Glossary
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CDP
Cisco Discovery Protocol: Cisco's proprietary protocol that is used to
tell a neighbor Cisco device about the type of hardware, software version,
and active interfaces that the Cisco device is using. It uses a SNAP frame
between devices and is not routable.
CDP holdtime
The amount of time a router will hold Cisco Discovery Pro-
tocol information received from a neighbor router before discarding it if the
information is not updated by the neighbor. This timer is set to 180 seconds
by default.
CDP timer
The amount of time Cisco Discovery Protocol is transmitted
out of all router interfaces, by default. The cdp timer is 90 seconds, by
default.
CDVT
Cell Delay Variation Tolerance: A QoS parameter for traffic man-
agement in ATM networks specified when a connection is established. The
allowable fluctuation levels for data samples taken by the PCR in CBR trans-
missions are determined by the CDVT. See also: CBR and PCR.
cell
In ATM networking, the basic unit of data for switching and mul-
tiplexing. Cells have a defined length of 53 bytes, including a 5-byte
header that identifies the cell's data stream and 48 bytes of payload. See
also: cell relay.
cell payload scrambling
The method by which an ATM switch maintains
framing on some medium-speed edge and trunk interfaces (T-3 or E-3 cir-
cuits). Cell payload scrambling rearranges the data portion of a cell to main-
tain the line synchronization with certain common bit patterns.
cell relay
A technology that uses small packets of fixed size, known as
cells. Their fixed length enables cells to be processed and switched in hard-
ware at high speeds, making this technology the foundation for ATM and
other high-speed network protocols. See also: cell.
Centrex
A local exchange carrier service, providing local switching that
resembles that of an on-site PBX. Centrex has no on-site switching capa-
bility. Therefore, all customer connections return to the CO. See also: CO.
CER
cell error ratio: The ratio in ATM of transmitted cells having errors to
the total number of cells sent in a transmission within a certain span of time.
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