Basic Definitions
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| ABR | Area Border Router. A Router using OSPF in which the router has interfaces in multiple areas.
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| access link | In Frame Relay, the physical serial link that connects a Frame Relay DTE (data terminal equipment), usually a router, to a Frame Relay switch. The access link uses the same physical layer standards as do point-to-point leased lines.
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| access rate (AR) | In Frame Relay, the speed at which bits are sent over an access link.
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| administrative distance | In Cisco routers, a means for one router to choose between multiple routers to reach the same subnet when those routes are learned by different routing protocols. The lower the administrative distance, the more preferred the source
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| administrative mode (trunking administrative mode) | The configured trunking setting on a Cisco switch interface as configured with switchport mode command
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| alternate port | In RSTP 802.1w, a port role used to denote an interface that is currently recieving an inferior Hellow BPDU - making it a possible replacement for root port
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| ARP | Address Resolution Protocol. Used to map an IP address to a MAC address. Defined in RFC826
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| ASBR | Autonomous System Border Router. A router using OSPF which learns routes via another source, typically another routing protocol exchanging routes that are external to OSPF with OSPF domain
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| asynchronous | Describes a convention for sending data with digital signals. Sender and receiver operate at same speeds, but no attempt is made to dynamically cause one to adjust speeds based on the other.
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| autosummarization | Feature in which a router that connects to more than one classful network advertises summarized routes for each entire classful network when sending updates out interfaces connected to other classful networks.
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| backup designated router | An OSPF router connected to a multiaccess network that monitors the work of the designated router (DR) and takes over the work of the DR if the DR fails.
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| backup port | In RSTP 802.1w, a port role used when multiple interfaces on one switch connect to a single collision domain. This makes one interface the designated port (DP) and one or more become backup
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| balanced hybrid | Refers to one of three general types of routing protocol algorithms. The two are distance vector and link-state. EIGRP is the only routing protocol that CISCO classifies as using a balanced hybrid algorithm
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| Bc | Committed Burst. A Frame Relay term referring to the number of bits that can be sent during a defined time interval. This helps measure if/when the DTE has, on average, sent more data over a VC than the speed defined in the traffic contract.
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| BECN | Backward explicit congestion notification. The bit in the Frame Relay header that implies that congestion is occurring in the opposite (backward) direction from the frame. Switches and DTEs can react by slowing the rate bits are sent in that direction.
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| Blocking State | In 802.1d STP, a port state in which no received frames are processed, and the switch forwards no frames out the interface, with the exception of STP messages.
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| Boolean AND | A math operation performed on a pair of one-digit binary numbers. The result is another one-digit binary number. 1 AND 1 yields 1; all other combinations yield 0
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| BPDU | Bridge Protocol Data Unit. The generic name for Spanning Tree Protocol messages
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| BPDU Guard | A CISCO switch feature that listens for incoming STP BPDU messages, disabling the interface if any are received. The goal is to prevent loops when a switch connects to a port expected to only have a host connected to it.
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| BRI | Basic Rate Interface. An ISDN interface composed of two bearer channels and one data (D) channel for circuit-switched communication of voice, video, and data
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| bridge ID (BID) | An 8-byte identifier for bridges and switches used by STP and RSTP. It is composed of a 2-byte priority field followed by a 6-byte System ID field that is usually filled with a MAC address.
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| broadcast domain | A set of all devices that receive broadcast frames originating from any device in the set. Devices in the same VLAN are in the same broadcast domain.
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| broadcast subnet | When subnetting a Class A, B, or C network, the one subnet in each classful network for which all subnet bits have a value of binary 1. The subnet broadcast address in this subnet has the same numeric value as the classful network's broadcast address.
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| CHAP | Challenge Handshake Authentication Protocol. Security Feature defined by PPP that allows either or both endpoints on link to authenticate the other device as a particular authorized device.
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| CIDR | An RFC-standard tool for global IP address range assignment. It reduces the size of Internet routers' IP routing tables.
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| CIR - classless interdomain routing | Committed Information Rate. In Frame Relay and ATM, average speed at which bits can be transferred over a virtual circuit according to the business contract between the customer and the service provider.
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| circuit switching | Dedicated physical circuit path that must exist between the sender and the receiver for the duration of the "call". Used heavily in telephone company network.
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| classful addressing | concept in IPv4 addressing that defines a subnetted IP address in three parts: network, subnet, and host.
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| classful network | An IPv4 Class A, B, or C network. Called Classful because these networks are defined by class rules for IPv4 addressing.
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| classful routing | A variation of the IPv4 forwarding (routing) process that defines the particulars of how the default route is used. the default route is used only if the classful network in wich the packet's destination address resides is missing from the router's table
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| classful routing protocol | Routing protocol does not send subnet masks in its routing tables. Unble to support VLSM and manual route summarization.
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| classless addressing | A concept in IPv4 addressing that defines a subnetted IP address as a having two parts: prefix (subnet) and host
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| classless routing | A variation of the IPv4 forwarding process that defines the particulars of how a default route is used. The default route is always used for packets whose destination IP address does not match any other routes
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| classless routing protocol | An inherent characteristic of a routing protocol. Must send subnet masks in its routing updates. Allows support for VLSM and manual route summarization
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| contiguous network | In IPv4 a internetwork design in which packets forwarded between any two subnets of a single classful network only pass through the subnets of that classful network
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| convergence | The time required for routing protocols to react to changes in the network, removing bad routes and adding new better routes so that the current ets routes are in all the routers' tables
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| counting to infinity | An unfortunate side-effect of distance vector routing protocols in which the routers slowly increase the metric for a failed route until the metric reaches that protocol's finite definition of maximimum metric (called infinity)
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| CSU/DSU | Channel service unit/data service unit. Device connects to a physical circuit installed by telco to some CPE device, adapting between the voltages, current, framing and connectors used on the circuit to the physical interface supported by DTE
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| database description | An OsPF packet type that lists brief descriptions of the LSAs in the OSPF LSDB
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| DLCI | data-link connection identifier. Frame Relay address that identifies a VC on a particular access link.
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| DCE | Data communications equipment. Physical Layer: the device providing the clocking on a WAN link, typically a CSU/DSU, is the DCE. Packet Switching: service provider's switch
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| DE | discard eligible. the bit in the Frame Relay header that, if frames must be discarded, signals a switch to choose this frame to discard instead of another frame without the DE bit set
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| dead timer | In OSPF, a timer used for each neighbor. A router considers the neighbor to have failed if no Hellos are received from that neighbor in the time defined by the timer
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| deny | An action taken with an ACL that implies that the packet is discarded
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| designated port | In STP and RSTP, a port role used to determine which of multiple interfaces, each connected to the same segment or collision domain, should forward frames to the segment. The switch advertising the lowest cost Hello BPDU is the DP
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| designated router | OsPF, on multiaccess network, the router that wins an election and is therefore responsible for managing a streamlined process for exchanging OSPF topology information between all routers attached to that network
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| Diffie-Hellman Key Exchange | A protocol in which two devices can exchange information over a public network.
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| Dijkstra Shortest Path First Algorithm | SPF - the name of the algorithm used by link-state routing protocols to analyze the LSDB and find the least-cost routes from hat router to each subnet
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| directed broadcast address | same as subnet broadcast address
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| disabled port | In STP, a port role for nonworking interfaces
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| discarding state | RSTP: interface state in which no received frames are processed, and the switch forwards no frames out the interface, with the exception of RSTP messages.
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| discontiguous network | IPv4: a internetwork design in which packets being forwarded between two subnets of a single classful network must pass throught he subnets of another classful network
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| distance vector | The logic behind RIP and IGRP (and others). Each router sends its entire routing table in each update, but only to neighbors.
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| DTE | Data terminal equipment. Layer 1: DTE synchronizes clock based on the clock sent by the DCE. Packet-switch: DTE is the device outside of the service provider's network, typically a router
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| DUAL | Diffusing Update Algorithm. Convergence used in EIGRP when a route fails and a router does not have a feasible successor route. DUAL causes routers to send EIGRP Query and Reply messages to discover alternate loop-free routes
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| dual stacks | IPv6: a mode of operation in which a host or router runs both IPv4 and IPv6
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| dynamic ACL | ACL beyond traditional IP ACLs . Dynamically permits traffic from host if the host's user first connects to the router via Telnet and passes authentication process
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| EIGRP | Enhanced Interior Gateway Routing Protocol. An advanced version of IGRP developed by Cisco. Provides superior convergence properties and operating efficiency and combines the advantages of link-state protocols with advantages of distance vector protocol
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| encoding | Conventions for how a device varies the electrical or optical signals sent over a cable to imply a particular binary code.
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| EtherChannel | A CISCO-proprietary feature in which up to eight parallel Ethernet segments between the same two devices, each using the same speed, can be combined to act as a single link for forwarding and STP logic
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| extended access list | A list of IOS access-list global configuration commands that can match multiple parts of an IP packet, including the source and dest IP addresses and TCP/IP ports, for deciding which packets to discard and which to allow
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| extended ping | An IOS command in which the ping command accepts many other options besides just the destination IP address
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| EGP - exterior gateway protocol | A routing protocol that was designated to exchange routing information between different autonomous systems
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| Feasibility condition | EIGRP: when a router has learned of multiple routes to reach one subnet, if the best route's metric is X, the feasibility condition is another route whose reported distance is <_ X.
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| feasible distance | EIGRP: the metric of the best route to reach a subnet
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| feasible successor | EIGRP: route that is not the best route (successor route) but that can be used immediately if the best route fails,m without causing a loop. Such a route meets the feasibility condition
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| FECN | Forward Explicit Congestion Notification. The bit in the Frame Relay header that signals to anything receiving the frame (switches and DTEs) that congestion is occurring in the same direction as the frame
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| FTP | File Transfer Protocol. An application protocol, part of TCP/IP protocol stack, used to transfer files between network nodes. Defined in RFC959
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| filter | process or device that screens network traffic for certain characteristics
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| forward | To send a frame toward its ultimate destination by way of internetworking
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| forward delay | An STP timer, defaulting to 15 seconds, used to dictate how long an interface stays in both the Listening and Learning State.
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| forwarding state | An STP and RSTP port state in which an interface operates unrestricted by STP
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| forward route | From one host's perspective, the route over which a packet travels from that host to some other host
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| Frame Relay | A data-link protocol that defines the capabilities to create a frame-switched (packet-switched) service allowing DTE devices (typically routers) to send data to many other devices using a single physical connection to the Frame Relay service
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| Frame Relay DTE | The customer device connected to a Frame Relay access link, typically a router
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| Frame Relay mapping | Information that correlates, or maps a Frame Relay DLCI to the Layer 3 address of the DTE on the other end of the VC identified by the local DLCI
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| framing | Conventions for how Layer2 interprets the bits sent according to OSI Layer 1.
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| full duplex | Any communication in which two communicating devices can concurrently send and receive data
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| Full State | In OSPF, a neighbor state that implies that the two routers have exchanged the complete (full) contents of their respective LSDBs
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| full update | IP routing protocols: general concept that a routing protocol update lists all known routes
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| full adjacent | OSPF: a characterization of the state of a neighbor in which the two neighbors have reached the Full state
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| global unicast address | type of unicast IPv6 address that has been allocated from a range of public globally unique IP addresses as registered through ICANN, its member agencies, and other registries or ISPs
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| HDLC | High Level Data-Link Control. Bit-oriented synchronous data link layer protocol developed by the International Organization for Standardization
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| Hello | 1) OSPF and EIGRP protocol for routers to discover, establish, and maintain neighbor relationship. 2) STP: name of the periodic message sourced by the root bridge
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| Hello BPDU | STP/RSTP: message used for majority of STP communications, listing the root's Bridge ID, sending device's Bridge ID and sending device's cost with which to reach the root
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| Hello interval | OSPF and EIGRP: interface timer that says how often the router should send Hello messages
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| Hello timer | STP: time interval at which the root switch should send Hello BPDU
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| holddown | Distance Vector protocol state assigned to a route placed so that routers neither advertise the route nor accept advertisements about it for a specific length of time. Used to flush bad info about a route from all routers in netword.
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| IEEE 802.11 | base standard for windows LAN
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| IEEE 802.1ad | The IEEE standard for the fundamental equivalent of the Cisco-proprietary
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| IEEE 802.1d | The IEEE standard for the original Spanning Tree Protocol
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| IEEE 802.1q | The IEEE-standard VLAN trunking protocol, 802.1Q includes the concept of a native VLAN, for which no VLAN header is added, and a 4-byte VLAN header is inserted after the original frame's type/length field
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| IEEE 802.1s | The IEEE standard for Multiple Instances of Spanning Tree (MIST), must allows for load balancing of traffic among different VLANs
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| IEEE 802.1w | The IEEE standard for an enhanced version of STP, called Rapid STP which speeds convergence
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| IEEE 802.3 | The IEEE base standard for Ethernet-like LANs
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| IGRP | Interior Gateway Routing Protocol. Old, no-longer-supported Interior Gateway Protocol (IGP) developed by CISCO
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| inferior Hello | When comparing two or more received Hello BPDUs, a Hello that lists numerically larger root Bridge ID than another Hello, or a Hello that lists the same root Bridge ID but with a larger cost
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| infinity | In the context of IP routing protocols, a finite metric value defined by the routing protocol that is used to represent an unusable route in a routing protocol update
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| inside global | A NAT term referring to the IP address used for a host inside the trusted part of the network, but in packets as they traverse the the global (untrusted) part of the network.
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| inside local | A NAT term referring to the IP address used for a host inside the trusted part of the network, but in packets as they travers the local (trusted) part of the network
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| interior gateway protocol (IGP) | A routing protocol designed to be used to exchange routing information inside a single autonomous system
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| Inter-Switch Link (ISL) | The CISCO-proprietary VLAN trunking protocol that predated 802.1Q by many years. ISL defines a 26-byte header that encapsulates the original Ethernet frame
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| Inverse ARP | A Frame Relay protocol with which a router announces its Layer 3 address over a VC, thereby informing the neighbor of useful Layer-3-to-Layer-2 mapping information
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| IP Control Protocol (IPCP) | A control protocol defined as part of PPP for the purpose of initializing and controlling the sending of IPv4 packets over a PPP link.
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| IPsec | The term referring to the IP Security Protocols, which is an architecture for providing encryption and authentication services, typically when creating VPN services through an IP network.
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| ISDN | Integrated Services Digital Network. A communication protocol offered by telephone companies that permits telephone networks to carry data, voice, and video.
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| ISP prefix | In IPv6, the prefix that describes an address block that has been assigned to an ISP by some Internet registry.
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| keepalive | A feature of many data-link protocols in which the router sends messages periodically to let the neighboring router know that the first router is still alive and well
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| LAPF | Link Access Procedure Frame Bearer Services. Defines the basic Frame Relay header and trailer. The header includes DLCI, FECN, BECN, and DE bits.
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| learn | Transparent bridges and switches learn MAC addresses by examining the source MAC addresses of frames they receive. The add each new MAC address along with the port number of the port on which it learned of the MAC address, to an address table.
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| Learning State | In STP, a temporary port state in which the interface does not forward frames, but it can begin to learn MAC address from frames received on the interface
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| leased line | A transmission line reserved by a communications carrier for a customer's private use. A dedicated line.
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| Link Control Protocol | A control protocol defined as part of PPP for the purpose of initializing and maintaining a PPP link.
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| link local address | A type of unicast IPv6 address that represents an interface on a single data link.
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| link-state | A classifiacation of the underlying algorithm used in some routing protocols. Link-state protocols build a detailed database that lists links (subnets) and their state (up,down), from which the best routes can then be calculated
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| link-state advertisement (LSA) | In OSPF, the name of the data structure that resides inside the LSDB and describes in detail in various components in a network, including routers and links (subnets).
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| link-state database (LSDB) | In OSPF, the data structure in RAM of a router that holds the various LSAs, with the collective LSAs representing the the entire topology of the network.
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| link-state request | An OSPF packet used to ask a neighboring router to send a particular LSA
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| link-state update | An OSPF packet used to send an LSA to a neighboring router
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| Listening State | A temporary STP port state that occurs immediately when a blocking interface must be moved to a forwarding state. The switch times out MAC table entries during this state. It also ignores frames received on the interface and doesn't fwd frames out int
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| Local Management Interface (LMI) | A Frame Relay protocol used between a DTE (router) and DCE (Frame Relay Switch). LMI acts as a keepalive mechanism.
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