting data. This service routes messages to destinations independently of other messages.

cluster

In BGP, the amalgamation of a route reflector and its clients .

cluster ID

In BGP, an identifier that enables a route reflector to recognize updates from other route reflectors in the same cluster.

cluster list

In BGP, a list or sequence of cluster IDs through which the route has passed.

CMNS (Connection-Mode Network Service)

One of two types of OSI Network layer services available at OSI Layer 4 (transport).

community

A BGP attribute that is used to organize BGP speakers according to shared destinations and common policies.

composite metric

The process of using more than one metric to determine the optimal path to a remote network, for example, bandwidth and delay with IGRP and EIGRP.

confederation

An extension of using route reflectors that enables one to divide an AS into sub-ASs running EBGP between them.

connectionless

Network protocols that enable a host to transmit a message without establishing a connection with the destination host. Examples of connectionless protocols include Ethernet, Internetwork Packet Exchange (IPX), and UDP.

connection-oriented

Network protocols that require a communication channel to be established between the sender and receiver before any messages are sent.

CONP (Connection-Oriented Network Protocol)

An OSI Network layer protocol that transports upper-layer information and error messages over connection-oriented links.

convergence

The amount of time that it takes for a router to update its routing tables.

core

See [backbone]

cost

An arbitrary value used by the routing protocol to compare paths through an internetwork. This value is often assigned by a network administrator.

count to infinity

A network problem that happens with slowly converging algorithms where the hop count keeps increasing.

CSNP (Complete Sequence Number PDU)

A list of the link-state packets stored by the router, which in IS-IS is implemented to distribute the router's complete Link-State database.

DDR (Dial-on-Demand Routing)

A Cisco routing process that permits a user to use existing telephone lines to generate a WAN.

default route

A static routing table entry that sends frames that do not have a next -hop entry in the routing table to a default destination.

delay

The time between the start of a transaction by a sender and the first reply received by the sender. Delay is also defined as the time it takes to transport a packet from source to destination over a particular path.

destination address

The physical or logical address of a device that is the recipient of data.

DIS (designated intermediate system)

The elected designated router for an IS-IS network. The DIS is configurable by priority value and then MAC address.

distance vector

A category of routing algorithm that demands that each router send all or part of its routing table to its directly connected neighbors. Also known as a Bellman-Ford algorithm.

distribute list

An access list that is used to filter inbound and outbound route table entries on a router.

DR (designated router)

A router running an OSPF process that is elected to generate the LSAs for a multi-access network, such as Ethernet.

See also [BDR (backup designated router)]

DUAL ( diffusing update algorithm)

The algorithm used by Cisco's EIGRP. DUAL offers a loop-free environment because it engages in constant route recomputation.

dynamic address resolution

The process of implementing an address resolution protocol to establish and maintain address information that is to be available on demand.

dynamic routing

A process of adaptive routing that automatically adjusts to traffic or physical network changes.

E1

An autonomous system external type 1 entry that originates from the ASBR and describes routes to the destination external system.

E2

An autonomous system external type 2 entry that originates from the ASBR and describes routes to destinations external to the autonomous system.

EBGP (External BGP)

The scenario in which BGP is operating between routers in different autonomous systems.

edge device

A routing device that enables packets to be forwarded between legacy interfaces and ATM or Frame Relay interfaces based on Layer 2 and Layer 3 processes. Edge devices do not run Layer 3 routing protocols.

EGP (Exterior Gateway Protocol)

A general term for Internet protocol that exchanges routes among autonomous systems.

EIGRP (Enhanced Interior Gateway Routing Protocol)

An advanced version of Cisco's IGRP. EIGRP offers better convergence and efficient routing with a combination of characteristics of link-state and distance vector protocols.

encapsulation

The process in which data is wrapped inside a protocol header. In networking, the equivalent of tunneling.

encryption

The translation of data into a secret format. This is the most effective way to secure data over a network.

ES (end system)

Any non-routing host or node in an OSI IS-IS environment.

ES-IS (End System “to “Intermediate System)

A discovery protocol that performs functions between end systems (ESs) and intermediate systems (ISs), commonly referred to as Level 0 routing. ES-IS is similar to the ARP protocol used in IP.

ESH (end system hello)

Sent by ESs in an IS-IS environment.

exterior routing protocol

A particular type of routing protocol that links two autonomous systems.

FD ( feasible distance)

In EIGRP, the lowest -cost route to a destination.

filter

A mechanism or device that screens or filters network traffic for specific characteristics and decides whether to forward or drop the packets based on established rules.

flapping

A situation in which an advertised route between two devices "flaps" back and forth between two paths because of the intermittent failure of an interface.

flash update

An asynchronous update packet sent in response to a change in the network topology.

floating static route

A static route with a higher AD value that can be overridden by a route entry learned from a dynamic routing protocol.

flooding

The process of passing traffic out of all interfaces of a networking device, with the exception of the interface on which the data was received.

frame relay

A packet-switching protocol that links network devices on a WAN and supports data transfer speeds of T1 (1.544Mbps) and T3 (45Mbps).

FS (feasible successor)

A route stored in the topology table and injected into the routing table if the current successor fails.

full mesh

A topology in which every node in a network has a link connecting it to every other node. Full mesh is expensive to implement but offers the highest degree of redundancy. Full mesh is typically used in backbone networks.

gateway

A node on a network that serves as an entry point to another network.

HDLC (High-level Data Link Control)

A protocol used at OSI Layer 2 (Data Link) for data communications. HDLC embeds information into a data frame so that network devices can control data flow and correct errors.

header

In network transmissions, the portion of the data packet that contains transparent information about the data or the transmission controls.

hello

A protocol used by OSPF routing devices for establishing, selecting, and maintaining neighbor relationships and adjacencies.

hello packet

A multicast data packet used for neighbor discovery, maintenance, and recovery.

hello PDU

A data packet used in IS-IS to generate and maintain peer adjacencies using ESH, ISH, or IS-IS Hello messages.

hierarchical addressing

An addressing scheme that uses a logical sequence of commands to determine a network's location. IP addresses consist of a hierarchy of network numbers, subnet numbers, and host numbers that send data packets to the proper destination.

hierarchical network

A network configuration of multiple segments that offers a single path from source segments through intermediate segments on to destination segments.

hold-down

A router state that is characterized by the fact that it can neither advertise a route nor accept updates about a route for a particular period of time. This process is used to prevent bad information from propagating through the network.

hold time

In EIGRP, the amount of time in seconds that a router waits to hear from its neighbor before designating it as down.

hop

A segment of the trip a data packet takes between any two nodes on a network.

hop count

A type of routing metric used by RIP in which the distance is calculated between a source and a destination.

host address

The logical address that is configured on a host device by an administrator and that logically identifies the device on a network.

IETF (Internet Engineering Task Force)

A governing body that consists of more than 80 groups working together to develop Internet standards.

IGP (Interior Gateway Protocol)

Any protocol, such as RIP, IGRP, and OSPF, used by the Internet to exchange routing data packets within a networking system.

IGRP (Interior Gateway Routing Protocol)

A proprietary distance vector protocol designed by Cisco.

inter-area routing

The process of routing between two or more logical areas.

internetwork

Generally speaking, a collection of networks that are interconnected by networking devices to function as a single network. Although an internetwork is sometimes referred to as an internet, it is not to be confused with the global Internet.

internetworking

The general process of connecting networks to each other. For example, when networks are connected to a router, an internetwork is technically created.

intra-area routing

Routing processes that take place within a single logical area.

IP (Internet Protocol)

A Network layer protocol that is a core component of the TCP/IP networking stack, allowing for connectionless service. IP provides addressing, type-of-service specification, fragmentation, and security.

IP address

This 32-bit address, also called an Internet address, uniquely defines any device (host) on the Internet or TCP/IP network.

IP multicast

A mechanism used for routing IP traffic from a single source to a logical group of endpoints, or from multiple sources to many destinations.

IP unnumbered

A Cisco proprietary protocol that permits two point-to-point connections to communicate without using an IP address.

IPv6 (Internet Protocol version 6)

The current official name of the next generation of IP is IPv6, where the v6 stands for version 6. The existing version of IP is version 4, sometimes referred to as IPv4.

IR (internal router)

A router that resides completely inside an IS-IS and does not have a link on the boundary between areas.

IS (intermediate system)

The term used for a router in OSI ES-IS and IS-IS protocols.

ISO-IGRP (International Organization for Standardization ”Interior Gateway Routing Protocol)

The Cisco proprietary version of a protocol implemented in a pure OSI (CLNS) environment.

LAN (local area network)

Generally, any network linking two or more nodes within a limited geographical area.

latency

The amount of time that it takes for a data packet to get from one location to another. In the context of networking, it can be the elapsed time (delay) between the execution of a device's request for access to a network and the time the mechanism actually is permitted transmission. It can also be defined as the elapsed time between when a mechanism receives a frame and the time that frame is forwarded out of the destination port.

leased lines

Permanent links between two endpoints leased from a telephone company.

Level 1 IS

The IS-IS equivalent of an OSPF internal non-backbone area router that has the duty of routing to ESs within an area.

Level 2 IS

The IS-IS equivalent of an OSPF backbone router that can route only between areas.

Level 1 “2 IS ( L1L2 )

The IS-IS equivalent of an OSPF ABR that routes between the backbone and other areas.

link state

The condition or state of a connection or link between two networking devices.

link-state router

A type of routing device that uses a protocol that incrementally sends partial route updates.

link-state routing algorithm

A routing algorithm that enables a router to broadcast (or multicast) information about the cost of reachability of all its neighbors to every node in the internetwork. Link-state algorithms offer a constant picture of the network topology and are not vulnerable to routing loops .

load

In networking, load is the amount of data (traffic) being carried by a network.

loopback interface

A virtual interface created and configured with Layer 3 addressing that remains active for router operations even if an individual physical interface goes down.

LSA (link-state advertisement)

A message stored inside a link-state packet. These are typically multicast packets that contain information about neighbors and path costs, which is used by link-state protocols.

LSAck (LSA acknowledgement)

A packet exchanged with OSPF neighbors and the originating OSPF router to acknowledge receipt of an LSA.

LSA flooding

The OSPF rapid convergence process where the network is flooded with LSAs when a change occurs in the network.

LSU (link-state update)

A term used in OSPF to describe link-state advertisements that are exchanged when a change occurs in the topology.

MAC address

A data-link layer hardware address that every port or device uses to connect to a LAN segment. MAC addresses are defined by the Institute of Electrical and Electronics Engineers (IEEE) standard and have a length of 6 bytes.

metric

A standard gauge of measurement used in routing to determine whether the primary goals of network maintenance have been met. For instance, this can be a performance factor or a path determination factor.

multicast

Any communication between one sender and multiple receivers. Multicast messages are sent to a defined subset of the network address, defined in the packet's destination address field.

multicast address

A single address that identifies more than one device on a network. Creating a multicast address is accomplished by designating a special nonexistent MAC address for the particular multicast protocol.

multihomed AS

The term used to describe an AS that is connected to two or more Internet service providers (ISPs).

multihoming

The scenario of being linked to two or more networks or having two or more network addresses. For example, a network server may be connected to a serial line and a LAN or to multiple LANs.

multiplexing

A method of converting multiple logical signals into a single physical signal for transmission over a single physical channel.

neighbor

A network device that shares a common network with another network device.

See also [neighboring routers]

neighboring routers

Two OSPF routers that both have interfaces linked to a common network.

network address

The portion of the logical address used to identify the network segment of an internetwork.

Network layer

Layer 3 of the OSI reference model. This is the layer where routing is implemented, enabling connections and path selection between two end systems.

non-stub area

In OSPF routing, a resource-consuming area carrying a default route, intra-area routes, inter-area routes, static routes, and external routes.

notification message

In BGP, a type of message that a router transmits when an error state is detected . Notification messages include error codes, sub-codes, and error data.

NSAP (network service access point)

A hierarchical address used at the OSI Network layer that consists of a conceptual point between the Transport and Network layers .

NSSA ( not-so-stubby area)

An OSPF area that allows the injection of a limited set of external routes, that is, only the routes needed to provide connectivity to backbone areas.

open message

The first message sent by each speaker in a BGP network after a TCP connection has been established.

originator ID

A BGP identifier that designates the router that originated the message.

OSI (Open Systems Interconnection)

An international program designed by the International Organization for Standardization (ISO) and International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) to develop data networking standards that facilitate multi-vendor equipment interoperability.

OSI reference model

A seven-layer conceptual archetype by which the ISO defines how any combination of devices can be connected for the purpose of communication.

OSPF (Open Shortest Path First)

A link-state, hierarchical IGP routing algorithm resulting from an earlier version of the IS-IS protocol. The characteristics include multipath routing, load balancing, and least-cost routing.

OSPF areas

Subsets within an autonomous system that share routing information.

packet

The essential logical unit of data information being transferred. A packet consists of a certain number of data bytes, encapsulated with headers and/or trailers containing information such as packet source and destination.

packet switching

A network mechanism involving the exchange of data packets. By dividing a continuous stream of data into smaller units, data can be simultaneously transferred through multiple devices on a network along the same communication channel.

partial mesh

A form of networking topology where some network nodes constitute a situation where every node has either a physical or a virtual circuit linking it to every other network.

path determination

The process of selecting the optimal route to a destination network or host. With RIP, routing decisions are made according to hop count. OSPF, on the other hand, uses a cost value based on the speed of the connection.

path vector protocol

Protocols such as BGP are known as path vector protocols because they exchange network reachability information called path vectors (or attributes).

PDU (protocol data unit)

The designation of the data process at each layer of the OSI model. For example, a PDU at the Network layer is a packet or datagram.

peer group

In BGP, neighbors that are logically grouped together through configuration commands based on a common routing policy.

pinhole congestion

Two links to the same remote network with equal hops but with different bandwidths. As a result, a distance-vector routing protocol attempts to load balance and therefore waste bandwidth by utilizing the slower link as well.

point-to-multipoint link

A communication path that connects a single root node to systems at multiple points of a destination called leaves .

point-to-point link

A communication channel that can be directed either one way or two ways between two end systems.

poison reverse updates

Update messages that are transmitted by a router back to the originating device (and that ignore the split-horizon rule) when route poisoning has occurred. Routes are poisoned when a neighbor router is deemed to be down. Typically used by distance vector routing algorithms to overcome routing loop problems.

See also [route poisoning]

prefix length

The number of bits used to represent the network and subnet in a CIDR address.

prefix list

A mechanism available in Cisco IOS release 20.0 and later that is used as an alternative to access lists for filtering BGP routes.

prefix routing

A routing protocol used in classless routing that includes subnet mask information with route update transmission.

process switching

The process that ensues when a packet to be forwarded arrives at a router and is copied to the router's process buffer. The router performs a lookup on the network address. The route table associates an exit interface with the destination address. The processor then forwards the packet, along with the additional information, to the exit interface. Subsequent packets, destined for the same target address, follow the same path at the first packet.

protocol

The designation and standardization of a set of rules for a certain type of network communication. This term is also used to reference the software that implements a particular protocol.

Proxy ARP (Proxy Address Resolution Protocol)

Used to permit redundancy if a failure with the configured default gateway on a host occurs. With Proxy ARP, an intermediate device sends an ARP response on behalf of an end node to the requesting host.

Pseudonode

Another name for a DIS.

redistribution

A command that is used on a Cisco router to inject one routing protocol path into another type of routing protocol.

redundancy

The duplication or replication of links, devices, or services that are used as failsafe solutions in case the primary connections, devices, or services fail.

relay system

Another term for a routing device.

reliability

A measurement of the quality of a network link and one of the possible metrics for making routing decisions.

RFC (Request for Comments)

A published Internet document that is used to put forward and define networking industry standards.

RIP (Routing Information Protocol)

A distance vector interior gateway protocol that uses hop count as a routing metric for optimal path determination.

RIP version 2

A newer , more up-to-date, and full-featured version of the original RIP.

route map

A mechanism that is used to manage and modify routing information by defining conditions for redistributing routes from one protocol to another in BGP.

route poisoning

A mechanism used by distance vector protocols to overcome routing loops and provide explicit information concerning the times when a subnet or network is not accessible. Route poisoning is usually accomplished by setting the IP count value to one greater than the maximum.

routed protocol

A routed protocol, such as IP, IPX, or AppleTalk, that can be routed by a Network layer device. This device must be able to understand the logical internetwork as designated by the rules of the particular routed protocol. The protocol also includes the underlying addressing scheme that a routing protocol uses to determine the best path to a destination.

route redistribution

The translation and conversion of routing information from one routing protocol to another.

route reflector

BGP routers that are configured to propagate routes learned by IBGP to other IBGP neighbors (clients) to lower TCP sessions and reduce routing traffic.

route reflector client

IBGP neighbors that have a partial peering relationship with route reflectors to receive advertisements.

See also [route reflector]

route summarization

The aggregation of publicized subnetwork addresses so that a single summary route is advertised to other areas by an area border router. It is implemented in several dynamic routing protocols, such as OSPF, EIGRP, and IS-IS.

router

A network (Layer 3) software or hardware mechanism that uses one or more metrics to determine the best path to use for transmission of network traffic. This device has historically been referred to as a gateway.

router ID

In OSPF, the identifying factor of a router, which is typically the highest configured IP address of an active interface or a loopback address on the router.

routing

The method used for transmitting logically addressed packets from a local subnetwork to a destination network host. In large internetworks, a packet may travel through a complex array of intermediary devices and destinations before reaching the ultimate destination.

routing domain

A group of end systems and intermediate systems that function under a common administrative policy.

routing metric

Any value that is used by routing protocols and algorithms to determine whether one route is better than another. Metrics can include bandwidth, delay, hop count, path cost, load, maximum transmission unit (MTU), reliability, and communication cost.

routing policy

In BGP, the process of restricting the routing information learned or advertised by filtering updates between certain BGP peers.

routing protocol

A set of standard procedures that defines algorithms used for updating routing tables and exchanging information between network devices.

routing table

A database stored in a router or other internetworking device that maintains a record of only the best possible routes to certain network destinations and the associated metrics or costs for those routes.

RTO (Retransmission Timeout)

The length of time in milliseconds that the routing software will delay before retransmitting a packet to an EIGRP neighbor.

seed metric

The initial metric used when a router is advertising to a directly connected link, usually derived from the characteristics of the interface and passed to other routers.

shortest path first algorithm

A link-state algorithm used to calculate the shortest path to each node, based on the topography of an internetwork that is constructed by each node.

SIA (stuck in active)

In EIGRP, the state that a router enters if it does not receive a reply to all its unresolved queries within a three-minute time interval.

split horizon

A mechanism for preventing routing loops implemented by distance vector protocols, where routing information is prevented from exiting the router interface by which that same information was received.

SRTT (smooth round trip time)

In EIGRP, the amount of time it takes in milliseconds for an EIGRP packet to be sent to a peer and for the local router to receive a subsequent acknowledgement.

standard IP access list

An IP access list that uses only the source IP address of a host or network for the purposes of filtering.

static route

The process of purposefully injecting into a routing table a route that takes priority over those selected by a dynamic routing protocol.

stub area

An OSPF area that carries a default route, intra-area routes, and inter-area routes, but no external routes.

stub AS

An area that accepts only default routes.

stub network

A network that has only one link to a router.

subinterface

One of many potential virtual interfaces available on a single physical interface of a networking device.

subnet address

The part of an IP address that is exclusively identified by the subnet mask as the subnetwork.

subnet mask

A 32-bit address mask used in IP to designate the bits of an IP address that are used for the subnet portion of the address.

subnetwork

Any network, also known as a subnet, that is part of a larger IP network and is identified by the subnet address. In OSI networks, the term refers to a collection of ESSs and ISSs, controlled by only one administrative domain, and that uses a single network connection protocol.

successor

In EIGRP, the best route to a destination, chosen from the topology table and injected into the routing table.

supernetting

Also called CIDR, an IP addressing scheme that uses a single IP address to designate many unique IP addresses. A supernetted IP address looks like a typical IP address, except that it ends with a slash followed by a number, called the IP prefix.

switching

The process of forwarding packets to the appropriate port, based on the packet's address.

synchronization

The rule enforced in BGP in which a BGP router should not advertise to or use an IBGP-learned route to an external neighbor unless the route is local or learned from the IGP being currently deployed.

TCP (Transmission Control Protocol)

A connection-oriented protocol offering reliable delivery of data. TCP is defined at the Transport layer (Layer 4) of the OSI reference model.

TCP/IP (Transmission Control Protocol/Internet Protocol)

The suite of protocols supporting the Internet and other internetworks. TCP and IP are the most widely known protocols in the group.

topology table

A database of all learned routes maintained by an EIGRP router for each configured routed protocol.

totally stubby area

An OSPF area that does not accept external AS routes or summary routes from other areas within the AS.

transit AS

In BGP, an AS through which data from one AS must traverse to get to another AS.

Transport layer

The fourth layer of the OSI reference model, used for reliable communication between end nodes over a network. It supplies mechanisms for establishing, maintaining, and terminating virtual circuits, for fault detection and recovery, and for flow control.

UDP (User Datagram Protocol)

A connectionless Transport layer (Layer 4) protocol in the TCP/IP protocol suite that allows datagrams to be exchanged without acknowledgement or guarantee of delivery.

unicast

A process used for direct host-to-host communication that originates from only one source and is directed at only one destination.

uptime

The amount of elapsed time that has passed since a local EIGRP router has heard from a particular neighbor in the routing table.

virtual link

An OSPF connection that provides a logical path between the backbone and a disconnected or discontiguous area.

VLSM (variable-length subnet mask)

Used to subnet a subnet by optimizing available address space and designating a different subnet mask for the same network number on various subnets.

WAN (wide area network)

LANs linked together over a DCE (data communications equipment) network. It is usually a leased line or dial-up connection across a PSTN network. Examples of WAN protocols include frame relay, Point-to-Point Protocol (PPP), Integrated Services Digital Network (ISDN), and High-Level Data Link Control (HDLC).

X.25

An ITU-T packet-relay standard that defines communication between data terminal equipment (DTE) and data communications equipment (DCE) network devices. X.25 has been made almost completely obsolete in North America by frame relay technology. X.25 uses a reliable Data Link layer protocol called HDLC Link Access Procedure Balanced (LAPB).