Rules applied to a router that will determine traffic patterns for data.

A value that ranges from 0 through 255, which determines the priority of a source’s routing information.

A routing protocol that combines the strengths of the distance vector and link state routing protocols. Cisco Enhanced Interior Gateway Routing Protocol (EIGRP) is considered an advanced distance vector protocol.

The highest layer of the OSI model (Layer 7). It is closest to the end user and selects appropriate network services to support end-user applications such as email and FTP.

A protocol used to map a known logical address to an unknown physical address. A device performs an ARP broadcast to identify the physical address of a destination device. This physical address is then stored in cache memory for later transmissions.

A group of networks under common administration that share a routing strategy.

A dedicated-connection switching technology that organizes digital data into units and transmits them over a physical medium using digital signal technology.

A term that refers to the reduction in strength of a signal. Attenuation occurs with any type of signal, whether digital or analog. Sometimes referred to as signal loss.

A header used with IPSec that provides integrity and authentication.

The available capacity of a network link over a physical medium.

A Frame Relay message that notifies the sending device that there is congestion in the network. A BECN bit is sent back in the direction from where the frame was sent (the source).

An exterior routing protocol that exchanges route information between autonomous systems.

The lowest four binary digits of a configuration register. The value of the boot field determines the order in which a router searches for Cisco IOS software.

Data messages that are exchanged across the switches within an extended LAN that uses a spanning-tree protocol topology.

A device used to segment a LAN into multiple physical segments. A bridge uses a forwarding table to determine which frames need to be forwarded to specific segments. Bridges isolate local traffic to the originating physical segment, but forward all nonlocal and broadcast traffic.

A data frame that’s sent to every node on a local segment.

A signal received on a router interface that indicates whether the physical layer connectivity is operating properly.

A Cisco proprietary protocol that operates at the data link layer. CDP enables network administrators to view a summary protocol and address information about other directly connected Cisco routers (and some Cisco switches).

A single communications path on a system. In some situations, channels can be multiplexed over a single connection.

An authentication protocol for the Point-to-Point Protocol (PPP) that uses a three-way, encrypted handshake to force a remote host to identify itself to a local host.

A field that performs calculations to ensure the integrity of data.

Implemented to resolve the rapid depletion of IP address space on the Internet and to minimize the number of routes on the Internet. CIDR provides a more efficient method of allocating IP address space by removing the concept of classes in IP addressing. CIDR enables routes to be summarized on powers-of-two boundaries; therefore, it reduces multiple routes into a single prefix.

The rate at which a Frame Relay link transmits data, averaged over time. CIR is measured in bits per second. This is the committed rate that the service provider guarantees for a Frame Relay connection.

Categorizes IP addresses into ranges that are used to create a hierarchy in the IP addressing scheme. The most common classes are A, B, and C, which can be identified by looking at the first three binary digits of an IP address.

Classless addressing does not categorize addresses into classes and is designed to deal with wasted address space.

The local telephone company office where all local loops in an area connect.

A numeric value (typically displayed in hexadecimal form) used to specify certain actions on a router.

A situation that occurs during data transfer if one or more computers generate network traffic faster than it can be transmitted through the network.

A terminal attached directly to the router for configuring and monitoring the router.

The process by which all routers within an internetwork route information and eventually agree on optimal routes through the internetwork.

A routing problem in which the distance metric for a destination network is continually increased because the internetwork has not fully converged.

Terminating equipment such as telephones and modems supplied by the service provider, installed at the customer site, and connected to the network.

An error-checking mechanism by which the receiving node calculates a value based on the data it receives and compares it with the value stored within the frame from the sending node.

A physical specification used in wireless networks to provide contention-based frame transmission. A sending device first listens to detect if there is any activity and, if it is clear, sends the frame. The sending device will send a signal telling other devices not to transmit.

A physical specification used by Ethernet to provide contention-based frame transmission. CSMA/CD specifies that a sending device must share physical transmission media and listen to determine whether a collision occurs after transmitting. In simple terms, this means that an Ethernet card has a built-in capability to detect a potential packet collision on the internetwork.

A method of forwarding frames based on the first six bytes contained in the frame. Cut-through switching provides higher throughput than store-and-forward switching because it requires only six bytes of data to make the forwarding decision. Cut-through switching does not provide error checking like its counterpart store-and-forward switching.

The device at the network end of a user-to-network connection that provides a physical connection to the network, forwards traffic, and provides a clocking signal used to synchronize data transmission between the DCE and DTE devices.

The process by which a destination peer layer removes and reads the control information sent by the source peer layer in another network host.

A binary or decimal representation of the number of bits used to identify an IP network. The class of the IP address defines the default mask. A default mask is represented by four octets of binary digits. The mask can also be presented in dotted decimal notation.

A network route (that usually points to another router) established to receive and attempt to process all packets for which no route appears in the route table.

The amount of time necessary to move a packet through the internetwork from source to destination.

The point of demarcation is between the carrier’s equipment and the customer premise equipment (CPE).

The algorithm used to securely exchange secret shared keys used in IPSec.

A bit that can be set to indicate that a frame can be dropped if congestion occurs within the Frame Relay network.

An interior routing protocol that relies on distance and vector or direction to choose optimal paths. A distance vector protocol requires each router to send all or a large part of its route table to its neighboring routers periodically.

A value that specifies a permanent virtual circuit (PVC) or switched virtual circuit (SVC) in a Frame Relay network.

A system used to translate fully qualified hostnames or computer names into IP addresses, and vice versa.

A method of representing binary IP addresses in a decimal format. Dotted decimal notation represents the four octets of an IP address in four decimal values separated by decimal points.

The device at the user end of the user-to-network connection that connects to a data network through a data communications equipment (DCE) device.

A network route that adjusts automatically to changes within the internetwork.

A routing protocol that conveys information between autonomous systems; it is widely used within the Internet. The Border Gateway Protocol (BGP) is an example of an exterior routing protocol.

A Cisco-proprietary routing protocol that includes features of both distance vector and link state routing protocols. EIGRP is considered an advanced distance vector protocol.

An IPSec header that provides confidentiality, authentication, and integrity.

Generally speaking, encapsulation is the process of wrapping data in a particular protocol header. In the context of the OSI model, encapsulation is the process by which a source peer layer includes header and trailer control information with a Protocol Data Unit (PDU) destined for its peer layer in another network host. The information encapsulated instructs the destination peer layer how to process the information.

The user interface for executing Cisco router commands.

Extra characters added to a frame for error control purposes. FCS is the result of a cyclic redundancy check (CRC).

A Frame Relay message that notifies the receiving device that there is congestion in the network. An FECN bit is sent in the same direction in which the frame was traveling, toward its destination.

Router memory that stores the Cisco IOS image and associated microcode. Flash is erasable, reprogrammable ROM that retains its content when the router is powered down or restarted.

A mechanism that throttles back data transmission to ensure that a sending system does not overwhelm the receiving system with data.

A switched data link layer protocol that supports multiple virtual circuits using High-Level Data Link Control (HDLC) encapsulation between connected devices.

A method of tagging a frame with a unique user-defined virtual local area network (VLAN). The process of tagging frames allows VLANs to span multiple switches.

A protocol used to copy a file from one host to another host, regardless of the physical hardware or operating system of each device. FTP identifies a client and server during the file-transfer process. In addition, it provides a guaranteed transfer by using the services of the Transmission Control Protocol (TCP).

The physical transmission process on a network device by which one pair of wires transmits data while another pair of wires receives data. Full-duplex transmission is achieved by eliminating the possibility of collisions on an Ethernet segment, thereby eliminating the need for a device to sense collisions.

A router mode that enables simple router configuration commands—such as router names, banners, and passwords—to be executed. Global configuration commands affect the whole router rather than a single interface or component.

The physical transmission process whereby one pair of wires is used to transmit information and the other pair of wires is used to receive information or to sense collisions on the physical media. Half-duplex transmission is required on Ethernet segments with multiple devices.

The process of one system making a request to another system before a connection is established. Handshakes occur during the establishment of a connection between two systems, and they address matters such as synchronization and connection parameters.

A bit-oriented, synchronous data link layer protocol that specifies data encapsulation methods on serial links.

Control information placed before the data during the encapsulation process.

A routing environment that relies on several routers to compose a backbone. Most traffic from nonbackbone routers traverses the backbone routers (or at least travels to the backbone) to reach another nonbackbone router. This is accomplished by breaking a network into a hierarchy of networks, where each level is responsible for its own routing.

The state into which a route is placed so that routers will not advertise or accept updates for that route until a timer expires.

The number of routers a packet passes through on its way to the destination network.

A logical name given to a router.

A physical standard designed for serial connections that require high data transmission rates. The HSSI standard allows for high-speed communication that runs at speeds up to 52Mbps.

A protocol that communicates error messages and controls messages between devices. Thirteen types of ICMP messages are defined. ICMP enables devices to check the status of other devices, to query the current time, and to perform other functions such as ping and traceroute.

An organization whose primary function is to define standards for network LANs.

The dialog used to configure a router the first time it is booted or when no configuration file exists. The initial configuration dialog is an optional tool used to simplify the configuration process.

The term to describe your inside addresses after they have been translated with network address translation (NAT). Inside global addresses are registered addresses that represent your inside hosts to your outside networks.

The addresses on the inside of your network before they are translated with network address translation (NAT).

Router components that provide the network connections in which data packets move in and out of the router. Depending on the model of router, interfaces exist either on the motherboard or on separate, modular interface cards.

A routing protocol that exchanges information within an autonomous system. Routing Information Protocol (RIP) and Open Shortest Path First (OSPF) are examples of interior routing protocols.

A component of IPSec that is used to dynamically and securely exchange secret keys. IKE uses Diffie-Helman to exchange keys.

One of the many protocols maintained in the TCP/IP suite of protocols. IP is the transport mechanism for Transmission Control Protocol (TCP), User Datagram Protocol (UDP), and Internet Control Message Protocol (ICMP) data. It also provides the logical addressing necessary for complex routing activity.

An access list that provides a way of filtering IP traffic based on the source IP address, destination IP address, TCP port, UDP port, IP precedence field, TOS field, ICMP-type, ICMP-code, ICMP-message, IGMP-type, and TCP-established connections.

An access list that provides a way of filtering IP traffic on a router interface based on the source IP address or address range.

A suite of security protocols that is used to provide a secure VPN. IPSec can operate in tunnel mode, where a new IP header is added, or transport mode, where the original IP header is used.

A protocol used to enable virtual local area networks (VLANs) to span multiple switches. ISL is used between switches to communicate common VLANs between devices.

Protocol Data Units (PDUs) transmitted at the data link layer that indicate whether the proper frame type is configured.

Protocols that identify Layer 2 protocols used for the transmission of data within a local area network (LAN). The three most popular LAN protocols used today are Ethernet, Token Ring, and Fiber Distributed Data Interface (FDDI).

A protocol that configures, tests, maintains, and terminates Point-to-Point Protocol (PPP) connections. LCP is a sublayer of the Point-to-Point Protocol (PPP).

A packet that contains the status of a router’s links or network interfaces.

An interior routing protocol in which each router sends only the state of its own network links across the network, but sends this information to every router within its autonomous system or area. This process enables routers to learn and maintain full knowledge of the network’s exact topology and how it is interconnected. Link state protocols use a “shortest path first” algorithm.

A sublayer of the data link layer. The LLC sublayer provides the software functions of the data link layer.

A set of enhancements to the Frame Relay protocol specifications used to manage complex networks. Some key Frame Relay LMI extensions include global addressing, virtual circuit status messages, and multicasting.

An indication of how busy a network resource is. CPU utilization and packets processed per second are two indicators of load.

The line from the customer’s premises to the telephone company’s central office (CO).

Network layer addressing is most commonly referred to as logical addressing (versus the physical addressing of the data link layer). A logical address consists of two parts: the network and the node. Routers use the network part of the logical address to determine the best path to the network of a remote device. The node part of the logical address is used to identify the specific host to forward the packet on the destination network.

A process of comparing two sets of binary numbers to result in one value representing an IP address network. Logical ANDing is used to compare an IP address against its subnet mask to yield the IP subnet on which the IP address resides. ANDing is also used to determine whether a packet has a local or remote destination.

A physical address used to define a device uniquely.

A sublayer of the data link layer that provides the hardware functions of the data link layer.

The relative cost of sending packets to a destination network over a specific network route. Examples of metrics include bandwidth, delay, and reliability.

A database that maintains statistics on certain data items. The Simple Network Management Protocol (SNMP) uses MIBs to query information about devices.

A process of using one IP address to represent a group of IP addresses. Multicasting is used to send messages to a subset of IP addresses in a network or networks.

A routing protocol that load balances over multiple optimal paths to a destination network when the costs of the paths are equal.

A method of flow control used by the transport layer in which application conversations are combined over a single channel by interleaving packets from different segments and transmitting them.

The process of translating your multiple, internal IP addresses to a single registered IP address on the outside of your network.

A multiaccess network that either does not support broadcasts or for which sending broadcasts is not feasible.

A collection of protocols that establishes and configures different network layer protocols for use over a Point-to-Point Protocol (PPP) connection. NCP is a sublayer of PPP.

A common session layer interface specification from IBM and Microsoft that enables applications to request lower-level network services.

A board that provides network communication capabilities to and from a network host.

A memory area of the router that stores permanent information, such as the router’s backup configuration file. The contents of NVRAM are retained when the router is powered down or restarted.

A layered networking framework developed by the International Organization for Standardization. The OSI model describes seven layers that correspond to specific networking functions.

A hierarchical link state routing protocol that was developed as a successor to the Routing Information Protocol (RIP).

A process by which a router moves a packet from one interface to another.

An authentication protocol for the Point-to-Point Protocol (PPP) that uses a two-way, unencrypted handshake to enable a remote host to identify itself to a local host.

A unit of measure that refers to data that is transmitted between two peer layers within different network devices. Segments, packets, and frames are examples of PDUs.

A form of communication that occurs between the same layers of two different network hosts.

A tool for testing IP connectivity between two devices. Ping is used to send multiple IP packets between a sending and a receiving device. The destination device responds with an Internet Control Message Protocol (ICMP) packet to notify the source device of its existence.

A physical location where a carrier has installed equipment to interconnect with a local exchange carrier.

A standard protocol that enables router-to-router and host-to-network connectivity over synchronous and asynchronous circuits such as telephone lines.

Layer 6 of the OSI model. The presentation layer is concerned with how data is represented to the application layer.

An extensive administrative and management mode on a Cisco router. This router mode permits testing, debugging, and commands to modify the router’s configuration.

A formal description of a set of rules and conventions that defines how devices on a network must exchange information.

The circuit-switching facilities maintained for voice analog communication.

A virtual circuit that is permanently established and ready for use.

A memory area of a router that serves as a working storage area. RAM contains data such as route tables, various types of caches and buffers, as well as input and output queues and the router’s active configuration file. The contents of RAM are lost when the router is powered down or restarted.

This protocol provides mapping that is exactly opposite to the Address Resolution Protocol (ARP). RARP maps a known physical address to a logical address. Diskless machines that do not have a configured IP address when started typically use RARP. RARP requires the existence of a server that maintains physical-to-logical address mappings.

A metric that allows the network administrator to assign arbitrarily a numeric value to indicate a reliability factor for a link. The reliability metric is a method used to capture an administrator’s experience with a given network link.

A widely used distance vector routing protocol that uses hop count as its metric.

An area of router memory that contains a version of the Cisco IOS image—usually an older version with minimal functionality. ROM also stores the bootstrap program and power-on diagnostic programs.

A mode on a Cisco router that allows basic functions such as changing the configuration register value or uploading an IOS via xmodem.

The process of combining multiple IP address networks into one superset of IP address networks. Route aggregation is implemented to reduce the number of route table entries required to forward IP packets accurately in an internetwork.

A routing technique by which a router immediately marks a network as unreachable as soon as it detects that the network is down. The router broadcasts the update throughout the network and maintains this poisoned route in its route table for a specified period of time.

An area of a router’s memory that stores the network topology information used to determine optimal routes. Route tables contain information such as destination network, next hop, and associated metrics.

A protocol that provides the information required for the routing protocol to determine the topology of the internetwork and the best path to a destination. The routed protocol provides this information in the form of a logical address and other fields within a packet. The information contained in the packet enables the router to direct user traffic. The most common routed protocols include Internet Protocol (IP) and Internetwork Packet Exchange (IPX).

The router identifier used with OSPF. The router ID is selected as the highest IP address among all loopback interfaces. If loopback interfaces are not configured, the router ID is the highest IP address of any active physical interface at the moment that OSPF is initialized.

Modes that enable the execution of specific router commands and functions. User, privileged, and setup are examples of router modes that allow you to perform certain tasks.

Well-defined rules that aid routers in the collection of route information and the determination of the optimal path.

An event in which two or more routers have not yet converged and are propagating their inaccurate route tables. In addition, they are probably still switching packets based on their inaccurate route tables.

Routing protocols use algorithms to generate a list of paths to a particular destination and the cost associated with each path. Routers use routing protocols to communicate among each other the best route to use to reach a particular destination.

A physical standard used to identify cabling types for serial data transmission for speeds of 19.2Kbps or less. RS-232 connects two devices communicating over a serial link with either a 25-pin (DB-25) or 9-pin (DB-9) serial interface. RS-232 is now known as EIA/TIA-232.

The current configuration file that is active on a router.

A router-maintenance mode that enables router recovery functions when the IOS file in flash has been erased or is corrupt.

A protocol that allows for secure communication between a client and a router. It is a secure alternative to Telnet.

A common method of securing HTTP communication. It is also used for web-based VPNs where users are first authenticated via a web GUI before gaining access to secure web applications.

A 32-bit unique identifier that is used to name a wireless network.

As Layer 5 of the OSI model, the session layer establishes, manages, and terminates sessions between applications on different network devices.

The router mode triggered on startup if no configuration file resides in nonvolatile random access memory (NVRAM).

See link state protocol.

A method by which TCP dynamically sets the window size during a connection, enabling the receiving device involved in the communication to slow down the sending data rate.

A protocol used to pass mail messages between devices, SMTP uses Transmission Control Protocol (TCP) connections to pass the email between hosts.

The combination of the sending and destination Transmission Control Protocol (TCP) port numbers and the sending and destination Internet Protocol (IP) addresses defines a socket. Therefore, a socket can be used to define any User Datagram Protocol (UDP) or TCP connection uniquely.

A protocol used to eliminate all circular routes in a bridged or switched environment while maintaining redundancy. Circular routes are not desirable in Layer 2 networks because of the forwarding mechanism employed at this layer.

A routing mechanism that prevents a router from sending information that it received about a network back to its neighbor that originally sent the information. This mechanism is useful in preventing routing loops.

The backup configuration file on a router.

A network route that is manually entered into the route table. Static routes function well in simple and predictable network environments.

A method of forwarding frames by copying an entire frame into the buffer of a switch and making a forwarding decision. Store-and-forward switching does not achieve the same throughput as its counterpart, cut-through switching, because it copies the entire frame into the buffer instead of copying only the first six bytes. Store-and-forward switching, however, provides error checking that is not provided by cut-through switching.

One of possibly many virtual interfaces on a single physical interface.

A process of splitting a classful range of IP addresses into multiple IP networks to allow more flexibility in IP addressing schemes. Subnetting overcomes the limitation of address classes and allows network administrators the flexibility to assign multiple networks with one class of IP addresses.

Provides increased port density and forwarding capabilities as compared to bridges. The increased port densities of switches enable LANs to be microsegmented, thereby increasing the amount of bandwidth delivered to each device.

One of the many protocols maintained in the TCP/IP suite of protocols. TCP provides a connection-oriented and reliable service to the applications that use it.

A three-step process whereby a TCP session is established. In the first step, the sending device sends the initial sequence number with the SYN bit set in the TCP header. The receiver sends back a packet with the SYN and ACK bits set. In the third and final step, the sender sends a packet with the ACK bit set.

A method of increasing or reducing the number of acknowledgments required between data transmissions. This enables devices to throttle the rate at which data is transmitted.

A standard protocol that provides a virtual terminal. Telnet enables a network administrator to connect to a router remotely.

A protocol used to copy files from one device to another. TFTP is a stripped-down version of FTP.

An IP service that allows a user to utilize the services of the User Datagram Protocol (UDP) and the Internet Control Message Protocol (ICMP) to identify the number of hops between sending and receiving devices and the paths taken from the sending to the receiving device. Traceroute also provides the IP address and DNS name of each hop. Typically, traceroute is used to troubleshoot IP connectivity between two devices.

Control information placed after the data during the encapsulation process. See encapsulation for more detail.

As Layer 4 of the OSI model, it is concerned with segmenting upper-layer applications, establishing end-to-end connectivity through the network, sending segments from one host to another, and ensuring the reliable transport of data.

A switch port that connects to another switch to enable virtual local area networks (VLANs) to span multiple switches.

A tunnel takes packets or frames from one protocol and places them inside frames from another network system. See encapsulation.

One of the many protocols maintained in the TCP/IP suite of protocols, UDP is a Layer 4, best-effort delivery protocol and, therefore, maintains connectionless network services.

A display-only mode on a Cisco router. Only limited information about the router can be viewed within this router mode; no configuration changes are permitted.

A physical standard used to identify cabling types for serial data transmission for speeds up to 4Mbps. The V.35 standard was created by the International Telecommunication Union-Telecommunication (ITU-T) standardization sector.

A technique of assigning devices to specific LANs based on the port to which they attach on a switch rather than the physical location. VLANs extend the flexibility of LANs by allowing devices to be assigned to specific LANs on a port-by-port basis versus a device basis.

VLSM provides more flexibility in assigning IP address space. (A common problem with routing protocols is the necessity of all devices in a given routing protocol domain to use the same subnet mask.) Routing protocols that support VLSM allow administrators to assign IP networks with different subnet masks. This increased flexibility saves IP address space because administrators can assign IP networks based on the number of hosts on each network.

A protocol for configuring and administering VLANS on Cisco network devices. With VTP, an administrator can make configuration changes centrally on a single Catalyst series switch and have those changes automatically communicated to all the other switches in the network.

WANs use data communications equipment (DCE) to connect multiple LANs. Examples of WAN protocols include Frame Relay, Point-to-Point Protocol (PPP), and High-Level Data Link Control (HDLC).

A set of ports between 1 and 1,023 that are reserved for specific TCP/IP protocols and services.

A security protocol used in Wi-Fi networks that encrypts packets over radio waves. It offers 40-bit and 104-bit encryption (often referred to 64- and 128-bit encryption because of the added initialization vector in the algorithm).

Security protocols for Wi-Fi networks that provide greater security than WEP.