Appendix C - Glossary

You should familiarize yourself with the concepts and terms used in this implementation of Routing and Remote Access Service.

This appendix is divided into two parts. In the first part, "Definitions," you will find definitions and explanations of terms and concepts. In the second part, "Sources for Terms," you will find a table listing terms and concepts and a pointer to which Administrator's Guide chapter or appendix contains a reference to or discussion of that term or concept.

An adapter is a representation of the physical point of attachment to a network segment. A LAN or WAN card is defined as one adapter. Adapters are also called network cards.

Adapters are installed and configured through the Network Control Panel. Examples of adapters include Ethernet, FDDI, Token Ring, ISDN, Frame Relay, T1, and modem cards and devices. A complete list of supported adapters for Windows NT can be found at https://www.microsoft.com/hcl/default.asp 

A relationship formed between selected neighboring routers for the purpose of exchanging routing information. When the link state databases of two neighboring routers are synchronized, the routers are said to be adjacent. Not every pair of neighboring routers becomes adjacent.

Within an autonomous system, groups that comprise contiguous networks and the hosts attached to them.

A router that is attached to multiple areas. Area border routers maintain separate topological databases for each area.

A group of routers exchanging routing information by using a common routing protocol.

A router that exchanges routing information with routers that belong to other autonomous systems. The AS boundary router then advertises external routes throughout the autonomous system. AS boundary routers can be internal or area border routers, and migh or might not be connected to the backbone.

Auto-static updates are supported in RIP for IP and RIP for IPX, but are not available for use with OSPF. When you configure an interface to use the auto-static update mode, the router sends a request to other routers and inherits routes. The routes are saved in the routing table as auto-static routes and are kept even if the router is restarted or if the interface goes down.

An OSPF backbone is responsible for distributing routing information between areas. It consists of all area border routers, networks not wholly contained in any area, and their attached routers. The backbone must be contiguous.

A router that is connected to the backbone. This includes routers that are connected to more than one area (area border routers). However, backbone routers do not have to be area border routers. Routers that have all networks connected to the backbone are internal routers.

In communications, the difference between the highest and lowest frequencies in a given range. For example, a telephone line accommodates a bandwidth of 3000 Hz, the difference between the lowest (300 Hz) and highest (3300 Hz) frequencies it can carry. In computer networks, greater bandwidth indicates faster data-transfer capability and is expressed in bits per second.

The component responsible for relaying DHCP and BOOTP broadcast messages between a BOOTP/DHCP server and a client across an IP router. With a relay agent, it is not necessary to have a DHCP or BOOTP server on each network segment.

A routing protocol designed for use between autonomous systems and especially useful to detect routing loops. (See the BGP version 4 specification in RFC 1654.)

Destined for all hosts on a given network.

A network that supports more than two attached routers and has the ability to address a single physical message to all of the attached routers (broadcast). Ethernet is an example of a broadcast network.

An IP address that ranges from 1.0.0.0 to 126.0.0.0. Class A addresses are used by very large networks such as the ARPAnet.

An IP address that ranges from 128.1.0.0 to 191.254.0.0. Class B addresses are used by large corporate networks. The first two octets indicate the network, and the last two octets are used to indicate subnets and hosts on the network.

An IP address that ranges from 192.1.1.0 to 223.254.254.0. The first three octets indicate the network, and the last octet is used to indicate hosts on the network.

The process of stabilizing after changes occur in the network. If a route becomes unavailable, routers send update messages throughout the networks, re-establishing information about preferred routes. Slow routing convergence can cause loops.

A loop that happens when a link in a network goes down and routers on the network update their routing tables with incorrect hop counts.

For example, a loop can develop if the link to Router C goes down. Router B then advertises that the link is down and that it has no route to C anymore. Because Router A has a route to C with a metric of 2, it responds to Router B and sends its link to C. Router B then updates its table to include a link with metric 3, and the routers continue to announce and update their links to C until they reach the number 16. This is called a count to infinity.

A demand-dial connection initiates a link to a remote site when data or routing information must be sent to that site, and closes the connection when a specified amount of time has passed without data being sent over the link.

Ability to make switched connections to connect networks across WANs based on need. For example, connecting branch offices with a modem that only dials and establishes a connection when there is network traffic from one office to another. This feature enables saving of long-distance connection costs.

See BOOTP/DHCP Relay Agent.

A standard dial-up connection, such as telephone and ISDN lines.

The process of real-time routing changes in response to network changes. Dynamic routing software adjusts routes based on the routing update messages it receives, then distributes update messages about its new routes.

An interface represents the connection over the network adapter. In the Routing and RAS Admin tool, it is a visual representation of the network that can be reached over the LAN or WAN adapters.

Each interface has a unique name within the Routing and RAS Admin tool. Interfaces are manageable objects, each appearing as a row in the Interface Table of the appropriate SNMP MIB and as an entry in the Routing and RAS Admin Explorer view.

A router for which all the networks that it is connected to belong in the same area.

One member of the TCP/IP protocol suite. IP provides a best-effort, connectionless delivery system that does not guarantee that packets arrive at their destination or that they are received in the sequence in which they were sent. This protocol is used by the Internet.

A Novell communication protocol that sends data packets to requested destinations (such as workstations and servers). The Microsoft NWLink protocol provides IPX support for Windows NT–based networks.

A "map" of the network maintained by OSPF routers. It is updated after any change in the network topology. The link state database is used to compute the network routes, which must be computed again after any change in the topology.

A group of computers and other devices dispersed over a relatively limited area and connected by a communications link that enables any device to interact with any other on the network.

The type of physical wiring and lower-layer protocols used for transmitting and receiving packets; for example, Ethernet, FDDI, Token Ring.

The metric indicates the distance in number of hops to the destination, its "cost" to the router.

Destined for a set of hosts that belong to a multicast group.

Routing protocol used in medium-sized and large networks. This protocol is more complex than RIP, but enables better control and is more efficient in propagation of routing information.

A unit of information that can be sent from one computer to another. A packet contains user data or protocol information or both. Also called a "datagram."

Ability to prevent certain types of network packets from going from one network to another. This can be employed for security reasons (to prevent access from unauthorized users) or to improve performance by disallowing unnecessary packets from going over a slow connection.

The part of a packet that contains an identification number, source and destination addresses, and sometimes, error-control data.

A set of industry standard framing and authentication protocols. PPP negotiates configuration parameters for multiple layers of the OSI model.

A tunneling protocol that enables VPNs to span public networks (such as the Internet) without jeopardizing security. PPTP can enable cost savings by using the Internet as the virtual WAN.

Used with split horizon. Split horizon with poison reverse improves RIP convergence over simple split horizon by advertising all network IDs, but those network IDs learned in a given direction are advertised with a hop count of 16, indicating that the network is unavailable.

A general term used to describe a method and/or format of communication between two entities.

A scripting utility for Routing and Remote Access Service that is intended as a command-line alternative to the router administration user interface available through the Routing and RAS Admin tool (rtradmin.exe).

A computer that runs software to enable routing functionality. Routers make packet-forwarding decisions on the basis of addresses in the network layer of the OSI reference model (for example, IP addresses).

The process of connecting more than one network (LANs or WANs) and transferring packets between them.

Enables a router to exchange routing information with a neighboring router. RIP is used in small and medium-sized networks. There is a version of RIP for IP and IPX networks. RIP version 2 (RIPv2) is a newer and more efficient form of RIP for IP networks.

Protocols used for learning and propagating network addresses across networks to facilitate connectivity between the different networks.

Initialized and maintained by routing algorithms to aid the process of optimal path determination.

Used by servers to advertise their services and addresses on an IPX network.

Software and protocol used for managing networking components such as routers. SNMP enables different routers to be managed from one management application, such as HP OpenView.

Refers to rules that prevent routing loops. Information about a route is not sent back toward the source of the information.

Static routes are manually configured by an administrator. They change only if the network administrator changes them. If the routing protocol is configured to support auto-static routes, then the router can issue a request to a protocol to get an update of routing information on a specific interface. The results of such update will then be converted and kept as static routes.

An overview of networks and how they relate to routers. Identical for routers in the same area.

Triggered updates occur at 30-second intervals (or a user-defined interval) when the network topology changes and routing update messages are sent that reflect those changes.

A term given to a secure channel between two parts of a VPN, typically across a public network.

Destined for a single host.

Connection between two virtual interfaces.

A network that is not connected to any public network (like the Internet) for reasons such as security, network addressing, and traffic control.

A communications network that connects geographically separated areas.

Table C.1 lists routing terms and concepts and points to which Administrator's Guide chapter or appendix contains references to or discussions of those terms or concepts.

Table C.1 Terms and Their Sources in This Document