Qa

1. Which Cisco IOS command applies an access list to an interface?

ip access-group {access-list-number | access-list-name} [in | out]

2. Which Cisco IOS command enables SNMP and sets community string? snmp-server community name [rw | ro] [access-list-number]

3. List the commands that configure the NTP server and the NTP peer.

ntp server {ip-address} ntp peer {ip-address}

4. Which Cisco IOS command configures the router to timestamp log or debug messages with the local date and time?

service timestamps {log | debug} datetime localtime

5. Which Cisco IOS command enables (or disables) DHCP server functionality on the local router?

service dhcp

6. Which Cisco IOS interface configuration command converts the UDP broadcasts (with certain destination ports, such as 68, 67 for BootP) to unicast and sends them to the IP address specified?

ip helper-address {ip-address}

7. What are the URLs for the Web pages of the IETF, ITU, FRF, and ATM Forum? Internet Engineering Task Force—www.ietf.org

International Telecommunications Union—www.itu.int/home Frame Relay Forum—www.frforum.com ATM Forum—www.atmforum.com

8. What are the URLs for Cisco Systems Technologies Reference, Cisco's Technical Assistance Center, and CCO?

Cisco Systems Technologies Reference—www.cisco.com/univercd/home/home.htm Cisco's Technical Assistance Center—www.cisco.com/tac/ CCO—www.cisco.com

9. What information must you gather and have ready before contacting Cisco Technical Assistance Center?

You should have the following information prepared before contacting Technical Assistance Center:

• A complete network diagram, or at least the affected area. The IP address/mask of the IP devices should be shown if possible.

• All the information and any facts gathered.

• The output of the show tech-support command if the number of affected routers is fewer than four.

• Dial-in or Telnet access to the devices under investigation.

10. List the seven steps to correct problems occurring at the transport and application layers.

Following are the seven steps to correct problems occurring at the transport and application layers:

1. For the device(s) whose configuration you intend to change, ensure that there is a saved valid configuration.

2. Make the intended changes. Make one change at a time.

3. Evaluate and document the results of your changes.

4. Verify that your changes did not introduce new problems/symptoms.

5. Continue making changes until problems are fixed.

6. Seek assistance from outside sources, such as other colleagues, consultants, and Cisco's Technical Assistance Center, if necessary.

7. Document the solution.

access list A list kept by routers and switches to control access to or from the router or switch for a number of services (such as to prevent packets with a certain IP address from leaving a particular interface on the router or switch).

administratively down A state that an interface might be in. An interface with the shutdown command stays in this state until the no shutdown command is entered.

ANSI American National Standards Institute. A voluntary organization composed of corporate, government, and other members that coordinates standards-related activities, approves U.S. standards, and develops positions for the United States in international standards organizations. ANSI helps develop international and U.S. standards relating to, among other things, communications and networking. ANSI is a member of the IEC and the ISO.

application layer Layer 7 of the OSI reference model. This layer provides services to application processes (such as e-mail, file transfer, and terminal emulation) that are outside the OSI model. The application layer identifies and establishes the availability of intended communication partners and the resources required to connect with them. It also synchronizes cooperating applications and establishes an agreement on the procedures for error recovery and the control of data integrity.

ARP Address Resolution Protocol. An Internet protocol that is used to map an IP address to a MAC address. Defined in RFC 826.

ATM Asynchronous Transfer Mode. The 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.

ATM Forum An international organization jointly founded in 1991 by Cisco Systems, NET/ADAPTIVE, Northern Telecom, and Sprint. This organization develops and promotes standards-based implementation agreements for ATM technology. The ATM Forum expands on official standards developed by ANSI and ITU-T and develops implementation agreements in advance of official standards.

autonomous system A collection of networks under a common administration sharing a common routing strategy. An autonomous system must be assigned a unique 16-bit number.

availability The amount of time that a telephone system or other device is operational—that is, how long it processes telephone calls or other transactions. Availability is represented as the ratio of the total time a device is operational during a given time interval to the length of that interval.

bandwidth The difference between the highest and lowest frequencies available for network signals. Also, the rated throughput capacity of a given network medium or protocol. The frequency range necessary to convey a signal is measured in hertz (Hz); for example, voice signals typically require approximately 7 kHz of bandwidth, and data traffic typically requires approximately 50 kHz of bandwidth.

baseline All information about a network that is in normal operating condition; includes diagrams, address specifications and lists, list of protocols in use, performance information, and so on.

BGP Border Gateway Protocol. The interdomain routing protocol that replaces EGP. BGP exchanges reachability information with other BGP systems. It is defined in RFC 1163.

bottom-up troubleshooting approach An approach to troubleshooting that starts with the physical components of the network and works its way up the layers of the OSI model until the problem's cause is identified.

broadcast A data packet that is sent to all nodes within a single logical network. Broadcasts are identified by a broadcast address.

cache A piece of (fast) memory that is used to hold information that frequently needs to be accessed. Different types of cache are available, and they hold different types of information. ARP cache, Fast Switching cache, and Multilayer Switching cache are examples of different types of cache.

CDP Cisco Discovery Protocol. A 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. CDP runs on all media that support SNA systems, including LANs, Frame Relay, and ATM media.

CDP neighbors Neighbors that are discovered (adjacent) through receiving of their CDP multicast frames.

Cisco IOS Software Cisco operating system software that provides common functionality, scalability, and security for all Cisco products. Cisco IOS Software allows centralized, integrated, and automated installation and management of internetworks while ensuring support for a wide variety of protocols, media, services, and platforms.

Cisco Technical Assistance Center The focal point of all Cisco software and hardware maintenance and support services. Contact the Cisco Technical Assistance Center for help with installation and testing, performance, training, documentation, equipment repair, Return Material Authorization (RMA) service, and equipment specifications.

clock rate This command is used to set the clocking rate on a serial interface that has the DCE end of the serial cable plugged into it.

collision In Ethernet, the result of two nodes transmitting simultaneously. The frames from each device impact and are damaged when they meet on the physical medium.

congestion A condition caused by traffic in excess of network capacity.

Correct the Problem stage The stage of troubleshooting at which the isolated cause of the problem is corrected.

CPU Central Processing Unit. A complex microcomputer chip that controls most of the important tasks performed by other components of the computing device.

CRC Cyclic redundancy check. An error-checking technique in which the frame recipient calculates a remainder by dividing frame contents by a prime binary divisor and compares the calculated remainder to a value stored in the frame by the sending node.

data flow A grouping of traffic, identified by a combination of source address/mask, destination address/mask, IP next protocol field, and source and destination ports, where the protocol and port fields can have the values of any. In effect, all traffic that matches a specific combination of these values is grouped logically in a data flow. A data flow can represent a single TCP connection between two hosts, or it can represent all the traffic between two subnets. IPSec protection is applied to data flows.

data flow control layer Layer 5 of the architectural model. This layer determines and manages interactions between session partners, particularly data flow. Corresponds to the session layer of the OSI reference model.

datagram A logical grouping of information sent as a network layer unit over a transmission medium without prior establishment of a virtual circuit. IP datagrams are the primary information units in the Internet. The terms cell, frame, message, packet, and segment are also used to describe logical information groupings at various layers of the OSI reference model and in various technology circles.

DCE 1. data communications equipment (EIA expansion). Provides clocking to the data terminal equipment (DTE). 2. data circuit-terminating equipment (ITU-T expansion). Devices and connections of a communications network that compose 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 that synchronizes data transmission between DCE and DTE devices. Modems and interface cards are examples of DCE.

debugging A term that is used for tools and commands that help remove culprits from computing software or configuration files.

default gateway An address to which an IP host forwards the data whose destination is outside the IP host's local subnet.

default route A routing table entry that directs packets for which a next hop is not explicitly listed in the routing table.

default VLAN The VLAN to which all switch interfaces are associated, unless specified otherwise. Naturally, the default VLAN for Ethernet, Token Ring, and the rest is different. The Ethernet default VLAN is usually VLAN 1.

DHCP Dynamic Host Configuration Protocol. A protocol that provides a mechanism for allocating IP addresses dynamically so that addresses can be reused when hosts no longer need them.

divide-and-conquer troubleshooting approach This approach to troubleshooting selects a layer and performs tests in both directions from the starting layer. The troubleshooter chooses the starting layer based on his past experience and any information at hand about the problem. When a layer is identified as working, the layer above it is suspected and examined. If a layer is detected as faulty, the layer below it is examined.

DNS Domain Name System. A system used on the Internet to translate names of network nodes into addresses.

DNS server Domain Name System server.

DRAM dynamic random-access memory. RAM that stores information in capacitors that must be refreshed periodically. Delays can occur because DRAM is inaccessible to the processor when refreshing its contents. However, DRAM is less complex and has greater capacity than SRAM.

DTE data terminal equipment. A 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 (such as a modem) and typically uses clocking signals that the DCE generates. DTE includes devices such as computers, protocol translators, and multiplexers.

duplex Specifies whether a device's interface sends and receives data on the same circuit (not at the same time), or whether the send and receive circuits are distinct and the interface can send data at the same time it receives data.

EIGRP Enhanced Interior Gateway Routing Protocol. An advanced version of IGRP that Cisco developed. Provides superior convergence properties and operating efficiency and combines the advantages of link-state protocols with those of distance vector protocols.

EMI electromagnetic interference. Interference by electromagnetic signals that can cause reduced data integrity and increased error rates on transmission channels.

encapsulation The wrapping of data in a particular protocol header. For example, Ethernet data is wrapped in a specific Ethernet header before network transit. Also, when you are bridging dissimilar networks, the entire frame from one network is placed in the header used by the other network's data link layer protocol.

end system Servers, workstations, personal computers, and laptop computers are examples of end systems.

end system network configuration table This table is baseline documentation that shows accurate records of the hardware and software used in end systems.

EtherChannel technology Developed and copyrighted by Cisco Systems, this is a logical aggregation of multiple Ethernet interfaces used to form a single higher-bandwidth routing or bridging endpoint.

Fast Ethernet Any of a number of 100-Mbps Ethernet specifications. Fast Ethernet offers a speed increase 10 times that of the 10BASE-T Ethernet specification while preserving such qualities as frame format, MAC mechanisms, and MTU. Such similarities allow the use of existing 10BASE-T applications and network management tools on Fast Ethernet networks. Based on an extension to the IEEE 802.3 specification.

Faststart A feature that can be used on switch ports (of certain switches such as 19xx) to which end systems or nonbridging devices connect. Allows the port to go into the spanning tree forwarding state in a few seconds. (The similar command on other switches, such as 5xxx, is PortFast.)

FCS frame check sequence. Extra characters added to a frame for error-control purposes. Used in high-level data link control (HDLC), Frame Relay, and other data link layer protocols.

firewall A router or access server, or several routers or access servers, designated as a buffer between any connected public networks and a private network. A firewall router uses access lists and other methods to ensure the private network's security.

flapping A routing problem in which an advertised route between two nodes alternates (flaps) between two paths because of a network problem that causes intermittent interface failures.

flash memory A special type of electrically erasable programmable read-only memory (EEPROM) that can be erased and reprogrammed in blocks instead of one byte at a time. Many modern PCs have their BIOS stored on a flash memory chip so that it can be updated easily if necessary. Such a BIOS is sometimes called a Flash BIOS. Flash memory is also popular in modems because it lets the modem manufacturer support new protocols as they become standardized.

frame A logical grouping of information sent as a data link layer unit over a transmission medium. Often refers to the header and trailer, used for synchronization and error control, that surround the user data contained in the unit. The terms cell, datagram, message, packet, and segment also are used to describe logical information groupings at various layers of the OSI reference model and in various technology circles.

frame error One of many error statistics reported on the output of the Cisco IOS show interface command. This counter reports on the frames read (input frames) that had noninteger number of bytes. (The lengths of those frames were not integer numbers.)

Frame Relay An 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 generally is considered a replacement.

FTP File Transfer Protocol. An application protocol, part of the TCP/IP protocol stack, that transfers files between network nodes. FTP is defined in RFC 959.

gateway of last resource A routing table entry that all packets not matching against any other routing table entry will match to and be sent to their next hop address.

Gather Symptoms stage The stage of the troubleshooting process in which symptoms of the problem(s) are collected.

general troubleshooting process A three-stage troubleshooting model composed of the following: 1. Gather the Symptoms stage; 2. Isolate the Problem stage; 3. Correct the Problem stage.

helper address An address configured on an interface to which broadcasts received on that interface are sent. Helper addresses are typically used when clients are configured as DHCP clients, and the DHCP server is connected to a different logical network than the clients.

high-bandwidth application An application that, because of its nature, requires high bandwidth. Multimedia network applications are examples of such applications.

HTTP Hypertext Transfer Protocol. The protocol used by Web browsers and Web servers to transfer files, such as text and graphics.

hub 1. Generally, a term used to describe a device that serves as the center of a star-topology network. 2. A hardware or software device that contains multiple independent but connected modules of network and internetwork equipment. Hubs can be active (they repeat signals sent through them) or passive (they do not repeat, but merely split, signals sent through them). 3. In Ethernet and IEEE 802.3, an Ethernet multiport repeater, sometimes called a concentrator.

IANA Internet Assigned Numbers Authority. An organization that is operated under the auspices of the ISOC as part of the IAB. The IANA delegates authority for IP address space allocation and domain-name assignment to the InterNIC and other organizations. IANA also maintains a database of assigned protocol identifiers used in the TCP/IP stack, including autonomous system numbers.

ICMP Internet Control Message Protocol. A network-layer Internet protocol that reports errors and provides other information that is relevant to IP packet processing. Documented in RFC 792.

IETF Internet Engineering Task Force. A task force consisting of more than 80 working groups that is responsible for developing Internet standards. The IETF operates under the auspices of ISOC.

IGP Interior Gateway Protocol. An Internet protocol used to exchange routing information within an autonomous system. Examples of common Internet IGPs include IGRP, OSPF, and RIP.

interface 1. A connection between two systems or devices. 2. In routing terminology, a network connection. 3. In telephony, a shared boundary defined by common physical interconnection characteristics, signal characteristics, and meanings of interchanged signals. 4. A boundary between adjacent layers of the OSI model.

interface reset This is one of many counters/statistics reported on the output of the Cisco IOS show interface command. This counter increments every time the interface is cleared (using the clear interface interface command), when the shutdown command is issued for that interface, when the no shutdown command is issued for that interface, and when the state of the interface changes from up to down or from down to up. When an interface is reset for any of these reasons, its hardware buffers are cleared.

IP Internet Protocol. The network layer protocol in the TCP/IP stack that offers a connectionless internetwork service. IP provides features for addressing, type of service (ToS) specification, fragmentation and reassembly, and security. Defined in RFC 791.

IP address A 32-bit address that is assigned to hosts using TCP/IP. An IP address belongs to one of five classes—A, B, C, D, or E—and is written as four octets separated by periods (dotted-decimal format). Each address consists of a network number, an optional subnetwork number, and a host number. The network and subnetwork numbers together are used for routing, and the host number is used to address an individual host within the network or subnetwork. Also called an Internet address.

IPTV A multimedia, multicast-based software application.

IS-IS Protocol Intermediate System-to-Intermediate System Protocol. An 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.

ISOC Internet Society. An international nonprofit organization, founded in 1992, that coordinates the evolution and use of the Internet. In addition, ISOC delegates authority to other groups related to the Internet, such as the Internet Architecture Board (IAB). ISOC is headquartered in Reston, Virginia.

Isolate the Problem stage The stage of troubleshooting at which the cause of the problem is isolated (determined).

ITU International Telecommunication Union. An organization established by the United Nations to set international telecommunications standards and to allocate frequencies for specific uses.

jabber 1. An error condition characterized by a network device continually transmitting random, meaningless data onto the network. 2. In IEEE 802.3, a data packet whose length exceeds that prescribed in the standard.

jitter 1. Interpacket delay variance; that is, the difference between interpacket arrival and departure. Jitter is an important quality of service (QoS) metric for voice and video applications. 2. Analog communication line distortion caused by the variation of a signal from its reference timing positions. Jitter can cause data loss, particularly at high speeds.

keepalive interval A period of time between each keepalive message sent by a network device.

keepalive message A message sent by one network device to inform another network device that the virtual circuit between the two is still active.

latency 1. The delay between the time that a device requests access to a network and the time that it is granted permission to transmit. 2. The delay between the time that a device receives a frame and the time that the frame is forwarded out the destination port.

latency-sensitive application An application that cannot perform well or completely fails in the presence of network delays (latency).

LED light emitting diode. A semiconductor device that emits light produced by converting electrical energy. Status lights on hardware devices typically are LEDs.

line coding error An error related to the line coding setting/mismatch on T1/E1 or Primary Rate Interfaces (PRIs).

line protocol Line protocol is often interpreted as the layer immediately above the physical layer (Media Access Control in Ethernet terminology). This layer is responsible for submitting Layer 2 frames (or protocol data units in general) and receiving Layer 2 frames to and from the physical layer. The ability to send and receive frames requires more than just the physical layer's health. For example, an Ethernet interface that is not connected to the cable media will go into "line protocol down" state because the keepalives fail and the device concludes that it cannot send and receive frames to and from the cable media.

LMI Local Management Interface. A set of enhancements to the basic Frame Relay specification. LMI includes support for a keepalive mechanism that verifies that data is flowing; a multicast mechanism that 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.

loopback interface A logical interface (or sometimes called software interface) that is often created on routers with an IP address assigned to it. This interface is stable; if present, its IP address is used as the router ID for routing protocols such as BGP and OSPF.

MAC address A standardized data link layer address that is required for every 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, and physical address.

Mac OS Apple Macintosh operating system.

management alarm system Implemented in the form of beeping, sending e-mail, generating log messages, and so on, the management alarm system monitors such things as devices, CPUs, memories, and links. It notifies the support personnel of an event such as an error, crash, threshold exceeded, and so on.

management IP address A device's address that is used for managing the device. On routers, the loopback 0 address (and on Catalyst 5xxx switches, the SCO [system console] address) is often used as the management IP address. This address is used when you ping the device, Telnet into it, or communicate with it using SNMP.

management VLAN The VLAN that administrators are urged to reserve for network management, management traffic, and protocols such as VTP, CDP, HSRP, and so on.

media type A Cisco IOS interface configuration command that allows the administrator to configure the interface for connectivity through its RJ45, AUI, or any other type of connector (such as MII) that it might have.

multicast A stream of single packets copied by the network and sent to a specific subset of network addresses. These addresses are specified in the destination address field.

multicast application A network application that generates multicast traffic on the server and expects the traffic to be directed to the segments at which the clients that have joined the multicast group reside. IPTV is an example of a multicast application.

multilayer switch A switch that filters and forwards packets on the basis of Media Access Control (MAC) addresses and network addresses.

multiplexing A scheme that allows multiple logical signals to be transmitted simultaneously across a single physical channel.

native VLAN The VLAN that a switch port is associated to and that receives all the broadcasts released in that VLAN.

network layer Layer 3 of the OSI reference model. This layer provides connectivity and path selection between two end systems. The network layer is the layer at which routing occurs.

network topology diagram A graphical representation of a network. It illustrates how each device in the network is connected and reveals the logical architecture of the network.

NMS network management system. A system that is responsible for managing at least part of a network. An NMS is generally a reasonably powerful and well-equipped computer, such as an engineering workstation. NMSs communicate with agents to help keep track of network statistics and resources.

noise A set of undesirable communications channel signals.

NTP Network Time Protocol. A protocol built on top of TCP that ensures accurate local timekeeping with reference to radio and atomic clocks located on the Internet. This protocol can synchronize distributed clocks within milliseconds over long time periods.

NTP peer This Cisco IOS command allows you to specify the address of another time server device as a peer.

NTP server This Cisco IOS command allows you to specify the address of another device that plays the NTP server role.

NTP source This Cisco IOS command allows you to specify, on an NTP server, the IP address of which of its interfaces should be used when the device communicates with other NTP devices.

OSI reference model Open System Interconnection reference model. A network architectural model that ISO and ITU-T developed. The model consists of seven layers, each of which specifies particular network functions, such as addressing, flow control, error control, encapsulation, and reliable message transfer. The lowest layer (the physical layer) is closest to the media technology. The highest layer (the application layer) is closest to the user. From Layer 1 to Layer 7, the layers are as follows: physical, data link, network, transport, session, presentation, and application. The OSI reference model is used universally as a method of teaching and understanding network functionality.

OSPF Open Shortest Path First. A link-state, hierarchical Interior Gateway Protocol (IGP) routing algorithm proposed as a successor to the Routing Information Protocol (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 Intermediate System-to-Intermediate System (IS-IS) protocol.

packet A logical grouping of information that includes a header containing control information and (usually) user data. Packets most often refer to network layer units of data.

passive-interface A router configuration command used for RIP and (E)IGRP that prevents the routing process from sending routing updates out of the interface referenced by this command.

PDU protocol data unit. An OSI term for packet.

peak rate The maximum rate, in kilobits per second, at which a virtual circuit can transmit.

physical layer Layer 1 of the OSI reference model. The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems.

ping packet Internet groper. An Internet Control Message Protocol (ICMP) echo message and its reply. ping is often used in IP networks to test a network device's reachability.

policy map This command/structure associates class maps to QoS policies. A policy map is applied to router interfaces using the service policy command.

POP 1. point of presence. A physical location where an interexchange carrier has equipment installed to interconnect with a local exchange carrier (LEC). 2. Post Office Protocol. A protocol that client e-mail applications use to retrieve mail from a mail server.

port security This feature allows the administrator to restrict usage of a switch port to a particular or a specific set of MAC addresses.

PortFast This feature can be used on switch ports to which end systems or nonbridging devices connect. Allows the port to go into the spanning tree forwarding state in a few seconds.

PPP Point-to-Point Protocol. A successor to Serial Line Internet Protocol (SLIP) that provides router-to-router and host-to-network connections over synchronous and asynchronous circuits. PPP was designed to work with several network layer protocols, such as IP, IPX, and AppleTalk Remote Access (ARA). PPP also has built-in security mechanisms, such as Challenge Handshake Authentication Protocol (CHAP) and Password Authentication Protocol (PAP). PPP relies on two protocols: Link Control Protocol (LCP) and Network Control Protocol (NCP).

presentation layer Layer 6 of the OSI reference model. This layer ensures that information sent by one system's application layer can be read by another system's application layer. The presentation layer also is concerned with the data structures that programs use; therefore, it negotiates data transfer syntax for the application layer.

PVC permanent virtual circuit. A virtual circuit that is permanently established. PVCs save bandwidth associated with circuit establishment and teardown in situations in which certain virtual circuits must exist all the time.

queue 1. An ordered list of elements waiting to be processed. 2. A backlog of packets waiting to be forwarded over a router interface.

redundancy 1. 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. 2. The portion of the total information contained in a message that can be eliminated without losing essential information or meaning.

reliability 1. A determination of a network's or system's capability to continue operating if a single component fails. 2. A ratio of expected to received keepalives from a link. If the ratio is high, the line is reliable. Used as a routing metric.

RFC Request For Comments. A series of documents used as the primary means of communicating information about the Internet. The Internet Architecture Board (IAB) designates some RFCs as Internet standards. Most RFCs document protocol specifications, such as Telnet and File Transfer Protocol (FTP), but some are humorous or historical. RFCs are available online from numerous sources.

RMON Remote Monitoring. A Management Information Base (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.

router A network layer device that uses one or more metrics to determine the optimal path along which network traffic should be forwarded. Routers forward packets from one network to another based on network layer information.

router ID (RID) A protocol such as OSPF or BGP selects one of the router's IP addresses as its RID. The highest loopback interface's IP is preferred; otherwise, with no loopback addresses present, merely the highest IP address is chosen as the RID.

RTP 1. Routing Table Protocol. A Virtual Integrated Network Service (VINES) routing protocol based on Routing Information Protocol (RIP). Distributes network topology information and helps VINES servers find neighboring clients, servers, and routers. Uses delay as a routing metric. 2. Rapid Transport Protocol. A protocol that provides pacing and error recovery for Advanced Peer-to-Peer Networking (APPN) data as it crosses the APPN network. With RTP, error recovery and flow control are performed end-to-end rather than at every node. RTP prevents congestion rather than reacting to it. 3. Real-Time Transport Protocol. A protocol commonly used with IP networks. RTP is designed to provide end-to-end network transport functions for applications that transmit real-time data—such as audio, video, or simulation data—over multicast or unicast network services. RTP provides such services as payload type identification, sequence numbering, time-stamping, and delivery monitoring to real-time applications.

secondary IP address Cisco routers allow you to assign multiple IP addresses to an interface using the command ip address secondary. The addresses other than the primary IP address are thereby called the secondary IP addresses for that interface.

segment 1. A section of a network that is bound by bridges, routers, or switches. 2. In a LAN that uses a bus topology, a continuous electrical circuit that often is connected to other such segments with repeaters. 3. A term used in the TCP specification to describe a single transport layer unit of information.

server A node or software program that provides services to clients.

service timestamps A service that you can enable for logging or debugging on Cisco routers. It puts a timestamp beside every line of the log or debug output.

session layer Layer 5 of the OSI reference model. This layer establishes, manages, and terminates sessions between applications and manages the data exchange between presentation layer entities.

SMTP Simple Mail Transfer Protocol. An Internet protocol that provides e-mail services.

SNMP Simple Network Management Protocol. A network management protocol that is 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.

SNMP communities An authentication scheme that allows an intelligent network device to validate SNMP requests.

SNMP server This is the server that sets or clears SNMP parameters or accepts SNMP traps from SNMP agents.

SNMPv2 SNMP version 2. SNMP2 supports centralized as well as distributed network management strategies and includes improvements in the Structure of Management Information (SMI), protocol operations, management architecture, and security.

spanning tree A loop-free subset of a network topology.

spanning-tree algorithm The algorithm used by the Spanning-Tree Protocol to create a spanning tree. Sometimes abbreviated as STA.

speed Speed of an interface or port might be configurable (such as 10/100 Mbps). This command allows you to set the speed to the appropriate value.

split-horizon updates A routing technique in which information about routes is prevented from exiting the router interface through which that information was received. Split-horizon updates are useful in preventing routing loops.

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, like dynamic RAM (DRAM) does.

STP 1. Spanning-Tree Protocol. A bridge protocol that uses the spanning-tree algorithm, allowing a learning bridge to dynamically work around loops in a network topology by creating a spanning tree. Bridges exchange bridge protocol data unit (BPDU) messages with other bridges to detect loops; then they remove the loops by shutting down selected bridge interfaces. Refers to both the

IEEE 802.1 Spanning-Tree Protocol standard and the earlier Digital Equipment Corporation Spanning-Tree Protocol upon which it is based. The IEEE version supports bridge domains and allows the bridge to construct a loop-free topology across an extended LAN. The IEEE version generally is preferred over the Digital version. 2. signal transfer point. An element of a Signaling System 7 (SS7)-based intelligent network that performs routing of the SS7 signaling.

STP designated bridge When spanning tree runs, a designated device is chosen on each segment based on the best BPDU it advertises.

STP root bridge When spanning tree runs, one device is chosen as the root for the entire spanning tree.

STP state Based on the classic Spanning-Tree Protocol, a port can be in the Disabled, Listening, Learning Forwarding, or Blocking states.

subnet mask A 32-bit address mask used in IP to indicate the bits of an IP address that are used for the subnet address. Sometimes simply called a mask.

switch 1. A network device that filters, forwards, and floods frames based on each frame's destination address. The switch operates at the data link layer of the OSI reference model. 2. A general term applied to an electronic or mechanical device that allows a connection to be established as necessary and terminated when there is no longer a session to support. 3. In telephony, a general term for any device, such as a private branch exchange (PBX), that connects individual phones to phone lines.

T1 A digital WAN carrier facility. T1 carries DS1-formatted data at 1.544 Mbps through the telephone-switching network by using AMI or B8ZS coding.

TCP/IP Transmission Control Protocol/Internet Protocol. A common name for the suite of protocols developed by the U.S. Department of Defense in the 1970s to support the construction of worldwide internetworks. TCP and IP are the two best-known protocols in the suite.

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 associated with the channel has data to transmit.

Telnet The standard terminal emulation protocol in the TCP/IP protocol stack. Telnet is used for remote terminal connections, allowing users to log in to remote systems and use resources as if they were connected to those systems. Telnet is defined in RFC 854.

TFTP Trivial File Transfer Protocol. A simplified version of File Transfer Protocol (FTP) that allows files to be transferred from one computer to another over a network, usually without the use of client authentication (such as username and password).

throughput The rate at which information arrives at, and possibly passes through, a particular point in a network system.

top-down troubleshooting approach An approach to troubleshooting that starts with the end user application and works its way down from the upper layers of the OSI model until the problem's cause is identified.

topology A physical arrangement of network nodes and media within an enterprise networking structure.

traceroute A program available on many systems that traces the path a packet takes to a destination. It is used mostly to debug routing problems between hosts. A traceroute protocol is also defined in RFC 1393.

transport layer Layer 4 of the OSI reference model. This layer is responsible for reliable network communication between end nodes. The transport layer provides mechanisms for the establishment, maintenance, and termination of virtual circuits, transport fault detection and recovery, and information flow control. Corresponds to the transmission control layer of the Systems Network Architecture (SNA) model.

trunk 1. A physical and logical connection between two switches across which network traffic travels. A backbone is composed of a number of trunks. 2. A phone line between two central offices (COs) or between a CO and a private branch exchange (PBX).

tunnel interface A logical interface that is created and used to build tunnels, such as IP GRE tunnels.

UDP User Datagram Protocol. A connectionless transport layer protocol in the TCP/IP protocol stack. UDP is a simple protocol that exchanges datagrams without acknowledgments or guaranteed delivery, requiring that other protocols handle error processing and retransmission. UDP is defined in RFC 768.

unicast A message sent to a single network destination.

UNIX An operating system developed in 1969 at Bell Laboratories. UNIX has gone through several iterations since its inception, including UNIX 4.3 BSD (Berkeley Standard Distribution), developed at the University of California at Berkeley, and UNIX System V, Release 4.0, developed by AT&T.

VLAN virtual LAN. 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.

WAN wide-area network. A data communications network that serves users across a broad geographic area and often uses transmission devices that common carriers provide. Frame Relay, SMDS, and ATM are examples of WANs.

WAN circuit A circuit such as a PVC or SVC connecting two remote sites over the WAN.

WINS server A Microsoft service that facilitates NetBIOS name to IP resolution. Machines register with the WINS server dynamically. Any machine that needs the IP address of another machine (that is, its NetBIOS name) can send a request to the WINS server for that name.

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