Table Of Contents

Glossary

Numbers
1+1 APS	See APS.
10BaseT	10-Mbps baseband Ethernet specification using two pairs of twisted-pair cabling (Category 3, 4, or 5): one pair for transmitting data and the other for receiving data. 10BaseT, which is part of the IEEE 802.3 specification, has a distance limit of approximately 328 feet (100 meters) per segment. See also IEEE 802.3 and Ethernet.
100BaseT	100-Mbps baseband Fast Ethernet specification using UTP wiring. Like the 10BaseT technology on which it is based, 100BaseT sends link pulses over the network segment when no traffic is present. However, these link pulses contain more information than those used in 10BaseT. The 100BaseT specification is based on the IEEE 802.3 standard. See also 10BaseT, Fast Ethernet, and IEEE 802.3.
1000BaseLX/LH	1000-Mbps gigabit Ethernet specification using two strands of multimode or single mode fiber-optic cable per link. To guarantee proper signal recovery, a 1000BaseLX/LH link cannot exceed 1804 feet (550 meters) in length over multimode fiber or 32,810 feet (10 km) in length over single mode fiber. Based on the IEEE 802.3 standard with reach over single mode fiber extended from 5 km to 10 km. See also 1000BaseSX, 1000BaseZX, Gigabit Ethernet, and IEEE 802.3.
1000BaseSX	1000-Mbps gigabit Ethernet specification using two strands of multimode fiber-optic cable per link. To guarantee proper signal recovery, a 1000BaseSX link cannot exceed 1804 feet (550 meters) in length. The 1000Base SX specification is based on the IEEE 802.3 standard. See also 1000BaseLX/LH, 1000BaseZX, Gigabit Ethernet, and IEEE 802.3.
1000BaseX	1000-Mbps gigabit Ethernet specification that refers to the 1000BaseCX, 1000BaseSX, and 1000BaseLX standards for gigabit Ethernet over fiber-optic cabling. The 1000BaseX specification is based on the IEEE 802.3 standard. See also 1000BaseSX, 1000BaseLX/LH, 1000BaseZX, Gigabit Ethernet, and IEEE 802.3.
1000BaseZX	1000-Mbps gigabit Ethernet specification using two strands of single mode fiber-optic cable per link. To guarantee proper signal recovery, a 1000BaseZX link cannot be longer than 62.1 mi (100 km). This is a Cisco specification. See also 1000BaseSX, 1000BaseLX/LH, Gigabit Ethernet, and IEEE 802.3.
802.x	A set of IEEE standards for the definition of LAN protocols.

A
AAA	Authentication, authorization, and accounting.
AAL	ATM adaptation layer. Service-dependent sublayer of the data link layer. The AAL accepts data from different applications and presents it to the ATM layer in the form of 48-byte ATM payload segments. AALs consist of two sublayers: the convergence sublayer and the segmentation and reassembly sublayer. Four types of AAL recommended by the ITU-T are AAL1, AAL2, AAL3/4, and AAL5; the Cisco 10000 Series ESR uses AAL5.
Access list	List kept by routers to control access to or from the router for a number of services. For example, access lists can be used to prevent packets with a certain IP address from leaving a particular interface on the router.
Add drop multiplexer	See ADM.
ADM	Add Drop Multiplexer. A multiplexer that allows a signal to be added into or dropped out of a SONET span. See also SONET.
Alarm	A status condition that shows that a module or port is experiencing an abnormal operating condition. See also Critical alarm, Major alarm, and Minor alarm.
Application-specific integrated circuit	See ASIC.
APS	Automatic protection switching. A SONET switching mechanism that achieves network resiliency by automatically switching from a primary circuit to a secondary circuit. This switching process occurs if the primary circuit fails or if the error rate on the primary line exceeds a set threshold. The Cisco 10000 ESR supports 1+1 APS, which provides permanent electrical bridging to the service and protection equipment, placed at both ends of the circuit.
ASIC	Application-specific integrated circuit. A chip that is built for a specific application.
Asynchronous Transfer Mode	See ATM.
ATM	Asynchronous Transfer Mode. International standard for cell relay in which multiple service types (such as voice, video, or data) are conveyed in fixed-length (53-byte) cells. Fixed-length cells allow cell processing to occur in hardware, thereby reducing transit delays. ATM is designed to take advantage of high-speed transmission media such as E3, SONET, and T3.
Authentication	In security, the verification of the identity of a person or process.
Automatic protection switching	See APS.

B
Backplane	The circuit board at the back of the chassis that all components plug into. It provides the physical connection between an interface processor or line card, and the data and power distribution buses inside a chassis.
Backward explicit congestion notification	See BECN.
BECN	Backward Explicit Congestion Notification. Bit set by a Frame Relay network in frames traveling in the opposite direction of frames encountering a congested path. DTE receiving frames with the BECN bit set can request that higher-level protocols take flow control action as appropriate. Compare with FECN.
Bellcore	Bell Communications Research. Former name of the organization that performs research and development on behalf of the Regional Bell Operating Companies (RBOCs). Bellcore is now called Telcordia.
BER	Bit error rate. The ratio of received bits that contain errors.
BGP	Border Gateway Protocol. An interdomain routing protocol that replaces EGP. BGP exchanges connection information with other BGP systems. It is defined by RFC 1163.
Bit error rate	See BER.
Border Gateway Protocol	See BGP.
Broadcast	Data packet that is sent to all nodes on a network. Broadcasts are identified by a broadcast address. Compare with Multicast and Unicast.

C
CAR	Committed access rate. A QoS feature that classifies packets based on such things as access lists, a MAC address, and IP precedence. It measures traffic rates and can be configured to take actions such as dropping packets or changing the ToS value.
C-bit parity	A modification of the M23 framing method for DS3 that frees the C bits for additional uses. See also M23.
CB-WFQ	Class-based weighted fair queuing (WFQ). WFQ offers dynamic, fair queueing that divides bandwidth across queues of traffic based on weights. WFQ ensures that all traffic is treated fairly, given its weight. Class-based WFQ applies WFQ to packets in different traffic classes. See also WFQ.
CB-WRED	Class-based weighted random early detection. Class-based WRED applies WRED to packets in different traffic classes. See also WRED.
CCITT	Consultative Committee for International Telegraph and Telephone. International organization responsible for the development of communications standards. Now called the ITU-T. (See ITU-T.)
CDP	Cisco Discovery Protocol. Media- and protocol-independent device-discovery protocol that runs on all Cisco-manufactured equipment including routers, access servers, bridges, and switches. Using CDP, a device can advertise its existence to other devices and receive information about other devices on the same LAN or on the remote side of a WAN. Runs on all media that support SNAP, including LANs, Frame Relay, and ATM media.
CEF	Cisco Express Forwarding. An advanced Layer 3 IP switching technology designed for high-performance, highly resilient Layer 3 IP backbone switching. CEF optimizes network performance and scalability for networks with large and dynamic traffic patterns, such as the Internet, on networks characterized by intensive Web-based applications or interactive sessions.
Central office	See CO.
CGMP	Cisco Group Management Protocol. A protocol used on routers connected to Cisco Catalyst switches to perform tasks similar to those performed by IGMP.
Channel	Communication path. Multiple channels can be multiplexed over a single cable in certain environments.
Channel service unit	See CSU.
Cisco Discovery Protocol	See CDP.
Cisco Express Forwarding	See CEF.
Cisco Group Management Protocol	See CGMP.
Cisco IOS	Cisco system software that provides common functionality, scalability, and security for Cisco products. Cisco IOS allows centralized, integrated, and automated installation and management of internetworks, while ensuring support for a wide variety of protocols, media, services, and platforms.
Clear channel DS3	A framed DS3 signal that is not multiplexed from 28 DS1 signals. Sometimes referred to as unchannelized DS3.
CLI	Command line interface. Interface that allows the user to interact with the operating system by entering commands and optional arguments at the command prompt.
CO	Central office. The local telephone company office to which all local loops in a given area connect and in which circuit switching of subscriber lines occurs.
Command line interface	See CLI.
Committed access rate	See CAR.
Console	Data terminal equipment (DTE) through which commands are entered into a host.
CoS	Class of service. An indication of how an upper-layer protocol requires a lower-layer protocol to treat its messages.
CRC	Cyclic redundancy check. Error-checking technique in which the frame recipient calculates a remainder by dividing frame contents by a prime binary divisor and then compares the calculated remainder to a value stored in the frame by the sending node.
Critical alarm	An alarm condition that might affect most or all subscribers that connect to the reporting node. To obtain more information about a problem, use the show facility-alarm status command. See also Major alarm and Minor alarm.
CSMA/CD	Carrier sense multiple access/collision detect. Media-access mechanism wherein devices ready to transmit data first check the channel for a carrier. If no carrier is sensed for a specific period of time, a device can transmit. If two devices transmit at once, a collision occurs and is detected by all colliding devices. This collision subsequently delays retransmissions from those devices for some random length of time. CSMA/CD access is used by Ethernet and IEEE 802.3.
CSU	Channel service unit. Digital interface device that connects end-user equipment to the local digital telephone loop. Often referred to with DSU as CSU/DSU. See also DSU.
Cyclic redundancy check	See CRC.

D
Data service unit	See DSU.
Data terminal equipment	See DTE.
DCC	Data communications channel. Bytes in the SONET or SDH payload used for inband management. See also SDDC (Section DCC).
DCE	Data circuit-terminating equipment (ITU-T expansion). Devices and connections of a communications network that comprise the network end of the user-to-network interface. The DCE provides a physical connection to the network, forwards traffic, and provides a clocking signal used to synchronize data transmission between DCE and DTE devices. Modems and interface cards are examples of DCE. Compare with DTE.
Differentiated service code point	See DSCP.
Distance Vector Multicast Routing Protocol	See DVMRP.
DLCI	Data link connection identifier. Value that specifies a PVC or SVC in a Frame Relay network. In the basic Frame Relay specification, DLCIs are locally significant (connected devices might use different values to specify the same connection). In the LMI extended specification, DLCIs are globally significant (DLCIs specify individual end devices). See also LMI.
DRAM	Dynamic random access memory. RAM that stores information in capacitors that must be periodically refreshed. Delays can occur because DRAMs are inaccessible to the processor when refreshing their contents. However, DRAMs are less complex and have greater capacity than SRAMs. See also SRAM.
DS0	Digital signal level 0. Framing specification used in transmitting digital signals over a single channel at 64 kbps on a T1 facility. Compare with DS1 and DS3.
DS1	Digital signal level 1. Framing specification used in transmitting digital signals at 1.544 Mbps on a T1 facility (in the United States) or at 2.108 Mbps on an E1 facility (in Europe). Compare with DS0 and DS3.
DS3	Digital signal level 3. Framing specification used for transmitting digital signals at 44.736 Mbps on a T3 facility. Compare with DS0 and DS1.
DSCP	Differentiated service code point. Specifies a precedence value for handling packets belonging to the specified class.
DSU	Data Service Unit. Device used in digital transmission that adapts the physical interface on a DTE device to a transmission facility such as T1 or E1. The DSU is also responsible for such functions as signal timing. Often used with CSU, as in CSU/DSU. See also CSU.
DTE	Data terminal equipment. Device at the user end of a user-network interface that serves as a data source, destination, or both. DTE connects to a data network through a DCE device (for example, a modem) and typically uses clocking signals generated by the DCE. DTE includes devices such as computers, protocol translators, and multiplexers.
DVMRP	Distance Vector Multicast Routing Protocol. An internetwork gateway protocol, largely based on RIP, that implements a typical dense mode IP multicast scheme. DVMRP uses IGMP to exchange routing datagrams with its neighbors. See also IGMP.
Dynamic random access memory	See DRAM.

E
E1	Wide-area digital transmission scheme used predominantly in Europe that carries data at a rate of 2.048 Mbps. E1 lines can be leased for private use from common carriers. Compare with T1; see also DS1.
Edge services router	See ESR.
EIGRP	Enhanced Interior Gateway Routing Protocol. Advanced version of IGRP developed by Cisco. Provides superior convergence properties and operating efficiency, and combines the advantages of link state protocols with those of distance vector protocols. Compare with IGRP.
Electromagnetic interference	See EMI.
Electrostatic discharge	See ESD.
EMI	Electromagnetic interference. Interference by electromagnetic signals that can cause reduced data integrity and increased error rates on transmission channels.
EMP	Electromagnetic pulse. Caused by lightning and other high-energy phenomena. Capable of coupling enough energy into unshielded conductors to destroy electronic devices.
Enhanced IGRP	See EIGRP.
ESD	Electrostatic discharge. Discharge of stored static electricity that can damage electronic equipment and impair electrical circuitry, resulting in complete or intermittent failures.
ESF	Extended Superframe format. Framing type used on T1 circuits that consists of 24 frames of 192 bits each, with the 193rd bit providing timing and other functions. ESF is an enhanced version of SF. See also SF.
ESR	Edge services router. A router that aggregates and routes traffic from thousands of low- and medium-bandwidth subscriber connections into a few high-bandwidth connections to the Internet core.
Ethernet	Baseband LAN specification. Ethernet networks use CSMA/CD and run over a variety of cable types at 10 Mbps, 100 Mbps, and 1000 Mbps. Ethernet is similar to the IEEE 802.3 series of standards. See also Fast Ethernet, Gigabit Ethernet, IEEE 802.3.
Extended Superframe Format	See ESF.

F
Facility data link	See FDL.
Fast Ethernet	Any of a number of 100-Mbps Ethernet specifications. Fast Ethernet offers a speed increase 10 times that of the 10BaseT Ethernet specification, while preserving qualities such as frame format, MAC mechanisms, and MTU. Existing 10BaseT applications and network management tools can be used on Fast Ethernet networks. The Fast Ethernet specification is based on an extension to the IEEE 802.3 specification. Compare with Ethernet and Gigabit Ethernet. See also 100BaseT and IEEE 802.3.
FDL	Facility data link. Embedded communications channel in ESF DS1 framing. Used to convey both bit-oriented and message-oriented signals.
FECN	Forward explicit congestion notification. Bit set by a Frame Relay network to inform DTE receiving the frame that congestion was experienced in the path from source to destination. DTE receiving frames with the FECN bit set can request that higher-level protocols take flow-control action as appropriate. Compare with BECN.
Fiber-optic cable	Physical medium capable of conducting modulated light transmission. Compared with other transmission media, fiber-optic cable is more expensive, but it is not susceptible to electromagnetic interference and is capable of higher data rates. Sometimes called optical fiber.
Field-replaceable unit	See FRU.
Flash memory	Nonvolatile storage that can be electrically erased and reprogrammed so that software images can be stored, booted, and rewritten as necessary. Flash memory was developed by Intel and is licensed to other semiconductor companies.
Forward explicit congestion notification	See FECN.
Frame Relay	Industry-standard, switched data link layer protocol that handles multiple virtual circuits using HDLC encapsulation between connected devices. Frame Relay is more efficient than X.25, the protocol for which it is generally considered a replacement.
Frame Relay Traffic Shaping	See FRTS.
FRTS	Frame Relay Traffic Shaping. A method for managing network traffic congestion on Frame Relay permanent virtual circuits (PVCs) and switched virtual circuits (SVCs).
FRU	Field-replaceable unit. A component that can be removed from a network device and replaced in the field. Line cards, power modules, and fan modules are typically FRUs.

G
GBIC	Gigabit interface converter. An interface module used by gigabit Ethernet and FibreChannel to convert the serial electrical signals to the transmission medium's physical layer signaling, which is typically optical. GBIC modules can be hot-swapped and contain ID and system information that a switch or router can use to determine the network device's capabilities. Different GBICs handle different types of fiber cable. See 1000BaseLX/LH, 1000BaseSX, and 1000BaseZX.
General Traffic Shaping	See GTS.
Generic Routing Encapsulation	See GRE.
Gigabit Ethernet	Gigabit Ethernet. Ethernet running at a transmission speed of 1 billion bits per second.
Gigabit interface converter	See GBIC.
GRE	Generic Routing Encapsulation. Tunneling protocol developed by Cisco that can encapsulate a wide variety of protocol packet types inside IP tunnels, creating a virtual point-to-point link to Cisco routers at remote points over an IP internetwork. By connecting multiprotocol subnetworks in a single-protocol backbone environment, IP tunneling using GRE allows network expansion across a single-protocol backbone environment.
GTS	General Traffic Shaping. GTS provides a mechanism to control the flow of outbound traffic on a particular interface. It reduces outbound traffic flow to avoid congestion by constraining specified traffic to a particular bit rate. Traffic adhering to a particular profile can be shaped to meet downstream requirements, eliminating bottlenecks in topologies with data rate mismatches.

H
HDLC	High-Level Data Link Control. Bit-oriented synchronous data link layer protocol developed by ISO. Derived from SDLC, HDLC specifies a data encapsulation method on synchronous serial links using frame characters and checksums. See also SDLC.
High-Level Data Link Control	See HDLC.
Hop count	Routing metric used to measure the distance between a source and a destination. RIP uses hop count as its sole metric.
Hot-swapping	Feature that permits the addition, replacement, or removal of cards without interrupting the system power, entering console commands, or causing other software or interfaces to shut down. Sometimes called online insertion and removal (OIR).

I
ICMP	Internet Control Message Protocol. Network layer Internet protocol that reports errors and provides other information relevant to IP packet processing. Documented in RFC 792.
IEEE	Institute of Electrical and Electronics Engineers. Professional organization whose activities include the development of communications and network standards. IEEE LAN standards are the currently predominant LAN standards.
IEEE 802.3	IEEE LAN protocol that specifies an implementation of the physical layer and the MAC sublayer of the data link layer. IEEE 802.3 uses CSMA/CD access at a variety of speeds over a variety of physical media. Extensions to the IEEE 802.3 standard specify implementations for Fast Ethernet and gigabit Ethernet.
IGMP	Internet Group Management Protocol. Tracks membership of hosts in multicast groups between hosts on a LAN and the routers on that LAN.
IGP	Interior Gateway Protocol. Internet protocol used to exchange routing information within an autonomous system. Examples of common Internet IGPs include IGRP, OSPF, and RIP. See also IGRP, OSPF, and RIP.
IGRP	Interior Gateway Routing Protocol. Protocol developed by Cisco to address the issues associated with routing in large, heterogeneous networks. Compare with EIGRP.
Institute of Electrical and Electronics Engineers	See IEEE.
Interior Gateway Protocol	See IGP.
Interior Gateway Routing Protocol	See IGRP.
Intermediate reach	See IR.
Intermediate System to Intermediate System Protocol	See ISIS.
Internet Control Message Protocol	See ICMP.
Internet Group Management Protocol	See IGMP.
Internet service provider	See ISP.
IOS	Internet Operating System. See Cisco IOS.
IP	Internet Protocol. Network layer protocol in the TCP/IP stack offering a connectionless internetwork service. IP provides features for addressing, type-of-service specification, fragmentation and reassembly, and security. Defined in RFC 791.
IP multicast	Routing technique that allows IP traffic to be propagated from one source to a number of destinations or from many sources to many destinations. Rather than sending one packet to each destination, one packet is sent to a multicast group identified by a single IP destination group address.
IP precedence	An IP protocol feature that lets you specify a packet's class of service using the three precedence bits in the IPv4 header's ToS field. Other features configured throughout the network can then use these bits to determine how to treat the packet.
IPSec VPN service module (VSM)	See VSM.
IPv6	IP version 6. Replacement for the current version of IP (version 4). IPv6 includes support for flow ID in the packet header, which can be used to identify flows. Formerly called IPng (next generation).
IR	Intermediate reach. SONET/SDH specification for transmit power and receive sensitivity that achieves a 9.3-mile (15-km) reach.
ISIS	Intermediate System to Intermediate System. OSI link-state hierarchical routing protocol based on DECnet Phase V routing, whereby ISs (routers) exchange routing information based on a single metric to determine network topology.
ISP	Internet service provider. A company that provides Internet access to other companies and individuals.
ITU-T	International Telecommunication Union Telecommunication Standardization Sector. International body that develops worldwide standards for telecommunications technologies. The ITU-T carries out the functions of the former CCITT.

K
Keepalive message	Message sent by one network device to inform another network device that the virtual circuit between the two is still active.
Kerberos	Developing standard for authenticating network users. Kerberos offers two key benefits: It functions in a multivendor network, and it does not transmit passwords over the network.

L
LAIS	Line Alarm Indication Signal. A SONET port status indicator that activates when an LAIS defect occurs and does not clear throughout the alarm integration period, which is typically 2.5 seconds. An LAIS defect occurs when bits 6, 7, and 8 of the K2 byte are 111 for three consecutive frames. This occurrence begins the alarm integration period. If this period elapses without the detection of three consecutive frames in which K2 bits 6, 7, and 8 show any pattern other than 111, the LAIS indicator activates. The LAIS indicator clears when an LAIS defect does not occur for a time interval equal to the alarm deactivation period (typically 10 seconds).
LAN	Local-area network. High-speed, low-error data network covering a relatively small geographic area (up to a few thousand meters). LANs connect workstations, peripherals, terminals, and other devices in a single building or some other geographically limited area. LAN standards specify cabling and signaling at the physical and data link layers of the OSI model. Ethernet, FDDI, and Token Ring are widely used LAN technologies. Compare with MAN and WAN.
Line card	Any I/O card that can be inserted in a modular chassis.
LMI	Local Management Interface. A set of enhancements to the basic Frame Relay specification. LMI includes support for a keepalive mechanism, which verifies that data is flowing; a multicast mechanism, which provides the network server with its local DLCI and the multicast DLCI; global addressing, which gives DLCIs global rather than local significance in Frame Relay networks; and a status mechanism, which provides an ongoing status report on the DLCIs known to the switch. Known as LMT in ANSI terminology.
Local-area network	See LAN.
LOF	Loss of Frame. A SONET port status indicator that activates when an LOF defect occurs and does not clear for an interval of time equal to the alarm integration period, which is typically 2.5 seconds. An LOF defect occurs when an out-of-frame (OOF) condition occurs and does not clear for more than 3 microseconds (ms). This occurrence begins the alarm integration period. (OOF occurs when four consecutive frames do not contain a valid frame word. OOF clears when two valid consecutive frames are detected.) The LOF indicator clears when an LOF defect is not detected for a time interval that is equal to the alarm deactivation period (typically 10 seconds).
Loopback test	A test in which signals are sent and then directed back toward their source from some point along the communications path. Loopback tests are often used to test network interface usability.
LOS	Loss of Signal. A SONET port status indicator that activates when an LOS defect occurs and does not clear throughout the alarm integration period, which is typically 2.5 seconds. An LOS defect occurs when the OC-3 port receives all zeros for 20 microseconds (+.3 ms). This occurrence begins the alarm integration period. If this period elapses without the detection of two consecutive frames in which there are no 20-ms periods of signal loss, the LOS indicator activates. The LOS indicator clears when an LOS defect is not detected for an interval equal to the alarm deactivation period (typically 10 seconds).

M
M13	Generic term for equipment that multiplexes DS1s into DS3s. Sometimes used to describe a specific DS3 multiplex format. Some standards use this term to describe a synchronous multiplexing format also know as SYNTRAN. In many cases M13 does not refer to the SYNTRAN format, but instead refers to the format also known as M23.
M23	A method of multiplexing four DS1 signals into a DS2 signal, then multiplexing seven DS2 signals into a DS3 signal.
MAC	Media Access Control. Lower of the two sublayers of the data link layer defined by the IEEE. The MAC sublayer handles access to shared media.
MAC address	Standardized data link layer address that is required for each port or device that connects to a LAN. Other devices in the network use these addresses to locate specific ports in the network, and to create and update routing tables and data structures. MAC addresses are 6 bytes long and are controlled by the IEEE. Also known as a hardware address, MAC-layer address, or physical address.
Maintenance data link	See MDL.
Major alarm	One of a group of alarm conditions that are considered the second most severe of all reportable alarms. Major alarms affect several subscribers who connect to the reporting node. Use the show facility-alarm status command to obtain more information about the problem. See also Critical alarm and Minor alarm.
MAN	Metropolitan-area network. A network that spans a metropolitan area. Generally, a MAN spans a larger geographic area than a LAN, but a smaller geographic area than a WAN. Compare with LAN and WAN.
Management Information Base	See MIB.
Maximum transmission unit	See MTU.
MBONE	Multicast backbone of the Internet.
MDL	Maintenance Data Link. Embedded communications channel in C-bit parity DS3 framing. Used to convey message-oriented signals.
Media Access Control	See MAC.
Metropolitan-area network	See MAN.
MIB	Management Information Base. Database of network management information that is used and maintained by a network management protocol such as SNMP or CMIP. The value of a MIB object can be changed or retrieved using SNMP or CMIP commands, usually through a GUI-based network management system. MIB objects are organized in a tree structure that includes public (standard) and private (proprietary) branches.
Minor alarm	One of a group of alarm conditions that are considered the third most severe of all reportable alarms. Minor alarms affect a single or small number of subscribers who connect to the reporting node. Use the show facility-alarm status command to obtain more information about the problem. See also Critical alarm and Major alarm.
MLP	Multilink Point-to-Point Protocol. A method of splitting, recombining, and sequencing datagrams across multiple logical data links.
MMF	Multimode fiber. Optical fiber supporting propagation of multiple frequencies of light. See also Single-mode fiber.
MPLS	Multiprotocol Label Switching. An emerging industry standard upon which tag switching is based.
MRM	Multicast Routing Monitor. Provides network fault detection and isolation in a large multicast routing infrastructure.
MSP	Multiplex section linear protection. MSP improves network resiliency by providing a fast Layer 1 mechanism to support one-to-one redundant transmission circuits between SDH devices within a chassis. MSP allows rapid switchover from one fiber connection to another in the event of a fiber cut, module failure, signal failure, or signal degradation.
MTU	Maximum transmission unit. The maximum packet size, in bytes, that a particular interface can handle.
Multicast	Single packets copied by the network and sent to a specific subset of network addresses. These addresses are specified in the Destination Address field. Compare with Broadcast and Unicast.
Multicast Routing Monitor	See MRM.
Multilink Point-to-Point Protocol	See MLP.
Multimode fiber	See MMF.
Multiplexing	An activity in which multiple logical signals are transmitted simultaneously across a single physical channel.
Multiprotocol Label Switching	See MPLS.

N
NEBS	Network Equipment Building Systems. The Telcordia (formerly Bellcore) requirements for equipment deployed in a central office environment. Covers spatial, hardware, crafts person interface, thermal, fire resistance, handling and transportation, earthquake and vibration, airborne contaminants, grounding, acoustical noise, illumination, EMC, and ESD requirements.
Network Equipment Building Systems	See NEBS.
Network termination unit	See NTU.
Nonvolatile RAM	See NVRAM.
NTU	Network terminating unit. The network equipment that connects directly to the data terminal equipment.
NVRAM	Nonvolatile RAM. RAM that retains its contents when a unit is powered off.

O
OAM&P	Operations, administration, maintenance, and provisioning. A SONET term.
OC	Optical carrier. A series of physical protocols (OC-1, OC-2, OC-3, and so forth), defined for SONET optical signal transmissions. OC signal levels put STS frames onto fiber-optic lines at a variety of speeds. The base rate is 51.84 Mbps (OC-1); each signal level thereafter operates at a speed divisible by that number (thus, OC-3 runs at 155.52 Mbps). See also SONET and STS-1.
ODL	Optical data link.
OIR	Online insertion and removal. Feature that permits the addition, replacement, or removal of cards without interrupting the system power, entering console commands, or causing other software or interfaces to shut down. Same as hot-swapping.
Online insertion and removal	See OIR.
Open Shortest Path First	See OSPF.
Optical carrier	See OC.
Optical fiber	See Fiber-optic cable.
OSPF	Open Shortest Path First. Link-state, hierarchical IGP routing algorithm proposed as a successor to RIP in the Internet community. OSPF features include least-cost routing, multipath routing, and load balancing. OSPF was derived from an early version of the IS-IS protocol. See also IGP, ISIS, and RIP. See also Enhanced IGRP and IGRP.

P
Packet	Logical grouping of information that includes a header containing control information and (usually) user data. Packets are most often used to refer to network layer units of data. The terms datagram, frame, message, and segment are also used to describe logical information groupings at various layers of the OSI reference model and in various technology circles.
Packet over SONET	See POS.
Packet switching	Networking method in which nodes share bandwidth with each other by sending packets.
Parallel express Forwarding	See PXF network processors.
Payload	Portion of a cell, frame, or packet that contains upper-layer information (data).
PCMCIA Flash disk card	A portable (credit-card size), nonvolatile storage device. PCMCIA Flash disk cards use Flash technology to store data. PCMCIA stands for Personal Computer Memory Card International Association, which sets the standard for this technology. Also called PC card.
PEM	Power entry module. A hardware module that distributes power to the chassis.
Performance routing engine	See PRE.
Permanent Virtual Circuit	See PVC.
PIM	Protocol-Independent Multicast. Multicast routing architecture that allows the addition of IP multicast routing on existing IP networks. PIM is unicast routing protocol independent and can be operated in two modes: dense and sparse.
Point of presence	See POP.
Point-to-Point Protocol	See PPP.
Policy map	A data structure assigning specific QoS service policies to classes of traffic.
POP	Point of presence. A physical location where an interexchange carrier installed equipment to interconnect with a local exchange carrier.
POS	Packet over SONET. A high-speed means of transmitting data over a SONET fiber-optic transmission system through a direct fiber connection to a data switch or router. POS is a point-to-point dedicated leased-line approach intended purely for high-speed data applications. POS allows a user organization to pass data in its native format, without the addition of any significant level of overhead in the form of signaling and control information.
POST	Power-on self test. A set of hardware diagnostics that runs on a hardware device when that device is powered on.
Power entry module	See PEM.
Power-on self test	See POST.
PPP	Point-to-Point Protocol. Provides router-to-router and host-to-network connections over synchronous and asynchronous circuits.
PRE	Performance routing engine. The central routing unit for the Cisco 10000 series ESR. The PRE performs all Layer 2 and Layer 3 packet manipulation related to routing and forwarding through the Cisco 10000 series ESR. Dual PREs can be configured in a single chassis for redundancy. See also PXF network processors.
Protocol Independent Multicast	See PIM.
PVC	Permanent virtual circuit or connection. A virtual circuit that is permanently established. PVCs save bandwidth associated with circuit establishment and teardown in situations where certain virtual circuits must exist all the time. In ATM terminology, called a permanent virtual connection. Compare with SVC.
PXF network processors	Parallel eXpress Forwarding network processors. A pair of programmable ASICs that perform parallel processing to support high-performance Layer 3 forwarding.

Q
QoS	Quality of service. A measure of performance for a transmission system that reflects its transmission quality and service availability.
QoS Policy Propagation on BGP	See QPPB.
QPPB	QoS Policy Propagation by BGP. A feature that classifies packets by IP precedence based on BGP community lists, BGP autonomous system paths, and access lists. After a packet is classified, other quality of service features such as committed access rate (CAR) and Weighted Random Early Detection (WRED) can specify and enforce policies to fit a business model.
Quality of service	See QoS.

R
RADIUS	Remote Dial-In User Service. Database for authenticating modem and ISDN connections and for tracking connection time.
RAM	Random-access memory. Volatile memory that can be read and written by a microprocessor.
Random access memory	See RAM.
Rate shaping	See Traffic shaping.
Read-only memory	See ROM.
Redundancy	In internetworking, the duplication of devices, services, or connections so that, in the event of a failure, the redundant devices, services, or connections can perform the work of those that failed.
Remote Dial-In User Service	See RADIUS.
Remote monitoring	See RMON.
Request for comments	See RFC.
Resource Reservation Protocol	See RSVP.
Reverse Path Forwarding	See RPF.
RFC	Request for comments. A document series used as the primary means for communicating information about the Internet. Some RFCs are designated by the IAB as Internet standards. Most RFCs document protocol specifications such as Telnet and FTP, but some are humorous or historical. RFCs are available online from numerous sources.
RIP	Routing Information Protocol. The IGP supplied with UNIX BSD systems. The most common IGP in the Internet, RIP uses hop count as a routing metric. See also hop count, IGP, and OSPF.
RMON	Remote Monitoring. MIB agent specification described in RFC 1271 that defines functions for the remote monitoring of networked devices. The RMON specification provides numerous monitoring, problem detection, and reporting capabilities.
ROM	Read-only memory. Nonvolatile memory that can be read, but not written, by the microprocessor.
Routing Information Protocol	See RIP.
RPF	Reverse Path Forwarding. Multicasting technique in which a multicast datagram is forwarded out of all but the receiving interface if the receiving interface is the one used to forward unicast datagrams to the source of the multicast datagram.
RPR	Route and forwarding processor redundancy.
RSVP	Resource Reservation Protocol. A protocol that supports the reservation of resources across an IP network. Applications running on IP end systems can use RSVP to indicate to other nodes the nature (bandwidth, jitter, maximum burst, and so forth) of the packet streams they want to receive. RSVP depends on IPv6. Also known as Resource Reservation Setup Protocol. See also IPv6.

S
SAR	Segmentation and reassembly. One of the two sublayers of the AAL common part convergence sublayer, responsible for dividing (at the source) and reassembling (at the destination) the protocol data units (PDUs) passed from the convergence sublayer. The SAR sublayer takes the PDUs processed by the convergence sublayer and, after dividing them into 48-byte pieces of payload data, passes them to the ATM layer for further processing. See also AAL.
SDDC	Section Data Communications Channel. A 192-kbps data communications channel embedded in the section overhead for OAM&P traffic between two SONET network elements. See also OAM&P and SONET.
SDH	Synchronous Digital Hierarchy. European standard that defines a set of rate and format standards that are transmitted using optical signals over fiber. SDH is similar to SONET, with a basic SDH rate of 155.52 Mbps, designated as STM-1. See also SONET and STM-1.
SDLC	Synchronous Data Link Control. SNA data link layer communications protocol. SDLC is a bit-oriented, full-duplex serial protocol that has spawned several similar protocols, including HDLC and LAPB.
Section DCC	See SDDC.
SF	Super Frame. Common framing type used on T1 circuits. SF consists of 12 frames of 192 bits each, with the 193rd bit providing error checking and other functions. SF has been superseded by ESF, but is still widely used. Also called D4 framing. See also ESF.
Short reach	See SR.
Simple Network Management Protocol	See SNMP.
Single-mode fiber	See SMF.
SMF	Single-mode fiber. Fiber-optic cabling with a narrow core that allows light to enter only at a single angle. Such cabling has higher bandwidth than multimode fiber, but requires a light source with a narrow spectral width (for example, a laser). Also called monomode fiber. See also Multimode fiber.
SNAP	Subnetwork Access Protocol. Internet protocol that operates between a network entity in the subnetwork and a network entity in the end system. SNAP specifies a standard method of encapsulating IP datagrams and ARP messages on IEEE networks. The SNAP entity in the end system makes use of the services of the subnetwork and performs three key functions: data transfer, connection management, and QoS selection.
SNMP	Simple Network Management Protocol. Network management protocol used almost exclusively in TCP/IP networks. SNMP provides a means to monitor and control network devices, and to manage configurations, statistics collection, performance, and security. See also SNMP2.
SNMP2	SNMP Version 2. Version 2 of the network management protocol. SNMP2 supports centralized as well as distributed network management strategies, and includes improvements in the SMI, protocol operations, management architecture, and security. See also SNMP.
SONET	Synchronous Optical Network. High-speed synchronous network specification developed by Bellcore (Telcordia) and designed to run on optical fiber. STS-1 is the basic building block of SONET. It was approved as an international standard in 1988. See also SDH and STS-1.
SPE	Synchronous Payload Envelope. The major portion of the SONET frame format used to carry the STS-1 signal; it is divided into an information payload section and a transport overhead system. SPE is used to address three payload structures: direct to STS-1 line rate muliplexing; asynchronous DS3 multiplexing; and synchronous DS3 multiplexing.
SR	Short reach. SONET/SDH specification for transmit power and receive sensitivity that achieves a 1.2-mile (2-km) reach.
SRAM	Static Random Access Memory. A type of RAM that retains its contents for as long as power is supplied. SRAM does not require constant refreshing as with DRAM. Compare with DRAM.
SSH	Secure shell.
STM-1	Synchronous Transport Module level 1. Basic building block signal of SDH, operating at 155.52 Mbps. Faster SDH rates are defined as STS-n, where n is a multiple of 155.52 Mbps. See also SDH.
STS-1	Synchronous Transport Signal level 1. Basic building block signal of SONET, operating at 51.84 Mbps. Faster SONET rates are defined as STS-n, where n is a multiple of 51.84 Mbps. See also SONET.
Subnetwork Access Protocol	See SNAP.
Subrate DS3	A generic term to describe a process in which the bandwidth of a clear channel DS3 is limited to a lower rate. Many proprietary formats exist.
Super Frame	See SF.
SVC	Switched virtual circuit. A virtual circuit that is dynamically established on demand and is torn down when transmission is complete. SVCs are used in situations in which data transmission is sporadic. Called a switched virtual connection in ATM terminology. Compare with PVC.
Switched virtual circuit	See SVC.
Synchronous Data Link Control	See SDLC.
Synchronous Digital Hierarchy	See SDH.
Synchronous Optical Network	See SONET.
Synchronous Payload Envelope	See SPE.

T
T1	Digital WAN carrier facility. T1 transmits DS1-formatted data at 1.544 Mbps through the telephone switching network.
T3	Digital WAN carrier facility. T3 transmits DS3-formatted data at 44.736 Mbps through the telephone switching network.
TAC	A Cisco Technical Assistance Center. There are four TACs worldwide.
TACACS	Terminal Access Controller Access Control System. Authentication protocol, developed by the DDN community, that provides remote access authentication and related services, such as event logging. User passwords are administered in a central database rather than in individual routers, providing an easily scalable network security solution.
TACACS+	Terminal Access Controller Access Control System Plus. Proprietary Cisco enhancement to Terminal Access Controller Access Control System (TACACS). Provides additional support for authentication, authorization, and accounting.
Tag switching	High-performance packet-forwarding technology that integrates network layer (Layer 3) routing and data link layer (Layer 2) switching and provides scalable, high-speed switching in the network core. Tag switching is based on the concept of label swapping, in which packets or cells are assigned short, fixed-length labels that tell switching nodes how data should be forwarded.
Telcordia	Organization that performs research and development on behalf of the Regional Bell Operating Companies (RBOCs). Formerly called Bellcore.
TDM	Time-division multiplexing. A technique in which information from multiple channels can be allocated bandwidth on a single wire based on preassigned time slots. Bandwidth is allocated to each channel regardless of whether the station has data to transmit.
Telnet	Standard terminal emulation protocol in the TCP/IP protocol stack. Telnet is used for remote terminal connection, enabling users to log in to remote systems and use resources as if they were connected to a local system. Telnet is defined in RFC 854.
Terminal Access Controller Access Control System	See TACACS.
Terminal Access Controller Access Control System Plus	See TACACS+.
TFTP	Trivial File Transfer Protocol. Simplified version of FTP that allows files to be transferred from one computer to another over a network.
Time-division multiplexing	See TDM.
ToS	Type of service. See CoS.
Traffic management	A technique for avoiding congestion and shaping and policing traffic. It allows links to operate at high levels of utilization by scaling back lower-priority, delay-tolerant traffic at the edge of the network when congestion begins to occur.
Traffic shaping	Use of queues to limit surges that can congest a network. Data is buffered and then sent into the network in regulated amounts to ensure that the traffic will fit within the promised traffic envelope for the particular connection. Traffic shaping is used in ATM, Frame Relay, and other types of networks. Also known as metering, shaping, or smoothing.

U
Unchannelized DS3	See Clear channel DS3.
Unicast	A message sent to a single network destination. Compare with Broadcast and Multicast.

V
Virtual Private Network	See VPN.
VLAN	Virtual LAN. A group of devices on one or more LANs that are configured (using management software) so that they can communicate as if they were attached to the same wire, when in fact they are located on a number of different LAN segments. Because VLANs are based on logical instead of physical connections, they are extremely flexible.
VoIP	Voice over IP. Voice over IP enables a router to carry voice traffic (for example, telephone calls and faxes) over an IP network.
VPN	Virtual private network. A secure, end-to-end private network constructed over a third-party or public network such as the Internet.
VSM	IPSec VPN service module. A module that works with the PRE card to bring scalable encryption services to the network. It occupies a line card slot and supports a single gigabit Ethernet interface, for encrypted and unencrypted data packets, based on gigabit interface converter (GBIC) technology.
VT-n	Virtual tributary level n. The SONET format for mapping a lower-rate signal into a SONET payload. For example, VT-1.5 is used to transport a DS1 signal. See also DS1 and SONET.

W
WAN	Wide-area network. A data communications network that serves users across a broad geographic area and often uses transmission devices provided by a common carrier (such as a telephone company or service provider).
Weighted Fair Queuing	See WFQ.
Weighted Random Early Detection	See WRED.
WFQ	Weighted Fair Queuing. Congestion management algorithm that identifies conversations (in the form of traffic streams), separates packets that belong to each conversation, and ensures that capacity is shared fairly between these individual conversations. WFQ is an automatic way of stabilizing network behavior during congestion and results in better performance and fewer retransmissions.
Wide-area network	See WAN.
WRED	Weighted Random Early Detection. RED uses an algorithm to randomly discard packets.The result of the drop is that the source detects the dropped traffic and slows its transmission. WRED combines the capabilities of the RED algorithm with IP precedence. This combination provides for preferential traffic handling for higher-priority packets. It can selectively discard lower-priority traffic when the interface starts to get congested and provide differentiated performance characteristics for different classes of service.