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Glossary
destination address
The address for the network
devices that will receive a packet.
DHCP
Dynamic Host Configuaration Protocol;
DHCP is a superset of the BootP protocol. This means
that it uses the same protocol structure as BootP, but
it has enhancements added. Both of these protocols
use servers that dynamically configure clients when
requested. The two major enhancements are address
pools and lease times.
directed broadcast
A data frame or packet that is
transmitted to a specific group of nodes on a remote
network segment. Directed broadcasts are known by
their broadcast address, which is a destination subnet
address with all the bits turned on.
discovery mode
Also known as dynamic configu-
ration, this technique is used by an AppleTalk inter-
face to gain information from a working node about
an attached network. The information is subsequently
used by the interface for self-configuration.
distance-vector routing algorithm
In order to find
the shortest path, this group of routing algorithms
repeats on the number of hops in a given route, requir-
ing each router to send its complete routing table with
each update, but only to its neighbors. Routing algo-
rithms of this type tend to generate loops, but they are
fundamentally simpler than their link-state counter-
parts. See also: link-state routing algorithm and SPF.
distribute lists
These lists are references to access-
lists and are applied to interfaces via a routing protocol.
distributed switching
Distributed switching hap-
pens on the VIP (Versatile Interface Processor) cards
(which have a switching processor onboard), so it's
very efficient. All required processing is done right on
the VIP processor, which maintains a copy of the
router's routing cache.
distribution layer
Middle layer of the Cisco three-
layer hierarchical model, which helps you design,
install, and maintain Cisco hierarchical networks. The
distribution layer is the point where access layer
devices connect. Routing is performed at this layer.
divide-by-half troubleshooting
Troubleshooting
method in which a point between two ends of a net-
work problem is used as a troubleshooting reference
point. Either half may be investigated first, thus nar-
rowing down the trouble location.
DLCI
Data-Link Connection Identifier: Used to iden-
tify virtual circuits in a Frame Relay network.
DNS
Domain Name System: Used to resolve host
names to IP addresses.
DSAP
Destination Service Access Point: The service
access point of a network node, specified in the desti-
nation field of a packet. See also: SSAP and SAP.
DSR
Data Set Ready: When a DCE is powered up
and ready to run, this EIA/TIA-232 interface circuit is
also engaged.
DSU
Data Service Unit: This device is used to adapt
the physical interface on a data terminal equipment
(DTE) mechanism to a transmission facility such as T1
or E1 and is also responsible for signal timing. It is
commonly grouped with the channel service unit and
referred to as the CSU/DSU. See also: CSU.
DTE
data terminal equipment: Any device located at
the user end of a user-network interface serving as a
destination, a source, or both. DTE includes devices
such as multiplexers, protocol translators, and com-
puters. The connection to a data network is made
through data channel equipment (DCE) such as a
modem, using the clocking signals generated by that
device. See also: DCE.
DTR
data terminal ready: An activated EIA/TIA-232
circuit communicating to the DCE the state of pre-
paredness of the DTE to transmit or receive data.
DUAL
Diffusing Update Algorithm: Used in
Enhanced IGRP, this convergence algorithm provides
loop-free operation throughout an entire route's com-
putation. DUAL grants routers involved in a topology
revision the ability to synchronize simultaneously,
while routers unaffected by this change are not
involved. See also: Enhanced IGRP.
DVMRP
Distance Vector Multicast Routing Proto-
col: Based primarily on the Routing Information Pro-
tocol (RIP), this Internet gateway protocol implements
a common, condensed-mode IP multicast scheme,
using IGMP to transfer routing datagrams between its
neighbors. See also: IGMP.
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