Glossary S

R

R reference point

Used with ISDN networks to identify the connection between an NT1 and an S/T device. The S/T device converts the four-wire network to the two-wire ISDN standard network.

RADIUS

Remote Authentication Dial-in User Service: A protocol that is used to communicate between the remote access device and an authentication server. Sometimes an authentication server running RADIUS will be called a RADIUS server.

RAM

random access memory: Used by all computers to store information. Cisco routers use RAM to store packet buffers and routing tables, along with the hardware addresses cache.

RARP

Reverse Address Resolution Protocol: The protocol within the TCP/IP stack that maps MAC addresses to IP addresses. See also: ARP.

rate queue

A value, assigned to one or more virtual circuits, that specifies the speed at which an individual virtual circuit will transmit data to the remote end. Every rate queue identifies a segment of the total bandwidth available on an ATM link. The sum of all rate queues should not exceed the total available bandwidth.

RCP

Remote Copy Protocol: A protocol for copying files to or from a file system that resides on a remote server on a network, using TCP to guarantee reliable data delivery.

redistribution

A process used in Cisco routers to inject the paths found from one routing protocol into another routing protocol. For example, networks found by RIP can be inserted into an IGRP network.

redundancy

In internetworking, the duplication of connections, devices, or services that can be used as a backup in the event that the primary connections, devices, or services fail.

reference point

Used to define an area in an ISDN network. Providers used these reference points to find problems in the ISDN network.

reliability

The measure of the quality of a connection. It is one of the metrics that can be used to make routing decisions.

reload

An event or command that causes Cisco routers to reboot.

remote access

A generic term that defines connectivity to distant resources using one of many technologies, as appropriate.

remote services

Network services close to users but not on the same network or subnet as the users. The users would have to cross a layer 3 device to communicate with the network services, but they might not have to cross the backbone.

rendezvous point

See: RP.

reverse Telnet

Maps a Telnet port to a physical port on the router or access device. This enables the administrator to connect to a modem or other device attached to the port.

RFC

Request for Comments: RFCs are used to present and define standards in the networking industry.

RIF

Routing Information Field: In source-route bridging, a header field that defines the path direction of the frame or token. If the Route Information Indicator (RII) bit is not set, the RIF is read from source to destination (left to right). If the RII bit is set, the RIF is read from the destination back to the source, so the RIF is read from right to left. It is defined as part of the token ring frame header for source-routed frames, which contains path information.

ring

Two or more stations connected in a logical circular topology. In this topology, which is the basis for Token Ring, FDDI, and CDDI, information is transferred from station to station in sequence.

ring topology

A network logical topology comprising a series of repeaters that form one closed loop by connecting unidirectional transmission links. Individual stations on the network are connected to the network at a repeater. Physically, ring topologies are generally organized in a closed-loop star. Contrast with: bus topology and star topology.

RIP

Routing Information Protocol: The most commonly used interior gateway protocol in the Internet. RIP employs hop count as a routing metric. See also: Enhanced IGRP, IGP, OSPF, and hop count.

RIP version 2

Newer, updated version of Routing Information Protocol (RIP). Allows VLSM. See also: VLSM.

RJ connector

registered jack connector: Used with twisted-pair wiring to connect the copper wire to network interface cards, switches, and hubs.

robbed bit signaling

Also known as Channel Associated Signaling, robbed bit signaling operates on a per-channel basis rather than having a dedicated signaling channel.

ROM

read-only memory: Chip used in computers to help boot the device. Cisco routers use a ROM chip to load the bootstrap, which runs a power-on self-test, and then find and load the IOS in flash memory by default.

root bridge

Used with the Spanning Tree Protocol to stop network loops from occurring. The root bridge is elected by having the lowest bridge ID. The bridge ID is determined by the priority (32768 by default on all bridges and switches) and the main hardware address of the device. The root bridge determines which of the neighboring layer 2 devices' interfaces become the designated and nondesignated ports.

round robin

A scheduling mechanism where queues are arranged as though in a logical ring around the output buffer. Packets are forwarded from queues as each rotates past the buffer.

routed protocol

Routed protocols (such as IP and IPX) are used to transmit user data through an internetwork. By contrast, routing protocols (such as RIP, IGRP, and OSPF) are used to update routing tables between routers.

route poisoning

Used by various DV routing protocols in order to overcome large routing loops and offer explicit information about when a subnet or network is not accessible (instead of merely suggesting that the network is unreachable by not including it in updates). Typically, this is accomplished by setting the hop count to one more than maximum. See also: poison reverse updates.

route summarization

In various routing protocols, such as OSPF, EIGRP, and IS-IS, the consolidation of publicized subnetwork addresses so that a single summary route is advertised to other areas by an area border router.

router

A Network layer mechanism, either software or hardware, using one or more metrics to decide on the best path to use for transmission of network traffic. Sending packets between networks by routers is based on the information provided on Network layers. Historically, this device has sometimes been called a 'gateway.'

router on a stick

A term that identifies a single router interface connected to a single distribution layer switch port. The router is an external router that provides trunking protocol capabilities for routing between multiple VLANs. See also: RSM and MSFC.

routing

The process of forwarding logically addressed packets from their local subnetwork toward their ultimate destination. In large networks, the numerous intermediary destinations that a packet might travel before reaching its destination can make routing very complex.

routing domain

Any collection of end systems and intermediate systems that operate under an identical set of administrative rules. Every routing domain contains one or several areas, all individually given a certain area address.

routing metric

Any value that is used by routing algorithms to determine whether one route is superior to another. Metrics include such information as bandwidth, delay, hop count, path cost, load, MTU, reliability, and communication cost. Only the best possible routes are stored in the routing table, while all other information may be stored in link-state or topological databases. See also: cost.

routing protocol

Any protocol that defines algorithms to be used for updating routing tables between routers. Examples include IGRP, RIP, and OSPF.

routing table

A table kept in a router or other internetworking mechanism that maintains a record of only the best possible routes to certain network destinations and the metrics associated with those routes.

RP

(1) rendezvous point: A router that acts as the multicast source in a multicast network. Primarily in a shared tree distribution. (2) Route Processor: Also known as a 'supervisory processor,' a module on Cisco 7000 series routers that holds the CPU, system software, and most of the memory components used in the router.

RSFC

Route Switch Feature Card: Used to provide routing between VLANs. The RSFC is a daughter card for the Supervisor engine II G and Supervisor III G cards. The RSFC is a fully functioning router running the Cisco IOS.

RSM

Route Switch Module: A route processor that is inserted into the chassis of a Cisco Catalyst 5000 series switch. The RSM is configured exactly like an external router.

RSP

Route/Switch Processor: A processor module combining the functions of RP and SP used in Cisco 7500 series routers. See also: RP and SP.

RSTP

Rapid Spanning Tree Protocol: The IEEE 802.1w protocol that defines how spanning- tree convergence can be speeded up by reducing the number of spanning-tree modes and introducing a Hello protocol enhancement.

RTS

Request to Send: An EIA/TIA-232 control signal requesting permission to transmit data on a communication line.