Network Glossary P

P router See provider router.

PAgP See Port Aggregation Protocol.

PAP See Password Authentication Protocol.

partial SPF calculation An SPF calculation for which a router does not need to run SPF for any LSAs inside its area, but instead runs a very simple algorithm for changes to LSAs outside its own area.

partial update A routing protocol feature by which the routing update includes only routes that have changed, rather than include the entire set of routes.

passive (EIGRP) A state for a route in an EIGRP topology table that indicates that the router believes that the route is stable, and it is not currently looking for any new routes to that subnet.

passive mode FTP Defines a particular behavior for FTP regarding the establishment of TCP data connections. In passive mode, an FTP server uses the FTP PORT command, over the FTP control connection, to tell the FTP client the port on which the server will be listening for a new data connection. The client allocates an unused port, and initiates a connection to the FTP server’s earlier-declared port.

passive scanning Each 802.11 station passively monitors each RF channel for a specific amount of time and listens for beacons. Stations use the signal strengths of found beacons to determine the access point or ad hoc network with which to attempt association.

Password Authentication Protocol An Internet standard authentication protocol that uses clear-text passwords and a two-way handshake to perform authentication over a PPP link.

PAT See Port Address Translation.

path attribute A term generally describing characteristics about BGP paths that are advertised in BGP Updates.

payload compression See Layer 2 payload compression.

PCM See pulse code modulation.

PDU See protocol data unit.

PE See provider edge.

peak information rate In two-rate policing, the second and higher rate defined to the policer.

peer group In BGP, a configuration construct in which multiple neighbors’ parameters can be configured as a group, thereby reducing the length of the configuration. Additionally, BGP performs routing policy logic against only one set of Updates for the entire peer group, improving convergence time.

penultimate hop popping An MPLS VPN term referring to the more efficient choice of popping the outer label at the second-to-last (penultimate) LSR, which then prevents the egress PE from having to perform two LFIB lookups to forward the packet.

Per-Hop Behavior With DiffServ, a DSCP marking and a related set of QoS actions applied to packets that have that marking.

permanent multicast group The multicast addresses assigned by IANA.

permanent virtual circuit A predefined VC. A PVC can be equated to a leased line in concept.

Per-VLAN Spanning Tree Plus A Cisco-proprietary STP implementation, created many years before IEEE 802.1s and 802.1w, that speeds convergence and allows for one STP instance for each VLAN.

PHB See Per-Hop Behavior.

PHP See penultimate hop popping.

PIM Hello message Sent by a PIM router, by default every 30 seconds, on every interface on which PIM is configured to discover neighbors, establish adjacency, and maintain adjacency.

PIM-DM See Protocol Independent Multicast dense-mode routing protocol.

PIM-SM See Protocol Independent Multicast sparse-mode routing protocol.

PIM-SM (S,G) RP-bit Prune When a PIM-SM router switches from RPT to SPT, it sends a PIM-SM Prune message for the source and the group with the RP bit set to its upstream router on the shared tree. RFC 2362 uses the notation PIM-SM (S, G) RP-bit Prune for this message.

PIR See peak information rate.

point coordination function An optional contention-free 802.11 access protocol that requires the access point to poll wireless stations before they are able to send frames. Not commonly implemented.

Point-to-Point Protocol An Internet standard serial data-link protocol, used on synchronous and asynchronous links, that provides data-link framing, link negotiation, Layer 3 interface features, and other functions.

poison reverse With RIP, the advertisement of a poisoned route out an interface, when that route was formerly not advertised out that interface due to split horizon rules.

policing rate The rate at which a policer limits the bits exiting or entering the policer.

policy map A term referring to the MQC policy-map command and its related subcommands, which are used to apply QoS actions to classes of packets.

policy routing Cisco IOS router feature by which a route map determines how to forward a packet, typically based on information in the packet other than the destination IP address.

Port Address Translation A NAT term describing the process of multiplexing TCP and UDP flows, based on port numbers, to a small number of public IP addresses. Also called NAT overloading.

Port Aggregation Protocol A Cisco-proprietary messaging protocol used to negotiate the dynamic creation of PortChannels (EtherChannels) and to choose which ports can be placed into an EtherChannel.

port security A switch feature that limits the number of allowed MAC addresses on a port, with optional limits based on the actual values of the MAC addresses.

PortFast Cisco-proprietary STP feature in which a switch port, known to not have a bridge or switch attached to it, transitions from disabled to forwarding state without using any intermediate states.

power-save mode A mechanism for conserving battery power in wireless stations. The access point buffers data frames destined to sleeping stations, which wake periodically to learn from information in the beacon frame whether or not data frames are waiting for transmission. The radio card receives applicable data frames and then goes back to sleep.

PPP See Point-to-Point Protocol.

PQ See priority queue and priority queuing.

prefix A numeric value between 0 and 32 (inclusive) that defines the number of beginning bits in an IP address for which all IP addresses in the same group have the same value. Alternative:The number of binary 1s beginning a subnet mask, written as a decimal value between 0 and 32, used as a more convenient form of representing the subnet mask.

prefix list A Cisco IOS configuration tool that can be used to match routing updates based on a base network address, a prefix, and a range of possible masks used inside the values defined by the base network address and prefix.

priority (OSPF) An administrative setting, included in Hellos, that is the first criteria for electing a DR. The highest priority wins, with values from 1–255, with priority 0 meaning a router cannot become DR or BDR.

priority queue Jargon referring to any queue that receives priority service, often used for queues in an LLQ configuration that have the priority command configured.

priority queuing A Cisco IOS queuing tool most notable for its scheduler, which always services the high-priority queue over all other queues.

private addresses RFC 1918-defined IPv4 network numbers that are not assigned as public IP address ranges, and are not routable on the Internet. Intended for use inside enterprise networks.

private AS A BGP ASN whose value is between 64,512 and 65,535. These values are not assigned for use on the Internet, and can be used for private purposes, typically either within confederations or by ISPs to hide the ASN used by some customers.

private VLAN A Cisco switch feature that allows separation of ports as if they were in separate VLANs, while allowing the use of a single IP subnet for all ports.

process switching A Layer 3 forwarding path through a router that does not optimize the forwarding path through the router.

promiscuous port With private VLANs, a port that can send and receive frames with all other ports in the private VLAN.

protocol data unit A generic term that refers to the data structure used by a layer in a layered network architecture when sending data.

Protocol Independent Multicast dense-mode routing protocol PIM-DM is a method of routing multicast packets that depends on a flood-and-prune approach. PIM Dense Mode gets its name from the assumption that there are many receivers of a particular multicast group, close together (from a network perspective). Does not depend on any particular unicast routing protocol to perform its multicast functions.

Protocol Independent Multicast sparse-mode routing protocol PIM-SM is a method of routing multicast packets that requires some intelligence in the network about the locations of receivers so that multicast traffic is not flooded into areas with no receivers. PIM Sparse Mode gets its name from the assumption that relatively few receivers of a particular multicast group, widely scattered (from a network perspective), want to receive that multicast traffic. Does not depend on any unicast routing protocol to perform its multicast functions.

provider edge An MPLS VPN term referring to any LSR that connects to customers to support the forwarding of unlabeled packets, as well as connecting to the MPLS network to support labeled packets, thereby making the LSR be on the edge between the provider and the customer.

provider router An MPLS VPN term referring to an LSR that has no direct customer connections, meaning that the P router does not need any visibility into the VPN customer’s IP address space.

proxy ARP A router feature used when a router sees an ARP request searching for an IP host’s MAC, when the router believes the IP host could not be on that LAN because the host is in another subnet. If the router has a route to reach the subnet where the ARP-determined host resides, the router replies to the ARP request with the router’s MAC address.

Prune Override On a multiaccess network, when a PIM-DM or PIM-SM router receives a Prune message, it starts a 3-second timer. If it receives a Join message on the multiaccess network from another router before the timer expires, it considers the message as an override to the previously received Prune message and continues forwarding the group traffic on the LAN interface; otherwise, it stops forwarding the traffic on the LAN interface.

pruning See VTP pruning.

public wireless LAN A wireless LAN that offers connections to the Internet from public places, such as airports, hotels, and coffee shops.

pulse code modulation An early standard from AT&T for encoding analog voice as a digital signal for transmission over a TDM network. PCM requires 64 kbps, and is the basis for the DS0 speed.

PVC See permanent virtual circuit.

PVST+ See Per-VLAN Spanning Tree Plus.