Glossary

2-WAY/DROTHER A DROTHER is a router that is neither the DR nor the BDR but has a neighbor relationship with another DROTHER. These two neighbors exchange Hello packets.

2-way state Designates that bidirectional communication has been established between two routers in OSPF. The term is also referred to as Two-Way state and occurs after the Init state. Bidirectional means that each router has seen the other’s Hello packet.

access control list (ACL) A series of ACEs that controls whether a router forwards or drops packets based on information found in the packet header.

active state In EIGRP, a state in which there is no feasible successor in the topology table. The router enters active state and queries its neighbors for routing information.

addressing table A table that captures device names, interfaces, IPv4 addresses, subnet masks, and default gateway addresses.

adjacency A relationship formed between selected neighboring routers and end nodes for the purpose of exchanging routing information. Adjacency is based on the use of a common media segment.

adjacency database Also called the neighbor table, an OSPF database that contains a list of all neighbors’ routers to which a router has established bidirectional communication. This table is unique for each router and can be viewed using the show ip ospf neighbor command.

administrative distance (AD) The feature that routers use to select the best path when there are two or more different routes to the same destination from two different routing protocols. The AD represents the “trustworthiness” or reliability of the route.

Advanced Research Projects Agency Network (ARPANET) One of the world’s first operational packet switching networks. It was also the first network to implement TCP/IP, and the precursor to the global Internet.

algorithm Well-defined rule or process for arriving at a solution to a problem. In networking, algorithms are commonly used to determine the best route for traffic from a particular source to a particular destination.

Area Border Router (ABR) A router interconnecting the areas in a multiarea OSPF network.

autonomous system (AS) A collection of routers under a common administration such as a company or an organization.

Autonomous System Boundary Router (ASBR) The OSPF router located between an OSPF autonomous system network and a non-OSPF network. ASBRs run both OSPF and another routing protocol, such as RIP. ASBRs must reside in a nonstub OSPF area.

auto-summarization Feature that consolidates networks and advertises them in classful network advertisements.

availability A measure of the probability that the network is available for use when it is required.

backbone router An OSPF router that has at least one interface in area 0.

backup designated router (BDR) A router that becomes the designated router if the current designated router fails. The BDR is the OSPF router with the second-highest priority at the time of the last DR election.

best path The path with the lowest metric to a destination network.

Border Gateway Protocol (BGP) The only Exterior Gateway Protocol (EGP) designed to exchange routing and reachability information between autonomous systems on the Internet.

border router A router that sits on the edge of two discontiguous classful networks. A border router can also be known as a router that sits on the edge of two different networks that have different routing protocols. Sometime the word boundary router is loosely used when discussing OSPF and Autonomous System Boundary Routers (ASBRs).

bounded triggered update Updates are sent immediately upon being discovered, without waiting for a timer to expire. Also known as a flash update.

Cisco Express Forwarding (CEF) An advanced, Layer 3 switching technology inside a router. CEF defines the fastest method by which a Cisco router forwards packets from ingress to egress interfaces.

classful routing protocols A routing protocol that does not carry subnet mask information in its routing updates.

classless Operates without pre-defined classes. In networking, the pre-defined classes refer to the legacy Class A, B, C, and D network address ranges with pre-determined subnet masks.

classless routing protocols A routing protocol that carries subnet mask information in its routing updates. Classless routing protocols can take advantage of VLSM and supernet routes.

console cable A cable connected between the serial port of the host and the console port on the device.

convergence Speed and ability of a group of internetworking devices running a specific routing protocol to agree on the topology of an internetwork after a change in that topology.

cost An arbitrary value, typically based on hop count, media bandwidth, or other measures, that is assigned by a network administrator and used to compare various paths through an internetwork environment. Routing protocols use cost values to calculate the most favorable path to a particular destination: the lower the cost, the better the path.

database description (DBD) packet A packet used in OSPF that contains link-state advertisement (LSA) headers only and describes the contents of the entire link-state database. Routers exchange DBDs during the Exchange State of adjacency creation. A DBD is an OSPF Type 2 packet.

default gateway Identifies the router to send a packet to when the destination is not on the same local network subnet.

default static route A route that matches all packets and identifies the gateway IP address to which the router sends all packets for which it does not have a learned or static route.

designated router (DR) OSPF router that generates link-state advertisements (LSAs) for a multiaccess network and has other special responsibilities in running OSPF. Each multiaccess OSPF network that has at least two attached routers has a DR that is elected by the OSPF Hello protocol. The DR enables a reduction in the number of adjacencies required on a multiaccess network, which in turn reduces the amount of routing protocol traffic and the size of the topological database.

Dijkstra’s algorithm The routing algorithm of the OSPF routing protocol.

directly connected network A network that can be reached by the local router.

discontiguous network A network that does not have a hierarchical scheme and thus has fragmented network addressing. It is impossible to summarize discontiguous networks.

distance vector routing protocols Protocols that use either the Bellman–Ford algorithm or the DUAL Finite State Machine (FSM) (in the case of Cisco EIGRP) to calculate paths. A distance vector routing protocol requires that a router inform its neighbors of topology changes periodically.

Down state The first OSPF neighbor state. It means that no information (Hello packets) has been received from this neighbor, but Hello packets can still be sent to the neighbor in this state.

DROTHER A router that is neither the DR nor the BDR. DROTHERs are the other routers in the OSPF network.

DUAL Finite State Machine The DUAL finite state machine embodies the decision process for all route computations. It tracks all routes advertised by all neighbors.

dynamic route A remote network in a routing table that has been automatically learned using a dynamic routing protocol such as EIGRP or OSPF.

dynamic routing protocols Allow network devices to learn routes dynamically. OSPF and EIGRP are examples of dynamic routing protocols.

dynamically assigned IP address IP address information is provided by a server using the Dynamic Host Configuration Protocol (DHCP).

equal cost load balancing When a router utilizes multiple paths with the same administrative distance and cost to a destination.

Evaluation Right-To-Use (RTU) licenses Evaluation RTU licenses are limited period-metered licenses that are valid for a predefined number of days. An RTU license scheme is an honor-based model for licensing. Licenses are not tied to a Unique Device Identifier (UDI), Product ID (PID), or Serial Number (SN).

event-driven updates Sent out by link-state routing protocols after the initial flooding of link-state packets (LSPs), and only when there is a change in the topology. The LSP contains only the information regarding the affected link. Unlike some distance vector routing protocols, link-state routing protocols do not send periodic updates.

Exchange state State in which OSPF routers exchange database description (DBD) packets. DBDs contain link-state advertisement (LSA) headers only and describe the contents of the entire link-state database.

ExStart state State in which the routers and their DR and BDR establish a master/slave relationship and choose the initial sequence number for adjacency formation. The router with the higher router ID becomes the master and starts the exchange.

extended ACL Filters traffic based upon multiple attributes, including protocol type, source IPv4 addresses, destination IPv4 addresses, source ports, and destination ports.

extended maintenance release (EM release) Cisco IOS Software 15 release that is ideal for long-term maintenance, enabling customers to qualify, deploy, and remain on the release for an extended period. Provides approximately 40 months of total support. Incorporates features delivered in previous releases plus incremental new feature enhancements and hardware support.

Exterior Gateway Protocol (EGP) A routing protocol used for routing between autonomous systems.

external route summarization Route summarization that is specific to external routes that are injected into OSPF via route redistribution. Ensuring the contiguity of the external address ranges that are being summarized is important. Generally, only ASBRs summarize external routes. See also route summarization.

feasibility condition The feasibility condition is met when the receiving router has a feasible distance (FD) to a particular network and it receives an update from another neighbor with a lower advertised distance (reported distance) to that network. Used in EIGRP.

feasible successor A next-hop router that leads to a certain destination network. The feasible successor can be thought of as a backup next hop if the primary next hop (successor) goes down. Used in EIGRP.

Flash Nonvolatile storage that can be electrically erased and reprogrammed. It provides permanent storage for the IOS and other system-related files. Developed by Intel and licensed to other semiconductor companies.

floating static route A static route used to provide a backup path to a primary static or dynamic route in the event of a link failure. Used only when the primary route is not available.

forwarding database List of OSPF routes generated when an algorithm is run on the link-state database. Each router’s routing table is unique and contains information on how and where to send packets to other routers. Can be viewed using the show ip route command.

FULL/BDR An OSPF router that is fully adjacent with the indicated BDR neighbor. These two neighbors can exchange Hello packets, updates, queries, replies, and acknowledgments.

FULL/DR An OSPF router that is fully adjacent with the indicated DR neighbor. These two neighbors can exchange Hello packets, updates, queries, replies, and acknowledgments.

FULL/DROTHER A DR or BDR that is fully adjacent with a router that is neither the DR nor the BDR. These two neighbors can exchange Hello packets, updates, queries, replies, and acknowledgments.

Full state State in which routers are fully adjacent with each other. All the router and network LSAs are exchanged and the routers’ databases are fully synchronized.

fully specified static route A static route in which both the output interface and next-hop address are identified.

global unicast addresses Globally unique and routable IPv6 addresses.

Hello keepalive mechanism A small Hello message that is periodically exchanged to maintain adjacencies with neighboring routers. This means a very low usage of network resources during normal operation, instead of the periodic updates.

Hello packet Type 1 OSPF packet used to establish and maintain adjacency with other OSPF routers.

High-Speed WAN Interface Card (HWIC) Single-wide interface card that provides Cisco modular and integrated services routers with additional line-rate Layer 3 routed ports.

Hold time The maximum time a router waits to receive the next Hello packet or routing update. Once the Hold time counter expires, that route will become unreachable.

interarea route summarization Route summarization that occurs on ABRs and applies to routes from within each area. It does not apply to external routes injected into OSPF via redistribution. To perform effective interarea route summarization, network addresses within areas should be assigned contiguously so that these addresses can be summarized into a minimal number of summary addresses. See also route summarization.

Interior Gateway Protocol (IGP) A routing protocol used for routing within an autonomous system (AS).

Interior Gateway Routing Protocol (IGRP) Legacy Cisco proprietary routing protocol that was replaced by EIGRP.

Intermediate System-to-Intermediate System (IS-IS) A dynamic link-state routing protocol that is based on a routing method known as DECnet Phase V routing. Routers are known as intermediate systems and exchange data routing messages using a single metric to determine the network topology. IS-IS was developed by the Organization for Standardization (ISO) as part of its Open Systems Interconnection (OSI) model.

internal router Internal OSPF router that has all of its interfaces in the same area. All internal routers in an area have identical link-state databases (LSDBs).

IP address The unique number ID assigned to one host or interface in a network.

level 1 parent route A level 1 route in the routing table that has subnets “catalogued” under it. A level 1 parent route does not contain any next-hop IP address or exit interface information.

level 1 route A route with a subnet mask equal to or less than the classful mask of the network address.

level 2 child route A route that is a subnet of a classful network address.

link-local address An IP address that is used for communications only within a network link. Packets sourced from or destined to a link-local address are not forwarded to other networks by routers. Link-local addresses play a significant role in IPv6.

link state The status of a link, including the interface IP address/subnet mask, type of network, cost of the link, and any neighbor routers on that link.

link-state acknowledgment (LSAck) packet Acknowledges receipt of LSA (link-state advertisement) packets. Link-state acknowledgment packets are Type 5 OSPF packets.

link-state advertisement (LSA) Broadcast packet used by link-state protocols that contains information about neighbors and path costs. LSAs are used by the receiving routers to maintain their routing tables.

link-state database A table used in OSPF that is a representation of the topology of the autonomous system. It is the method by which routers “see” the state of the links in the autonomous system.

link-state packet (LSP) See link-state advertisement.

link-state request (LSR) packet Used to request the pieces of the neighbor’s database that are more up to date. Link-state request packets are Type 3 OSPF packets.

link-state router A router that uses a link-state routing protocol.

link-state routing protocol A routing protocol in which routers exchange information with one another about the reachability of other networks and the cost or metric to reach the other networks. Link-state routers use Dijkstra’s algorithm to calculate shortest paths to a destination, and normally update other routers with which they are connected only when their own routing tables change.

link-state update (LSU) packet Carries a collection of link-state advertisements one hop farther from its origin. Link-state update packets are Type 4 OSPF packets.

load balancing The capability of a router to distribute traffic over all the router network ports that are the same distance from the destination address.

Loading state OSPF state in which the actual exchange of link-state information occurs. Based on the information provided by the DBDs, routers send link-state request packets. The neighbor then provides the requested link-state information in link-state update packets. During the adjacency, if a router receives an outdated or missing LSA, it requests that LSA by sending a link-state request packet. All link-state update packets are acknowledged.

logical topology The path over which the data is transferred in a network.

loopback address The IP address assigned to a loopback interface. It could also be the local host loopback interface address on a host computer. In IPv4 this is the 127.0.0.1 address, while in IPv6 this is the ::1 address.

loopback interface A software-only interface that emulates a physical interface. A loopback interface is always up and never goes down.

Message Digest 5 (MD5) A widely used cryptographic hash function producing a 128-bit (16-byte) hash value, typically expressed as a 32-digit hexadecimal number. MD5 has been utilized in a wide variety of security applications, including to check data integrity.

Message Digest 5 (MD5) authentication The most secure and recommended method of routing protocol authentication. MD5 authentication provides higher security because the password is never exchanged between peers. Instead, it is calculated using the MD5 algorithm. Matching results authenticate the sender.

metric The quantitative value used to measure the distance to a given network.

multiaccess network Network that allows multiple devices to connect and communicate simultaneously.

multiarea OSPF In an OSPF network design, one large autonomous system (AS) is divided into smaller areas, to support hierarchical routing. All areas must connect to the backbone area (area 0).

neighbor table OSPF table that lists all OSPF neighbor routers to which a router has established bidirectional communication. This table is unique for each router and can be viewed using the show ip ospf neighbor command.

nonbroadcast multiaccess (NBMA) network Multiaccess network that either does not support broadcasting or in which broadcasting is not feasible. Examples of NBMA networks include Frame Relay and ATM.

Non-Volatile Random Access Memory (NVRAM) Provides permanent storage for the startup configuration file (startup-config). NVRAM is non-volatile and does not lose its contents when power is turned off.

Null0 summary route Another mechanism to prevent routing loops. EIGRP always creates a route to the Null0 interface when it summarizes a group of network routes because there is a risk of looping packets if a default route is also configured.

OSPF area A logical set of network segments (CLNS-, DECnet-, or OSPF-based) and their attached devices. Areas are usually connected to other areas through routers, making up a single autonomous system.

OSPF authentication When configured on a router, the router authenticates the source of each routing update packet that it receives. This is accomplished by the exchange of an authenticating key (sometimes referred to as a password) that is known to both the sending and the receiving router. To exchange routing update information in a secure manner, enable OSPF authentication. OSPF authentication can either be none (or null), simple, or Message Digest 5 (MD5).

OSPF Hello and Dead intervals The Hello interval is the frequency at which a router sends Hello packets, and the Dead interval is the length of time that a router waits to hear from a neighbor before declaring the neighboring router out of service. These parameters are configurable on a per-interface basis. Unlike the EIGRP intervals of the same name, the OSPF intervals must match or a neighbor adjacency does not occur.

OSPF states Different states through which OSPF routers transition when adjacencies are being established. Specifically, OSPF routers transition through the Down state, Init state, Two-Way state, ExStart state, Exchange state, Loading state, and finally Full state.

OSPFv3 Version 3 of the OSPF routing protocol, used to support both IPv4 and IPv6 unicast address families.

passive interface An interface that does not take part in the advertisement of routing information. The passive-interface command enables the suppression of routing updates over some interfaces while allowing updates to be exchanged normally over other interfaces. A neighbor adjacency cannot be formed over a passive interface. This is because link-state packets cannot be sent or acknowledged.

passive state In EIGRP, the stable state to which a router transitions when it has identified the successor(s) for a certain destination.

periodic update An update that is transmitted between routers at the end of a certain time period. The periodic update for RIP is 30 seconds.

physical topology The arrangement of the cables, network devices, and end systems. It describes how the network devices are actually interconnected with wires and cables.

prefix length Another name for a network prefix.

process switching An older packet-forwarding mechanism still available for Cisco routers. In process switching the first packet is copied to the system buffer. The router looks up the Layer 3 network address in the routing table and initializes the fast-switching cache. The frame is rewritten with the destination address and sent to the outgoing interface that services that destination. Subsequent packets for that destination are sent by the same switching path.

Product Activation Key (PAK) Serves as a receipt of purchase for an IOS image and is used to obtain a Software Activation License.

protocol-dependent modules (PDMs) A feature of EIGRP in which individual modules are responsible for the tasks related to a specific routing protocol. EIGRP has PDMs for IP, IPX, AppleTalk, and IPv6.

quad-zero route An IPv4 default static route. Referred to as quad-zero route because of its syntax of 0.0.0.0 for the network address and 0.0.0.0 for the subnet mask.

Read-Only Memory (ROM) Provides permanent storage for bootup instructions, basic diagnostic software, and a limited IOS in case the router cannot load the full featured IOS. ROM is firmware and is referred to as non-volatile because it does not lose its contents when power is turned off.

recursive lookup Occurs when a router has to perform multiple lookups in a routing table before forwarding a packet.

reference bandwidth The number, measured in Mb/s, that is used by routing protocols to calculate a route’s metric. In OSPF, the default reference bandwidth is 100 Mb/s. Changing the reference bandwidth does not actually affect the bandwidth capacity on the link; rather, it simply affects the calculation used to determine the metric.

regular (non-backbone) area An area that connects users and resources. Regular areas are interconnected through the backbone area. Regular areas are usually set up along functional or geographical groupings. By default, a regular area does not allow traffic from another area to use its links to reach other areas. All traffic from other areas must cross a transit area.

reliability Indicates the dependability of the components that make up the network, such as the routers, switches, PCs, and servers.

Reliable Transport Protocol (RTP) Responsible for guaranteed, ordered delivery of EIGRP packets to neighbors.

remote network Network that can only be reached by forwarding packets to another router.

reported distance In EIGRP, the total metric along a path to a destination network as advertised by an upstream neighbor in EIGRP.

RIPng The IPv6 version of the RIPv2 dynamic routing protocol.

route summarization The process of taking multiple contiguous routes and representing them with a single route statement.

Router Advertisement (RA) message A message type used by an IPv6 router to provide IPv6 addressing information to clients. The router sends the message using the IPv6 all-nodes multicast address of FF02::1.

Router ID A field in an OSPF Hello packet that is a 32-bit value expressed in dotted-decimal notation (an IPv4 address) used to uniquely identify the originating router.

Router Priority Used in a DR/BDR election. The default priority for all OSPF routers is 1, but can be manually altered from 0 to 255. The higher the value, the more likely the router becomes the DR on the link.

Routing Information Protocol (RIP) One of the oldest distance-vector routing protocols, employs the hop count as a routing metric. RIP is a legacy dynamic routing protocol.

routing protocol messages Various types of messages used by routing protocols to discover neighboring routers, exchange routing information, and perform other tasks to learn and maintain accurate information about the network.

routing table A data file in RAM that is used to store route information about directly connected and remote networks.

shortest path first (SPF) Algorithm that uses accumulated costs along each path, from source to destination, to determine the total cost of a route. Commonly used in link-state routing algorithms. Sometimes referred to as Dijkstra’s algorithm because it was developed by Edsger Dijkstra.

single-area OSPF In an OSPF network design, all routers are in one area called the backbone area (area 0).

SPF tree Topology tree to which a link-state routing algorithm adds a destination network after determining the shortest path to that network using the shortest path first algorithm (Dijkstra’s algorithm).

split horizon A technique for preventing reverse routes between two routers. Information about the routing for a particular packet is never sent back in the direction from which it was received.

standard ACL ACL that filters traffic by using only the source IPv4 addresses.

standard maintenance release (T release) Beginning with Cisco IOS 15.0, enables new IOS features to be delivered before the next extended maintenance release (EM release) becomes available.

static route A remote network in a routing table that has been manually entered into the table by a network administrator.

statically assigned IP address The host is manually assigned the correct IP address, subnet mask, and default gateway.

stub network A network with only one exit point.

stub router A router that has only one exit interface from the routing domain and forwards all traffic to a central or distribution router.

subnet mask A dotted-decimal number that helps identify the structure of IP addresses. The mask represents the network and subnet parts of related IP addresses with binary 1s and the host part of related IP addresses with binary 0s.

successor A neighboring router that is used for packet forwarding and is the least-cost route to the destination network. In EIGRP, a successor is chosen using DUAL from all of the known paths or feasible successors to the end destination.

summary route Route summarization reduces the number of routes that a router must maintain. It is a method of representing a series of network numbers in a single summary address; for example, when multiple static routes that can be summarized with a common prefix length are replaced by one static route.

supernet route A route that has a subnet mask less than the classful mask. A summary address is an example of a supernet route.

switched virtual interface (SVI) Provides basic Layer 3 functions for a switch, which does not have a dedicated physical interface for IP addressing.

Terminal emulation software A software program required to establish a serial connection to the console port of a router.

TLV In EIGRP, the data portion of the EIGRP packet. All TLVs (Type, Length, Value) begin with a 16-bit Type field and a 16-bit Length field. Different TLV values exist according to the router protocol.

topology diagram Provides a visual reference that indicates the physical connectivity and logical Layer 3 addressing.

topology table Contains information regarding EIGRP routes received in updates and routes that are locally originated. EIGRP sends and receives routing updates from adjacent routers to which peering relationships (adjacencies) have been formed. The objects in this table are populated on a per-topology table entry (route) basis.

Two-Way state Designates that bidirectional communication has been established between two routers in OSPF. The term is also referred to as 2-Way state and occurs after the Init state. Bidirectional means that each router has seen the other’s Hello packet.

type 1 router LSA Contains a list of the directly connected interfaces, link types, and link states. All OSPF routers advertise their directly connected OSPF-enabled links in a type 1 LSA and forward their network information to OSPF neighbors.

type 2 network LSA Contains the router ID and IP address of the DR, along with the router ID of all other routers on the multiaccess segment. Type 2 LSAs only exist for multiaccess and nonbroadcast multiaccess (NBMA) networks where there is a DR elected and at least two routers on the multiaccess segment. The type 2 LSA contains the router ID and IP address of the DR, along with the router ID of all other routers on the multiaccess segment. A type 2 LSA is created for every multiaccess network in the area.

type 3 summary LSA Used by an ABR to advertise networks from other areas. ABRs collect type 1 LSAs in the LSDB. After an OSPF area has converged, the ABR creates a type 3 LSA for each of its learned OSPF networks. Therefore, an ABR with many OSPF routes must create type 3 LSAs for each network.

type 4 summary LSA A type 4 summary LSA is generated by an ABR only when an ASBR exists within an area. Used by an ABR to advertise an ASBR to other areas and provide a route to the ASBR. All traffic destined to an external autonomous system requires routing table knowledge of the ASBR that originated the external routes. A type 4 LSA is generated by the originating ABR and regenerated by other ABRs.

type 5 AS external LSA Describes routes to networks outside the OSPF autonomous system. Type 5 LSAs are originated by the ASBR and are flooded to the entire autonomous system and regenerated by other ABRs. A type 5 LSA link-state ID is the external network address.

unequal cost load balancing The capability of a router to distribute traffic over all the router network ports, even those that are different distances from the destination address. EIGRP supports unequal cost load balancing by using the variance command.

Unique Device Identifier (UDI) Combination of the Product ID, Serial Number, and hardware version.

variable-length subnet masking (VLSM) Allows for the use of different subnet masks for individual subnets, which allows a network space to be divided into unequal parts. With VLSM, the network is first subnetted, and then the subnets are subnetted again. This process can be repeated multiple times to create subnets of various sizes. Creates a more efficient use of address space.

vector Identifies the direction of the next-hop router or exit interface needed to reach a destination network.

wildcard mask A string of 32 binary digits used by the router to determine which bits of the address to examine for a match.