access layer In a network, a group of routers responsible for providing connectivity to external or remote sites. CPU-intensive functions, such as packet filtering, are implemented in the access layer of a network.
access rate The maximum speed, in bits per second, at which a network node may transmit or receive information over a physical link.
acknowledgment A packet or a frame used to indicate that information has been received.
active In EIGRP, a route is considered to be active state if a change in the topology has caused it not to have a successor available. During active processing, EIGRP attempts to find an alternative path to the target network.
AD (administrative distance) The relative weight assigned to each routing protocol by a Cisco router; used to select the preferred route if two or more protocols have the same prefix.
address summarization The method used to aggregate routing information into shorter prefixes.
AS (autonomous system) A group of routers with a common routing policy. In general, an autonomous system operates under a single administrative control and may use one or more interior gateway protocols.
ATM (asynchronous transfer mode) A data-switching technology that uses fixed-length (53-byte) cells.
autosummarization The automatic aggregation of routing information at major network boundaries, where the addressing in a network changes from one classful network to another.
bandwidth The speed (in bits per second) available for data transmission in a particular link or media. Also used by EIGRP as a component of the metric for path determination.
BGP (Border Gateway Protocol) A routing protocol used for interdomain routing.
broadcast The transmission mode used to send information to all the destinations in a particular network or subnet.
BUS (broadcast and unknown server) In ATM LANE, the network component that is used to forward traffic destined to a multicast or broadcast address. BUS also floods information addressed to an unknown destination.
CDP (Cisco Discovery Protocol) The media-independent protocol Cisco equipment uses to discover neighboring devices.
CEF (Cisco Express Forwarding) The switching method used for layer 3 forwarding in Cisco equipment.
CIR (committed information rate) The guaranteed bandwidth associated with a frame relay PVC.
classful/classless A classful protocol does not advertise a mask in its routing updates; a classless protocol does, making it capable of advertising supernets. In general, IP addresses are divided into classes (A, B, and C). A network that corresponds to one of these major classes is considered classful.
convergence time The time between a change in a network and the moment when all the nodes have been made aware of it and processed the information.
core layer In a network, a group of routers that represents the backbone. In general, routers in the core layer are expected to have complete routing information and to carry large amounts of traffic.
default network The network used to send all traffic for which an explicit route does not exist.
default route The route used to forward traffic for which an explicit route does not exist.
delay The time required for information to travel between two points in a network.
discontiguous network An addressing scheme in which the components of a major network (Class A, B, or C) are physically and logically separated by addresses corresponding to a different major network.
distance vector protocol A routing protocol that propagates information by announcing all its known destinations to local neighbors. EIGRP, IGRP, and RIP are examples of distance vector protocols.
distribution layer In a network, a group of routers that connects the access layer to the core layer. In general, common services, such as authentication and databases, are located at the distribution layer.
DLCI (data link connection identifier) A value that identifies a PVC or an SVC in a frame relay network.
DUAL (Diffusing Update Algorithm) The algorithm used to find loop-free paths through a network.
dual-homed remotes Remote routers that have connections to two different access-layer routers.
external routes Routes carried by a routing protocol that originated outside its domain. These routes may have been originated by other protocols or from static routes.
feasible distance (FD) The calculated metric, or cost, to a destination as reported by the successor.
feasible successor (FS) An EIGRP neighbor that advertised a guaranteed loop-free path to a destination but is not the best path.
floating static route A manually configured static route that is assigned a higher administrative distance than a dynamically learned route, so that if the dynamic route is removed from the routing table, the static route is installed.
frame relay WAN (wide area network) technology that allows a single physical link from a router to be shared by connections to multiple remote sites, using DLCIs.
HDLC (high-level data link control) A bit-oriented synchronous data link-layer protocol developed by ISO. HDLC specifies a data encapsulation method on synchronous serial links, using frame characters and checksums.
hello interval The frequency with which a router sends hello packets out of an interface. A hello interval has default values, based on interface type, or can be manually configured per interface.
hierarchy The method of network design that uses a layered approach based on function. Typically, the hierarchy comprises the core, distribution, and access layers.
holddown The amount of time a routing protocol will wait once it has lost a path to a specific destination before accepting another path of equal or higher cost to that destination.
hold time How long a router will wait without hearing an EIGRP packet from a neighbor before declaring that neighbor down. The hold time defaults to three times the hello interval but can be manually set per interface.
IGRP (Interior Gateway Routing Protocol) The Cisco-proprietary distance vector protocol that was the predecessor of EIGRP.
incremental updates The technique of sending only the changes instead of full information in routing updates to neighbors. EIGRP uses incremental updates.
information hiding A network design method that limits the information known by routers in the network to that necessary to properly route traffic; typically implemented through the use of summarization and/or route filtering.
IS-IS (Intermediate System to Intermediate System) A link state protocol defined by the International Standards Organization, originally to support OSI networks. Integrated IS-IS expanded IS-IS to include the ability to support IP networks.
ISDN (integrated services digital network) The connection method typically used for dialing over digital services.
K value Defines a multiplier for the components of the EIGRP metric in the metric calculation. Defaults to include only the minimum bandwidth and delay components but can be manually set to include load and reliability, using the metric-weight command. Not recommended.
LAN (local area network) A high-speed, low-error data network covering a relatively small geographic area. LAN standards specify cabling and signaling at the physical and data link layers of the OSI model. Ethernet, FDDI, and Token Ring are typical LAN technologies.
LANE (LAN emulation) Technology that permits asynchronous transfer mode (ATM) networks to function as a LAN backbone. LANE appears to the network layer to operate as an Ethernet and Token Ring LANs through the use of address mapping, emulated broadcast support, and so on.
link state protocol A routing protocol that floods information throughout an area and then performs a shortest-path first algorithm, such as Dijkstra’s algorithm, on the received information to create a shortest-path tree with itself as root. IS-IS and OSPF are both widely used link state protocols.
load The EIGRP metric component that describes the level of traffic passing across a link. The value is encoded as x/255; the larger the value of x, the greater the amount of traffic. Load is not used in the metric calculation by default, but it may be enabled via the metric-weight command.
MAC (media access control) address A link-layer address that identifies which device on a LAN is the source or destination of a packet. Also known as the hardware address.
metric The measurement a routing protocol uses to determine which path is best to reach a particular target. In EIGRP, the metric potentially comprises delay, minimum bandwidth, load, and reliability. By default, only delay and minimum bandwidth are used.
MTU (maximum transmission unit) The maximum size of a packet supported across a particular link/interface.
multicast The addressing method—at both the link and the network layer—that allows a single packet to be received and processed by more than one device at a time.
multipoint The type of frame relay network that allows multiple devices to connect using the same network-layer address range, as if sharing a subnet, but that uses separate permanent virtual circuits (PVCs) to reach each remote device.
NBMA (nonbroadcast multiaccess) A network that supports the connection to multiple devices to the same layer 3 network but doesn’t support the delivery of broadcast or multicast packets. Frame relay multipoint interfaces and X.25 networks are examples of NBMA networks.
neighbor A relationship whereby routers running the same routing protocol share one or more links.
ODR (on-demand routing) The mechanism that allows routes to be received from other routers via Cisco Discovery Protocol (CDP).
Offset list The method used to change the metric routes sent to or received from selected neighbors by increasing the delay component of the metric.
OSPF (Open Shortest Path First) A widely deployed hierarchical, standards-based link state interior gateway protocol.
pacing interval The method EIGRP uses to determine how quickly EIGRP packets can be sent out of an interface. By default, EIGRP will use no more than half the defined bandwidth of an interface.
PAP/CHAP passive The normal, stable state of a route in EIGRP.
point-to-point link The type of link that connects only two devices. HDLC links are typical point-to-point links.
point-to-point subinterfaces A type of frame relay configuration that emulates point-to-point links on a frame relay network. Each point-to-point subinterface connects only two devices across a PVC.
poison reverse The technique that occurs when a router, on receiving an advertisement for a destination through a given interface, readvertises that destination through the same interface as unreachable. Poison reverse prevents count-to-infinity routing loops in distance vector protocols.
PRI (primary rate interface) An ISDN interface with 23 data bearer (B) channels and an aggregate bandwidth equal to a T1.
PVC (permanent virtual circuit) A virtual circuit—a logical circuit multiplexed onto a series of physical links—permanently connected between two points on a network.
quality of service Providing service based on latency, jitter, bandwidth, or traffic-loss requirements to applications running over the network.
query In EIGRP, a request by one router to its peers for alternative paths to a given destination.
query scope The number of routers involved in a query event; the number of routers that will receive a query and be forced to reply before the router that originated the queries is answered.
redistribution Distributing information about reachable destinations learned through one routing protocol into another routing protocol.
redundancy Providing multiple paths through a network so a given number of failures will not prevent traffic from being delivered.
reliability A metric, measured as a factor of packets lost between the peers; the reliability of a link between two EIGRP peers.
reliable multicast A system of reliably sending multicast packets between two routers.
reported distance (RD) The distance, or cost, of reaching a given destination through a router as reported by that router. The link between the reporting router and the receiving router is not included in the reported distance.
RIP (Routing Information Protocol) Based on the Bellman-Ford algorithm, a distance vector protocol that uses periodic full updates between peers to provide routing information.
route filtering Preventing a router from advertising given destinations to its neighbors.
router ID A unique identifier for a router.
RTO (retransmission timeout) The amount of time a router running EIGRP will wait before retrying an unacknowledged transmission.
SIA (Stuck in Active) An EIGRP route that has been in active state longer than allowed by the Stuck in Active timer; typically, 3 minutes.
SMDS (switched multimegabit data service) A wide area network service similar to frame relay but higher in speed and capable of broadcast.
split horizon The technique of a router’s not advertising a destination through an interface being used to reach it.
SRTT (smooth round-trip time) The amount of time previously taken for a peer to acknowledge transmissions.
stub In a network, a point beyond which there is no connectivity back toward the network’s core; for instance, a remote site with only a few directly connected networks.
successor The EIGRP peer being used as the next hop toward a given destination.
summarization The process of consolidating several longer-prefix destination advertisements into one shorter-prefix advertisement.
topology The logical (IP or network layer) layout of the network.
topology database A repository of information a router knows about the network’s topology. Used by EIGRP.
traffic aggregation Combining several lower-speed links onto one higher-speed link.
transit paths Links through which traffic passes en route to its destination; traffic doesn’t terminate (or stop) on a transit link but just passes through.
unicast A packet transmitted from one host to another single host.
Update Information about destination and path and transmitted to a neighbor.
variance The amount of difference allowed between unequal-cost paths while still load sharing.
VLANs Virtual logical networks created on top of a set of physical links using local area network media; on WAN links, these are called PVCs.
VLSM (variable-length subnet masks) Having multiple prefix lengths within a single major network.
WAN (wide area network) Typically, a network that traverses long distances (outside of a campus environment).