Glossary Q



In data communications, the basic logical unit of information transferred. A packet consists of a certain number of data bytes, wrapped or encapsulated in headers and/or trailers that contain information about where the packet came from, where it's going, and so on. The various protocols involved in sending a transmission add their own layers of header information, which the corresponding protocols in receiving devices then interpret.

packet mode connections

Packet mode connections are typically passed through the router or remote access device. This includes Point-to-Point Protocol (PPP) sessions.

packet switch

A physical device that makes it possible for a communication channel to share several connections; its functions include finding the most-efficient transmission path for packets.

packet switching

A networking technology based on the transmission of data in packets. Dividing a continuous stream of data into small units-packets-enables data from multiple devices on a network to share the same communication channel simultaneously but also requires the use of precise routing information.


packet assembler and disassembler: Used to buffer incoming data that is coming in faster than the receiving device can handle it. Typically, only used in X.25 networks.


Port Aggregation Protocol: The communication process that switches use to determine if and how they will form an EtherChannel connection.


Password Authentication Protocol: In Point-to-Point Protocol (PPP) networks, a method of validating connection requests. The requesting (remote) device must send an authentication request, containing a password and ID, to the local router when attempting to connect. Unlike the more secure CHAP (Challenge Handshake Authentication Protocol), PAP sends the password unencrypted and does not attempt to verify whether the user is authorized to access the requested resource; it merely identifies the remote end. Compare to: CHAP.

parity checking

A method of error-checking in data transmissions. An extra bit (the parity bit) is added to each character or data word so that the sum of the bits will be either an odd number (in odd parity) or an even number (even parity).

partial mesh

A type of network topology in which some network nodes form a full mesh (where every node has either a physical or a virtual circuit linking it to every other network node), but others are attached to only one or two nodes in the network. A typical use of partial- mesh topology is in peripheral networks linked to a fully meshed backbone. See also: full mesh.

particle buffers

Buffers created from physically separate but logically contiguous small blocks of memory.


Port Address Translation: This process enables a single IP address to represent multiple resources by altering the source TCP or UDP port number. Sometimes referred to as NAT overload.

payload compression

Reduces the number of bytes required to accurately represent the original data stream. Header compression is also possible. See also: compression.


Peak Cell Rate: As defined by the ATM Forum, the parameter specifying, in cells per second, the maximum rate at which a source can transmit.


Public Data Network: Generally for a fee, a PDN offers the public access to a computer communication network operated by private concerns or government agencies. Small organizations can take advantage of PDNs, aiding them in creating WANs without investing in long- distance equipment and circuitry.


Protocol Data Unit: The name of the processes at each layer of the OSI model. PDUs at the Transport layer are called 'segments,' PDUs at the Network layer are called 'packets' or 'datagrams,' and PDUs at the Data Link layer are called 'frames.' The Physical layer uses 'bits.'

per-hop routing

Per-hop routing behavior refers to how packets are forwarded across an internetwork, with each router forwarding according to its own routing tables and priorities.


Policy Feature Card: The PFC can be paralleled with the NFFC used in Catalyst 5000 switches. It is a device that is capable of examining IP and Ethernet headers in order to establish flow caches.


Pretty Good Privacy: A popular public-key/private-key encryption application offering protected transfer of files and messages.

Physical layer

The lowest layer-layer 1-in the OSI reference model, it is responsible for converting data packets from the Data Link layer (layer 2) into electrical signals. Physical layer protocols and standards define, for example, the type of cable and connectors to be used, including their pin assignments and the encoding scheme for signaling 0 and 1 values. See also: Application layer, Data Link layer, Network layer, Presentation layer, Session layer, and Transport layer.


Protocol Independent Multicast: A multicast protocol that handles the IGMP requests as well as requests for multicast data forwarding.


Protocol Independent Multicast dense mode: PIM DM utilizes the unicast route table and relies on the source root distribution architecture for multicast data forwarding.


Protocol Independent Multicast sparse mode: PIM SM utilizes the unicast route table and relies on the shared root distribution architecture for multicast data forwarding.

PIM sparse-dense mode

An interface configuration that enables the interface to choose the method of PIM operation.


Packet Internet Groper: A Unix-based Internet diagnostic tool, consisting of a message sent to test the accessibility of a particular device on the IP network. The term's acronym (from which the 'full name' was formed) reflects the underlying metaphor of submarine sonar. Just as the sonar operator sends out a signal and waits to hear it echo ('ping') back from a submerged object, the network user can ping another node on the network and wait to see whether it responds.


Nearly synchronous, except that clocking comes from an outside source instead of being embedded within the signal as in synchronous transmissions. The T1 and E1 hierarchies are plesiochronous.


Packet Level Protocol: Occasionally called 'X.25 Level 3' or 'X.25 Protocol,' a Network layer protocol that is part of the X.25 stack.


Private Network-Network Interface: An ATM Forum specification for offering topology data used for the calculation of paths through the network, among switches and groups of switches. It is based on well-known link-state routing procedures and allows for automatic configuration in networks whose addressing scheme is determined by the topology.

point-to-multipoint connection

In ATM, a communication path going only one way, connecting a single system at the starting point, called the 'root node,' to systems at multiple points of destination, called 'leaves.' See also: point-to-point connection.

point-to-point connection

In ATM, a channel of communication that can be directed either one way or two ways between two ATM end systems. See also: point-to-multipoint connection.

poison reverse updates

These update messages are transmitted by a router back to the originator (thus ignoring the split-horizon rule) after route poisoning has occurred. Typically used with DV routing protocols in order to overcome large routing loops and offer explicit information when a subnet or network is not accessible (instead of merely suggesting that the network is unreachable by not including it in updates). See also: route poisoning.


When frames/packets arrive at an interface, they may be classified. In addition, queue sizes are configured to allow certain amounts of data through. Measuring whether data conforms to the permitted amount is called policing. See also: classification.


The procedure of orderly inquiry, used by a primary network mechanism, to determine whether secondary devices have data to transmit. A message is sent to each secondary, granting the secondary the right to transmit.


(1) Point of Presence: The physical location where an interexchange carrier has placed equipment to interconnect with a local exchange carrier. (2) Post Office Protocol (currently at version 3): A protocol used by client e-mail applications for recovery of mail from a mail server.

port density

Port density reflects the capacity of the remote access device regarding the termination of interfaces. For example, the port density of an access server that serves four T1 circuits is 96 analog lines (non-ISDN PRI).


The configuration option that tells the switch to move directly from blocking mode to forwarding mode. Only to be used when a single PC is connected to the port.

port security

Used with layer 2 switches to provide some security. Not typically used in production because it is difficult to manage. Allows only certain frames to traverse administrator- assigned segments.


plain old telephone service: This refers to the traditional analog phone service that is found in most installations.


Point-to-Point Protocol: The protocol most commonly used for dial-up Internet access, superseding the earlier SLIP. Its features include address notification, authentication via CHAP or PAP, support for multiple protocols, and link monitoring. PPP has two layers: the Link Control Protocol (LCP) establishes, configures, and tests a link; and then any of various Network Control Programs (NCPs) transport traffic for a specific protocol suite, such as IPX. See also: CHAP, PAP, and SLIP.

PPP callback

The point-to-point protocol supports callback to a predetermined number to augment security.


A compression technique supported by Cisco. See also: compression.

Presentation layer

Layer 6 of the OSI reference model, it defines how data is formatted, presented, encoded, and converted for use by software at the Application layer. See also: Application layer, Data Link layer, Network layer, Physical layer, Session layer, and Transport layer.


Primary Rate Interface: A type of ISDN connection between a PBX and a long-distance carrier, which is made up of a single 64Kbps D channel in addition to 23 (T1) or 30 (E1) B channels. Compare to: BRI. See also: ISDN.

priority queuing

A routing function in which frames temporarily placed in an interface output queue are assigned priorities based on traits such as packet size or type of interface.

processing delay

The delay associated with packets being forwarded across a router fabric.

process switching

As a packet arrives on a router to be forwarded, it's copied to the router's process buffer, and the router performs a lookup on the layer 3 address. Using the route table, an exit interface is associated with the destination address. The processor forwards the packet with the added new information to the exit interface, while the router initializes the fast- switching cache. Subsequent packets bound for the same destination address follow the same path as the first packet. Compare to: fast switching.


programmable read-only memory: ROM that is programmable only once, using special equipment. Compare to: EPROM and EEPROM.

propagation delay

The time it takes data to traverse a network from its source to its destination.


In networking, the specification of a set of rules for a particular type of communication. The term is also used to refer to the software that implements a protocol.

protocol stack

A collection of related protocols.

Proxy ARP

Proxy Address Resolution Protocol: Used to allow redundancy in case of a failure with the configured default gateway on a host. Proxy ARP is a variation of the ARP protocol in which an intermediate device, such as a router, sends an ARP response on behalf of an end node to the requesting host.


The act of trimming down the Shortest Path Tree. This deactivates interfaces that do not have group participants.


Packet Switch Exchange: The X.25 term for a switch.


packet-switched network: Any network that uses packet-switching technology. Also known as 'packet-switched data network (PSDN).' See also: packet switching.


Public Switched Telephone Network: Colloquially referred to as 'plain old telephone service' (POTS). A term that describes the assortment of telephone networks and services available globally.


PNNI Topology State Packet, used in ATM.


permanent virtual circuit: In a Frame-Relay network, a logical connection, defined in software, that is maintained permanently. Compare to: SVC. See also: virtual circuit.


permanent virtual path: A virtual path made up of PVCs. See also: PVC.

PVP tunneling

permanent virtual path tunneling: A technique that links two private ATM networks across a public network by using a virtual path; the public network transparently trunks the complete collection of virtual channels in the virtual path between the two private networks.


Per-VLAN Spanning Tree: A Cisco proprietary implementation of STP. PVST uses ISL and runs a separate instance of STP for each and every VLAN.


Per-VLAN Spanning Tree+: Allows CST information to be passed into PVST.

CCNP. Building Cisco Multilayer Switched Networks Study Guide (642-811)
CCNP: Building Cisco Multilayer Switched Networks Study Guide (642-811)
ISBN: 078214294X
EAN: 2147483647
Year: 2002
Pages: 174
Authors: Terry Jack © 2008-2017.
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