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Glossary

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discovery mode

Also known as dynamic configuration, this technique is

used by an AppleTalk interface 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,
requiring each router to send its complete routing table with each update,
but only to its neighbors. Routing algorithms of this type tend to generate
loops, but they are fundamentally simpler than their link-state counterparts.
See also: link-state routing algorithm and SPF.

distribution layer

Middle layer of the Cisco three-layer hierarchical

model, which helps you design, install, and maintain Cisco hierarchical net-
works. The distribution layer is the point where access-layer devices connect.
Routing is performed at this layer.

DLCI

Data-Link Connection Identifier: Used to identify virtual circuits in a

Frame Relay network.

DLSw

Data Link Switching: IBM developed Data Link Switching (DLSw)

in 1992 to provide support for SNA (Systems Network Architecture) and
NetBIOS protocols in router-based networks. SNA and NetBIOS are non-
routable protocols that do not contain any logical layer 3 network informa-
tion. DLSw encapsulates these protocols into TCP/IP messages that can be
routed and is an alternative to Remote Source-Route Bridging (RSRB).

DLSw+

Cisco's implementation of DLSw. In addition to support for the

RFC standards, Cisco added enhancements intended to increase scalability
and to improve performance and availability.

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 destination 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
T-1 or E-1 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.

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