Network Glossary D

D4 framing Another name for Superframe.

DAI See Dynamic ARP Inspection.

Data Carrier Detect On a serial cable, the pin lead set by the DCE to imply a working link.

data communications equipment DCE devices are one of two devices on either end of a communications circuit, specifically the device with more control over the communications. Frame Relay switches are DCE devices. DCEs are also known as data circuit-terminating equipment (DTE).

Database Description A type of OSPF packet used to exchange and acknowledge LSA headers.
Sometimes called DBD.

Data-link connection identifier A Frame Relay address used in Frame Relay headers to identify the VCdata plane In IP routing, a term referring to the process of forwarding packets through a router.

data Set Ready On a serial cable, the pin lead set by the DCE to imply that the DCE is ready to
signal using pin leadsdata terminal equipment From one perspective, DTE devices are one of two devices on either end of a communications circuit, specifically the device with less control over the communi-cations. In Frame Relay, routers connected to a Frame Relay access link are DTE devices.

Data Terminal Ready On a serial cable, the pin lead set by the DTE to imply that the DTE is ready to signal using pin leads.

DCD See Data Carrier Detect.

DCE See data communications equipment.

DD See Database Description.

DE See Discard Eligible.

Dead Time/Interval With OSPF, the timer used to determine when a neighboring router has failed, based on a router not receiving any OSPF messages, including Hellos, in this timer period.

default route A route that is used for forwarding packets when the packet does not match any more specific routes in the IP routing table.

dense-mode protocol A multicast routing protocol whose default action is to flood multicast packets throughout a network.

designated port With Spanning Tree Protocol, the single port on each LAN segment from which the best Hello BPDU is forwarded.

designated router With PIM on a multiaccess network, the PIM router with the highest IP address on the subnet. With OSPF, the OSPF router that wins an election amongst all current neighbors. The DR is responsible for flooding on the subnet, and for creating and flooding the type 2 LSA for the subnet.

DHCP See Dynamic Host Configuration Protocol.

DHCP snooping A switch feature in which the switch examines DHCP messages and, for untrusted ports, filters all messages typically sent by servers and inappropriate messages sent by clients. It also builds a DHCP snooping binding table that is used by DAI and IP Source Guard.

DHCP snooping binding database The list of entries learned by the switch DHCP snooping feature. The entries include the MAC address used as the device’s DHCP client address, the assigned IP address, the VLAN, and the switch port on which the DHCP assignment messages flowed.

Differentiated Services A set of QoS RFCs that redefines the IP header’s ToS byte, and suggests specific settings of the DSCP field and the implied QoS actions based on those settings.

Differentiated Services Code Point The first 6 bits of the DS field, used for QoS marking.

differentiated tail drop A term relating to Cisco LAN switch tail-drop logic, in which multiple tail-drop thresholds may be assigned based on CoS or DSCP, resulting in some frames being discarded more aggressively than others.

DiffServ See Differentiated Services.

Diffusing Update Algorithm A term referring to EIGRP’s internal processing logic.

Digital Signal Level 0 Inside telcos’ original TDM hierarchy, the smallest unit of
transmission at 64 kbps.

Digital Signal Level 1 Inside telcos’ original TDM hierarchy, a unit that combines multiple DS0s into a single channel—24 DS0s (plus overhead) for a T1, and 30 (plus overhead) for an E1.

Digital Signal Level 3 Inside telcos’ original TDM hierarchy, a unit that combines multiple DS1s into a single channel—28 DS1s (plus overhead) for a T3, and 16 E1 DS1s (plus overhead) for an E3.

Dijkstra Alternate name for the SPF algorithm, named for its inventor, Edsger W. Dijkstra.

direct sequence spread spectrum A type of spread spectrum that spreads RF signals over the frequency spectrum by representing each data bit by a longer code. 802.11b specifies the use of DSSS.

disabled state An 802.1d STP port state in which the port has been administratively disabled.

Discard Eligible A bit in the Frame Relay header that, when set to 1, means that if a device needs to discard frames, it should discard the frames with DE 1 first.

discarding state An 802.1w RSTP port state in which the port is not forwarding or receiving; covers 802.1d port states disabled, blocking, and listening.

distance vector The underlying algorithms associated with RIP.

Distance Vector Multicast Routing Protocol Operates in dense mode and depends on its own unicast routing protocol that is similar to RIP to perform its multicast functions.

distributed coordination function The mandatory contention-based 802.11 access protocol that is also referred to as CSMA/CA.

distribution list A Cisco IOS configuration tool for routing protocols by which routing updates may be filtered.

DLCI See data-link connection identifier.

DMVPN See Dynamic Multipoint VPN.

downstream router The router that will receive the group traffic when a multicast router forwards group traffic to another router.

DR See designated router.

DR election (OSPF) The process by which neighboring OSPF routers examine their Hello messages and elect the DR. The decision is based on priority (highest), or RID (highest) if priority is a tie.

DROther The term to describe a router that is neither the DR nor the BDR on a subnet that elects a DR and BDR.

DS field The second byte of the IP header, formerly known as the ToS byte and redefined by DiffServ.

DS0 See Digital Signal Level 0.

DS1 See Digital Signal Level 1.

DS3 See Digital Signal Level 3.

DSCP See Differentiated Services Code Point.

DSCP-to-CoS map A mapping between each DSCP value and a corresponding CoS value, often used in Cisco LAN switches when performing classification for egress queuing.

DSCP-to-threshold map A mapping between each DSCP value and a WRED threshold, often used in Cisco LAN switches when performing WRED.

DSR See Data Set Ready.

DSSS See direct sequence spread spectrum.

DTE See data terminal equipment.

DTIM interval The number of beacons that governs how often multicast frames are sent over a wireless LAN.

DTP See Dynamic Trunking Protocol.

DTR See Data Terminal Ready.

DUAL See Diffusing Update Algorithm.

Dual FIFO A Cisco IOS interface software queue queuing strategy implemented automatically when using either form of Frame Relay fragmentation. The system then interleaves packets from the high-priority queue between fragments of the medium-priority queue.

dual stack An IPv6 migration strategy in which a host or router supports both IPv4 and IPv6 natively.

dual token bucket A conceptual model used by CB Policing when using an excess burst.

dual-rate, three-color policer Policing in which two rates are metered, and packets are placed into one of three categories (conform, exceed, or violate).

DVMRP See Distance Vector Multicast Routing Protocol.

Dynamic ARP Inspection A switch feature with which the switch watches ARP messages, determines if those messages may or may not be part of some attack, and filters those that look suspicious.

Dynamic Host Configuration Protocol A standard (RFC 2131) protocol by which a host can dynamically broadcast a request for a server to assign to it an IP address, along with other configuration settings, including a subnet mask and default gateway IP address. DHCP provides a great deal of flexibility and functionality compared with RARP and BOOTP.

Dynamic Multipoint VPN A method of providing dynamically configured spoke-to-spoke VPN connectivity in a hub-and-spoke network that significantly reduces configuration required on the spoke routers compared to traditional IPsec VPN environments.

Dynamic Trunking Protocol A Cisco-proprietary protocol used to dynamically negotiate whether the devices on an Ethernet segment want to form a trunk and, if so, which type (ISL or 802.1Q).