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Glossary

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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 net-
work is unreachable by not including it in updates). See also: route poisoning.

polling

The procedure of orderly inquiry, used by a primary network

mechanism, to determine if secondary devices have data to transmit. A mes-
sage is sent to each secondary, granting the secondary the right to transmit.

POP

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).

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.

POTS

Plain Old Telephone Service: This refers to the traditional analog

phone service that is found in most installations.

PPP

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 proto-
cols, and link monitoring. PPP has two layers: the Link Control Protocol
(LCP) establishes, configures, and tests a link; and then any of various Net-
work 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 pre-

determined number to augment security.

Predictor

A compression technique supported by Cisco. See also:

compression.

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