001 ccnp glossary

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CCNP—Glossary

6 to 4 tunnel This is a tunnel used for IPv4 to IPv6 transition. It is a multipoint tunnel. A 6 to 4 tunnel uses static IPv6 routes since it does not support IPv6 routing protocols.

802.11 IEEE 802.11 defines the mechanical process of WLAN implementation for vendors to create compatible products. WLAN standards include 802.11a, 802.11b, 802.11g, and 802.11n.

802.1Q IEEE 802.1Q is a standard for trunking, which supports tagged and untagged frames. Untagged frames are used for the native VLAN.

802.2 The data link layer is split into two components. The IEEE 802.2. LLC is performed in software and identifies the encapsulated upper layer protocol. The 802.2 MAC component is performed in hardware.

802.3 IEEE 802.3 defines the physical and data link layer implementation of wired Ethernet. 802.3 is a technology that supports IEEE 802.1 network architecture.

802.3af IEEE 802.3af is a standard method of providing Power over Ethernet (PoE).

Access Control List (ACL)

An ACL is a list of commands to filter packets. ACLs are used to restrict telnet (VTY) access to a router, filter routing information, apply Layer 3 path control, and more.

Access Layer The access layer is the bottom layer of Cisco’s three-layer hierarchical model. It provides an initial connection to the network, normally via a switch.

Access Link In Frame Relay, an access link is the connection between the Frame Relay switch and the Frame relay customer router.

Access Point In wireless, an access point provides network connectivity for end devices such as PCs.

Address Resolution Protocol (ARP)

ARP uses a local broadcast to find the layer 3 IP address that corresponds to a layer 2 MAC address.

Administrative Distance

Administrative distance, a Cisco-proprietary mechanism, ranks the IP routing protocols. When there are two or more different route sources to reach the same destination, the source with the lowest administrative distance is selected.

Alternate Port State In Spanning Tree, ports that have an alternate path or paths and not currently in use are known as alternate port states. An alternate port can take over when a designated port fails. This speeds up STP convergence.

Anycast Address In IPv6, multiple nodes can be configured with the same anycast address. Traffic is forwarded/routed to the closest interface.

Application-Specific Integrated Circuit (ASIC)

ASICs are processors optimized to perform one or several tasks. ASICs are used in Cisco switches.

Adaptive Security Device Manager (ASDM)

ASDM is a GUI-based Cisco configuration tool for the Adaptive Security Appliance (ASA).


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Adjacency Table (AT) In CEF, the AT is the MAC table stored in TCAM. It is created from the ARP table.

Advanced Data Encryption (AES)

AES is the strongest encryption available on Cisco IOS, depending on the platform and IOS version.

Area Border Router (ABR)

In OSPF, this is a router that has interfaces connected to at least two different OSPF areas. It maintains separate LSDBs for each area.

Asynchronous transfer mode (ATM)

ATM was one of the first protocols developed to send both voice and data. Its datagrams are fixed at 53 bytes and are called cells.

Authentication Header (AH)

Protocol 51; in IPsec, AH is a protocol that provides authentication and data integrity.

Authentication, Authorization, and Accounting (AAA)

AAA is a feature that provides authentication to access a device, authorization to use resources, and/or accounting for track resource usage.

Automatic Private IP Addressing (APIPA)

If a host is configured to obtain an address from a DHCP server, but does not obtain one, it is assigned an APIPA address in the range 169.254.0.0/16.

Autonomous System (AS)

An AS is a group of networks, such as company’s network, division within a company, or a group of companies’ network, under a single administrative control.

Autonomous System Boundary Router (ASBR)

In OSPF, an ASBR is a router that connects the OSPF routing domain to another autonomous system.

Autonomous System Number (ASN)

In EIGRP, it is an arbitrary number used to differentiate instances of EIGRP. In BGP, it is a number assigned to a BGP process, either privately or by ICANN/IANA.

Backup Designated Router (BDR)

In OSPF, this is the backup for the Designated Router (DR).

Backup Port State In STP, a backup port is an alternate port in a Layer 1 environment, such as a hub. This is rare in today’s networks.

Blocking State In STP, blocking is handled differently depending on the STP version. A blocking port receives/processes BPDUs, but discards data frames and does not learn MAC addresses.

Border Gateway Protocol (BGP)

BGP is the routing protocol that ties the Internet together. It has two components: eBGP and iBGP.

Bootstrap program The bootstrap program brings the switch and router up. It proceeds to find and load the IOS image, file system, and configuration files.

Bridge ID In STP, the bridge ID is used to select the root switch. It consists of the bridge priority, Extended System ID if the STP is per-VLAN, and the bridge’s MAC address.

Bridge Protocol Data Unit (BPDU)

In STP, BPDUs are used to detect loops in the network topology. They are sent out as multicasts every two seconds by default.

Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA)

In wireless, CSMA/CA is used to transmit information. To avoid collisions, WLAN devices will use Ready-to-Send (RTS) and Clear-to-Send (CTS) signals.

Carrier Sense Multiple CSMA/CD is used in slower Ethernet networks. In half duplex, only one NIC at


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Access/Collision Detection (CSMA/CD)

a time can send a frame successfully. If more than one NIC sends a frame simultaneously, there will be a collision. Therefore, each device waits for a random amount of time and examines if the status is available before transmission.

Challenge Handshake Authentication Protocol (CHAP)

CHAP uses the concept of challenge and response, where the peer challenges the caller to authenticate its identity. The challenge includes a unique ID and a random number for authentication.

Channel Service Unit/Data Service Unit (CSU/DSU)

This is a device used to convert the data from the local router into a form acceptable to the WAN service provider, and vice versa.

Cisco Discovery Protocol (CDP)

CDP is a Cisco proprietary protocol to discover information about neighboring devices.

Cisco Express Forwarding (CEF)

This is Cisco’s most advanced route forwarding method. It builds tables to speed lookups.

Cisco IP Service-Level Agreement (IP SLA)

A Cisco IP SLA is an operation that provides various performance measurements between two Cisco devices or between a Cisco device and a non-Cisco device.

Cisco Inline Power (ILP)

ILP provides Power over Ethernet (PoE). It uses a test tone to detect attached devices.

Cisco Unified WLAN Network (CUWN)

CUWN uses split-MAC architecture where a Wireless LAN Controller (WLC) and lightweight APs take over much of the work of the regular autonomous APs.

Class of Service (CoS) CoS is a Layer 2 QoS mechanism. It can only be implemented in 802.1q trunks. The tag contains three priority bits called the CoS or the 802.1p user priority field.

Committed Information Rate (CIR)

In Frame Relay, this is the guaranteed amount of bandwidth. It is negotiated with the service provider for each PVC.

Common Spanning Tree (CST)

CST is IEEE 802.1D, the original spanning tree protocol.

Configuration Register The configuration register holds a series of four-digit hexadecimal values. These values determine the behavior of the device. For example, certain values determine the loading of the IOS image and configuration file.

Content-Addressable Memory (CAM) table

A switch CAM table holds the MAC addresses and VLANs of ports. It is used for Layer 2 forwarding.

Core Layer The core layer is the backbone of the network that provides a high-speed connection between the different distribution layer devices.

Database description (DBD)

DBD is a type of OSPF message that provides a summary of known routes. It is exchanged before individual LSAs are exchanged.

Data Communication Equipment (DCE)

DCE ends a physical connection. It provides synchronization and clocking of a connection between two sites.

Data Encryption Standard (DES)

DES is a standard for encryption that has been superseded by AES.

Data Link Connection Identifier (DLCI)

A DLCI is the Layer 2 address of a Frame Relay PVC. It is used to identify each Frame Relay VC uniquely on a physical interface.


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Data Termination Equipment (DTE)

DTE converts user information into signals or reconverts received signals. In serial interfaces, one side is DCE and the other is DTE.

Default Gateway The default gateway is a routed interface that serves as the first hop. For example, a PC is configured with an IP address, subnet mask, and default gateway in order to reach devices in other networks.

Default Route The default route is the route selected if a more specific route is not available in the IP routing table.

Denial of Service (DoS) attack

DoS attacks involve a flooding of packets designed to overwhelm the processing capabilities of the target network. This results in a denial of access to the network by authorized users.

Designated Port In STP, a designated port is a nonroot port that is in the forwarding state.

Designated Router (DR) In OSPF, this router handles the routing traffic centrally to minimize the exchange of routing updates in some network types.

DiffServ Code Point (DSCP)

The DSCP constitutes the six most significant bits of the Type of Service (ToS) byte in the IP header.

Differentiated Services Model (DiffServ)

DiffServ is the standard QoS model used for Layer 3. It is configured per-hop.

Diffie-Hellman Diffie-Hellman provides a secure way to exchange keys for encryption and hash for authentication. It is used in IPsec.

Diffusing Update Algorithm (DUAL)

DUAL is the algorithm to determine the best path in EIGRP.

Digital Subscriber Line (DSL)

DSL sends data traffic over a telephone local loop. The three major Layer 2 protocols used for DSL are bridging, PPPoA, and PPPoE.

Distance Vector Protocols

Distance (metric) and direction (vector) are used to find paths to destinations. An example is RIP.

Distribute List This is a path control tool. It requires an ACL. Distribute lists are used extensively in BGP.

Distribution layer The distribution layer is the second layer in Cisco’s Hierarchical Network Model for switching. It is responsible for high availability, load balancing, QoS, and security.

Domain Name System (DNS)

DNS resolves names which a user enters to IP addresses.

Dual Stacking Dual stacking is configuring two protocol stacks on a device. A Cisco router is dual stacked by default.

Duplex The method of receiving and transmitting frames is known as duplex. In half-duplex configurations, an interface can either receive or send frames, but not both at the same time. In full-duplex configurations, an interface can receive and send frames simultaneously.

Duplicate Address Detection (DAD)

In IPv6, NDP performs DAD to ensure assignment of a unique address.

Dynamic Host Configuration Protocol (DHCP)

A DHCP server provides network configuration parameters dynamically to DCHP clients. Such parameters include IP address and subnet mask, default gateway, DNS server, etc.

Dynamic Host This allows a DHCP server to be on another network, allowing centralization


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Configuration Protocol (DHCP) relay

of the DHCP function.

Dynamic Trunking Protocol (DTP)

DTP is used to dynamically form trunk connections between two Cisco switches.

Encapsulating Security Payload (ESP)

Protocol 50; in IPsec, ESP is a protocol that provides authentication, data integrity, confidentiality, and anti-replay.

Enhanced Interior Gateway Routing Protocol (EIGRP)

EIGRP is a Cisco-proprietary routing protocol. It is referred to as an enhanced distance vector routing protocol.

Errdisable By default, when a Catalyst switch detects certain error conditions, it shuts down the switch port. The port is considered in an errdisable state.

EtherChannel An EtherChannel allows multiple physical Ethernet links to act as a single logical link.

Ethernet Ethernet is a network technology that functions at the physical and data link layer. It can be used for WANs or LANs.

Ethernet over Multiprotocol Label Switching (EoMPLS)

This is a protocol that uses Ethernet at Layer 2 to connect to the ISP.

Extended Service Set (ESS)

An ESS is the service set represented by a type of wireless network.

Extended Unique Identifier (EUI-64)

An EUI-64 is a modified MAC address used to dynamically create IPv6 addresses.

Extensible Authentication Protocol (EAP)

A layer 2 process that allows a client to authenticate to a network. One variety of EAP is for wireless connections and the other is for LAN connections (called EAP over LAN—EAPoL).

Exterior Gateway Protocol (EGP)

An EGP is a routing protocol that routes between autonomous systems. Border Gateway Protocol (BGP) is the major EGP used today. The term EGP also refers to a deprecated routing protocol.

Extranet External users or business partners are often granted access to the extranet portion of a corporate network.

Feasible distance (FD)

In EIGRP, this is the RD added to the metric between this router and the next-hop router to yield the metric for the entire route.

Floating Static Route This is a static route which provides route redundancy.

Forwarding Information Base (FIB)

The FIB is the CEF routing table stored in TCAM. It is created from the routing table.

Frame check sequence (FCS)

An FCS is a cyclic redundancy check for a Layer 2 frame.

Frame Relay Frame Relay (FR) is a connection-oriented Layer 2 WAN protocol. It operates on a non-broadcast multi-access (NBMA) network, like X.25 and ATM.

Generic Routing Encapsulation (GRE)

This is a Cisco tunneling protocol. For IPv4 to/from IPv6, GRE requires a 4-byte GRE header between the IPv6 and v4 headers.

Global Load Balancing Protocol (GLBP)

GLBP is a first hop redundancy protocol providing load balancing over multiple default gateways. It is a Cisco proprietary protocol.

Gratuitous ARP A gratuitous ARP is an ARP reply generated without any corresponding request.


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Hierarchical Addressing For the routing information of a network to be summarized into a smaller set of routes, hierarchical addressing is used.

High-Level Data Link Control (HDLC)

HDLC is a Layer 2 protocol that defines the frame type and interactions between two devices. Cisco implements HDLC based on IOS standards, but has altered its frame format to make it proprietary.

Hashed Message Authentication Code (HMAC)

An HMAC is used to provide data integrity and authentication. Examples are MD5 and SHA-1.

Hold-Down Timer The hold-down mechanism is used where a time is set for the routers to transmit the poisoned route and ensure no routing loops occur during the transmission.

Internet Control Message Protocol (ICMP)

This is a Layer 3 protocol used to send messages. The IPv6 version is called ICMPv6.

Inside global IP address In NAT, this is a public address referencing an inside device. An example is a valid public (Internet routable) address that the inside host is given when it exits the NAT router.

Inside local IP address In NAT, this is a private address referencing an inside device. It is the address of the inside host as seen from inside your LAN.

Institute of Electrical and Electronics Engineers (IEEE)

The IEEE is a standards organization responsible for Ethernet standards among many others involving computers.

Interior Gateway Protocol (IGP)

A routing protocol that manages routing within a single autonomous system.

Internet Control Message Protocol (ICMP)

ICMP is a Layer 3 protocol which sends control and error information between TCP/IP devices. The ping and traceroute commands use ICMP.

Internet Key Exchange (IKE)

IKE is a step in the ISAKMP protocol. Two unidirectional IPsec SAs are established for data transfer using separate keys.

Internet Service Provider (ISP)

An ISP provides a connection to the Internet. There are many different providers such as cable, wireless, leased lines, etc.

Internetwork Operating System (IOS)

IOS is Cisco’s operating system.

Internet Security Association and Key Management Protocol (ISAKMP)

ISAKMP is a protocol that provides authentication and key exchange. It creates a Security Association (SA) between nodes.

Intranet With intranet, users can find information within the company itself, i.e., it is local to a company.

Intrasite Automatic Tunnel Address Protocol (ISATAP)

This is a type of IPv6 tunnel that can be used only within a site. ISATAP supports global unicast for all prefixes.

Inverse ARP In Frame Relay, inverse ARP in conjunction with MI status messages allow a router to associate Layer 3 and Layer 2 addresses.

Internetwork Protocol IPSec is a protocol suite to secure IP communications. It offers authentication,


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Security Protocol (IPSec)

data integrity, confidentiality, and anti-replay.

Inter-Switch Link (ISL) A Cisco-proprietary trunk encapsulation. It is no longer supported on the most recent Cisco switches.

Leased Line A leased line is a type of point-to-point WAN connection.

Light-Emitting Diode (LED)

LEDs are used to display the status of an interface and can change to various colors, such as green, amber or orange, red, or off, to indicated various states.

Link Aggregation Control Protocol (LACP)

LACP is the IEEE 802.3ad negotiation protocol to form an EtherChannel. Each EtherChannel interface is configured as active, passive, or on.

Link-Local Address A link-local address permits Layer 3 forwarding within a subnet. In IPv6, a link-local address can be automatically assigned to an interface.

Link State Advertisements (LSA)

In OSPF, there are various types of LSAs that communicate routing information between nodes.

Link State Database (LSDB)

In OSPF, this is another name for the topology database. It contains routing information received from other nodes as well as locally.

Link State Protocols An example of a link state protocol is OSPF. Link state protocols use the Shortest Path First algorithm to identify the best layer 3 path to a destination. They are aware of the entire topology of the network.

Local Area Network (LAN)

A LAN connects networking devices in a proximity area such as a floor of a building or a building itself.

Local Management Interface (LMI)

The LMI defines the interactivity between Frame Relay DTE and Frame Relay DCE. The LMI generates status messages for the virtual circuit.

Logical Address The Layer 3 address is a logical address, as opposed to a Layer 2 MAC, which is a physical address.

Logical Link Control The IEEE splits the Ethernet data link layer into two sublayers. The first layer is 802.3 Media Access Control. The second layer is 802.2 LLC. It handles communication between Layers 1 and 3.

Loopback Interface A logical and virtual interface on a router. It can be assigned addressing information.

Maximum transmission unit (MTU)

MTU is the largest Layer 3 packet that can be carried by a Layer 2 frame.

Manually Configured Tunnel (MCT)

This is a tunnel used for IPv4 to IPv6 transition. An MCT can carry packets involving many protocols. Its packet MTU is 1476.

Media Access Control (MAC) address

MAC addresses are hardware addresses used for communication at Layer 2. A MAC address is 48 bits in length and is represented as a 12-digit hexadecimal number.

Message-digest 5 (MD5)

MD5 is a cryptographic hash function that produces a 128-bit hash value. It is an authentication option for Cisco AAA and routing protocols.

Metric In routing, a metric is a measurement used to determine the best path by routing protocol, for example, hop count, cost, bandwidth, and delay.

Multicast A packet with a multicast address is forwarded to a group of devices.

Multi-Exit Discriminator (MED)

The MED attribute is called the BGP metric in Cisco IOS. It is used to influence routing from other autonomous systems when there are multiple entry points.

Multiple Spanning Tree IEEE 802.1s STP. In MST, multiple instances of RSTP run. Each instance is


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(MST) associated with a list of VLANs.

Multiprotocol BGP (MBGP)

MBGP is a version of BGP. It can handle both IPv4 and IPv6 at the same time by using a construct called address families.

Native VLAN In 802.1Q trunking, frames assigned to the native VLAN are untagged.

Neighbor Discovery Protocol (NDP)

In IPv6, NDP uses ICMPv6 messages to dynamically discover neighbors. It keeps track of the Layer 2 addresses. NDP also performs Duplicate Address Detection (DAD) to ensure assignment of a unique address.

Network A network connects a group of computers (hardware and software components) together.

Network Address Translation (NAT)

NAT translates addresses. It can be used for many purposes. NAT is used most often to translate private IP address to public IP address and vice versa.

Network Address Translation – Protocol Translation (NAT-PT)

NAT-PT is used for IPv4-only nodes to communicate with IPv6-only nodes and vice versa.

Network Address Translation (NAT) Exemption

NAT is configured to exclude traffic from NAT translation. This is often used for VPN traffic between the headquarters and a branch.

NAT-Traversal (NAT-T) In IPsec, this allows implementation of PAT. ESP traffic is tunneled inside of UDP port 4500.

Network-Based Application Recognition (NBAR)

NBAR is a feature used by some Cisco routers and switches to recognize a flow by inspecting some packets sent. NBAR does not support all applications.

Network layer reachability information (NLRI)

In BGP, NLRI is basically route information. It is communicated in BGP messages that also carry path attribute information.

Non-Broadcast Multi-Access (NBMA)

An NBMA network attaches multiple devices at Layer 2, such as Frame Relay. NBMA networks do not inherently support multicast or broadcast traffic.

Non-Volatile RAM (NVRAM)

The startup-config file of IOS is stored in NVRAM. When the device is switched off, the NVRAM contents are preserved.

Offset List An offset list is a path control tool used for distance vector routing protocols and EIGRP. They increase the metric for incoming or outgoing routes.

Open Shortest Path First (OSPF)

OSPF is a link state routing protocol. Multiple areas can be defined.

Open Systems Interconnection (OSI) Reference Model

The International Organization for Standardization (ISO) developed the OSI Reference Model to describe data communications processing.

Outside Global IP Address

In NAT, this is a public address referencing an outside device. An example is any Internet address.

Outside Local IP Address

In NAT, this is a private address referencing an outside device. An example is the Internet-routable IP address assigned to a host that resides on the Internet.

Oversubscription When a link is oversubscribed, the sum of the downlink bandwidths is greater than the sum of the uplink bandwidths.

Packet-Switched Connection

Packet-switched connections send data as packages of packets, as opposed to circuit-switched connections, which last for an entire conversation.


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Passive interface The effect of a passive interface is dependent on the routing protocol. In general, routing protocol advertisements are not sent out a passive interface.

Path attribute (PA) PAs are used to determine the best BGP path. Attributes are divided into two categories: well-known and optional.

Per-VLAN Spanning Tree Protocol (PVST)

PVST is a version of STP that supports one instance of STP for each VLAN. PVST can make use of redundant links by using them for other VLANs.

Permanent Virtual Circuit (PVC)

In Frame Relay, PVCs are permanent connections established between a source and a destination. These are configured manually between hosts that communicate frequently.

Physical Layer It is the bottommost layer of the OSI model. It manages the electrical and mechanical interfaces. The physical layer determines the type of interface used on the networking device, type of cable used for connecting devices, connectors used at the end of the cable, pinouts used, etc.

Physical Topology A physical topology indicates how multiple devices are placed in a network (LAN) and connected to each other physically, for example, using wires.

Point-to-Point Protocol (PPP)

PPP is a Layer 2 protocol used to connect two nodes. It provides features like authentication, error detection, and multilink.

Point-to-Point Protocol over ATM (PPPoA)

In DSL, PPPoE encapsulates PPP traffic in ATM cells for transport from an ISP to a customer.

Point-to-Point Protocol over Ethernet (PPPoE)

In DSL, PPPoE encapsulates PPP traffic in Ethernet frames for transport from an ISP to a customer.

Poison Reverse In routing protocols, a router actively advertises routes as unreachable through the interface over which they were learned. This helps prevent routing loops.

Policy-Based Routing (PBR)

PBR allows policy as opposed to metrics to determine the path in routing protocols. A variant is local PBR which allows policy to be applied to routes generated by the local router.

Port Address Translation (PAT)

PAT is a variation of NAT. One inside address is translated to a unique IP address and port number combination. This conserves public addresses required in a LAN.

Port Aggregation Protocol (PAgP)

In EtherChannel, PAgP is a Cisco-proprietary negotiation protocol. EtherChannel interfaces are configured as on, desirable, or auto.

Port Security This feature limits the MAC addresses that can access a switch port. Aging and sticky address learning can be configured.

PortFast A port configured with PortFast is moved quickly into the forwarding state. The time spent by the port in the listening and learning states is reduced.

Power-On Self Test (POST)

POST is a diagnostic test that is run when a Cisco device is started. Various checks on interfaces, lines, and memory components are performed. If there is a failure with any of the components, the Cisco device will not boot.

PPP Authentication Protocol (PAP)

This is a password authentication protocol where the source sends the username and password details to the destination. If the received details match with the currently stored credentials, an accept message is sent; else a reject message is sent back to the source.

Prefix List This is a Layer 3 path control tool. Unlike an ACL, it can be used to specify addresses by subnet length range.


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Presentation Layer This layer ensures that the audio, video, graphic, and text information is appropriately presented to the user. This is the sixth layer in the OSI model.

Pre-shared key (PSK) A PSK is an authentication key used for ISAKMP and is shared by the sender and receiver before transmitting data.

Private IP address The RFC 1918 document defines IPv4 addresses that can be used within a LAN but are not routable over the Internet.

Private Virtual Local Area Network (PVLAN)

A PVLAN is a switch configuration that restricts data transmission between devices. Devices can be grouped as isolated or community.

Privileged EXEC mode This mode provides full access to the IOS. It also includes the User EXEC commands and is used for troubleshooting. A # prompt indicates that you are in the Privileged EXEC mode.

Protocol A protocol is a standard or a format to establish communication between devices.

Protocol Data Unit (PDU)

PDUs are packages of data exchanged between devices. PDUs are referred to by different terms depending on the OSI Reference Model layer in which they are found, for example, segments, packets, and frames.

Public Switched Telephone Network (PSTN)

PSTN is the infrastructure of the telephone network provided for public use.

Quality of Service (QoS)

QoS helps to prioritize traffic when there is congestion. It involves classifying traffic types and marking packets.

Rapid Spanning Tree Protocol (RSTP)

RSTP is an enhancement of STP. It is IEEE standard 802.1w. It reduces the convergence time when there is a topology change.

Read-Only Memory (ROM)

The contents of this memory will not get erased when the system is turned off. In a Cisco device, this contains the firmware necessary to boot up the router.

Reliable Transport Protocol (RTP)

EIGRP uses RTP to manage EIGRP packets. Reliable messages require an ACK (acknowledgement); unreliable messages do not.

Remote Access Virtual Private Network (VPN)

Individual users can get a secure access to a remote private network across the Internet.

Remote Authentication Dial-In User Service (RADIUS)

UDP ports 1645/6 or 1812/13; RADIUS is a server/client protocol that provides authentication and authorization.

Reported Distance (RD)

In EIGRP, this is the metric between the next-hop router and the destination.

RIPng (Routing Information Protocol next generation)

RIPng is an IPv6 routing protocol based on IPv4 RIP version 2.

Rivest-Shamir-Adelman (RSA)

RSA refers to the developers of one of the first practical public key infrastructures. It is based on asymmetric keys.

Rollover cable It is a Cisco proprietary and cannot be used for non-Cisco network connections. It is used to connect to the console port of a router or a switch.

ROM Monitor (ROMMON)

When a Cisco router or switch is booted, the ROM Monitor tests the IOS device, performs password recovery procedure, and performs an upgrade if


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the IOS image is corrupted or missing. If the IOS does not load, the device remains in ROMMON.

Root Bridge or Switch In STP, the root bridge is located at the top of the spanning tree and the branches are connected to other switches. Depending on the version of STP, the root bridge transmits the changes in the network topology to other bridges.

Root Port In STP, the root port is the port on a switch that has the smallest root path cost. This is the link taken to reach the root bridge.

Root Path Cost The root path cost is the cumulative cost to reach the root switch. Each link on a switch is assigned a cost based on the link speed.

Route caching This is a multilayer switch forwarding method. The route processor processes a traffic flow’s first packet. Thereafter, the switch engine handles all subsequent packets in that traffic flow.

Route map This is a path control tool. Route maps can perform route filtering, like distribute lists can, and also set the metric.

Routing Information Protocol (RIP)

This protocol is used to exchange routing information. There are two versions of RIP: one for IPv4 and one for IPv6, called RIPng.

Routing table The information in the routing table is used to make decisions regarding how a packet would be routed to the destination.

Secure Shell (SSH) TCP/UDP port 22; SSH is server-client protocol for secure remote access.

Security Device Manager (SDM)

It is a web-based tool that provides administration and security features on Cisco routers. It helps the user configure and troubleshoot a router without having to use the command line interface.

Seed metric In redistribution, the seed metric is the metric assigned to a redistributed route. It should be larger than the largest metric in the target routing protocol to help prevent suboptimal routing.

Service Set Identifier (SSID)

To access an access point, an SSID, a naming scheme for WLANS, allows an administrator to group WLAN devices together.

Shortest Path First (SPF)

SPF is the algorithm used in a link-state protocol such as OSPF to find best path. SPF was developed by Dijkstra.

Simple Network Management Protocol (SNMP)

UDP port 161; SNMP is a protocol to monitor network devices. It includes a server, agent, and Management Information Base.

Site-local address In IPv6, site-local addresses are specific to a site and are not routable over the Internet. They are similar to RFC 1918 IPv4 addresses.

Site-to-site Virtual Private Network (VPN)

A site-to-site VPN connects two locations or sites together with a secure VPN.

Small Office/Home Office (SOHO)

A SOHO network is when a small number of people work from a small office or home.

Solicited Node Multicast

In IPv6, a solicited mode multicast address is automatically generated. It is used for NDP.

Spanning Tree Protocol (STP)

STP is used to prevent the formation of loops in layer 2. There are various standard and proprietary versions of STP.

Split Horizon Split horizon is used to reduce the possibility of routing loops. In this technique, if a route is sent by one router to its neighboring router, the router


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that receives this route will not share that information to the advertising router on the same interface.

Star Topology In a star topology, a central device or hub is connected to multiple devices through a point-to-point connection. This is the most widely used topology.

Stateful DHCP In IPv6, stateful DHCP works like DHCP for IPv4 except it uses multicast to address FF02::1:2 and the default router is learned by NDP.

Stateless Address AutoConfiguration (SLAAC)

In IPv6, this is an automatic address assignment technique. The Domain Name System (DNS) server is provided by DHCPv6 and the Neighbor Discovery Protocol (NDP) provides the prefix, prefix length, and default router.

Static Route Static routes are configured on a router manually.

Sticky Learning This is a port security feature that helps the switch dynamically learn a MAC address and then retain that MAC address in the running configuration.

Straight-Through cable A straight-through cable is used to connect two different types of devices, for example, a router and a switch. In these cables, the pinouts match on both ends of the cable, for example, pin 1 to pin 1, pin 2 to pin 2, etc.

Stuck-in-active (SIA) SIA is a condition in EIGRP. It can be mitigated by restricting query scope.

Subinterface A subinterface is a virtual interface associated with a particular physical interface. Subinterfaces are often used for Frame Relay and for routing-on-a-stick.

Subnet zero (subnet 0) The first subnet created in a subnetted network.

Subset Advertisement A VTP server responds to a client or server request to generate a subset advertisement, which follows summary advertisements. Subset advertisements contain all the information pertaining to VLANs.

Summary Advertisement

When there is a change to a VLAN in a catalyst, the catalyst generates a summary advertisement to inform the neighboring catalysts the current VTP domain name and the revised configuration. Summary advertisements are generated every 5 minutes to create a summary of VLAN changes.

Switch A switch is a layer 2 device used to connect multiple devices, for example, many end devices. A multilayer switch also provides Layer 3 connectivity.

Switch Port Analyzer (SPAN)

In a switch, SPAN is an IOS feature that provides port mirroring. There are VSPAN, RSPAN, and ERSPAN, and all provide different types of SPAN capabilities.

Switch virtual interface (SVI)

In a switch, an SVI is a Layer 3 virtual port defined on a VLAN. A Layer 2 switch provides one SVI; a multilayer switch provides many.

Terminal Access Controller Access-Control System Plus (TACACS+)

TCP port49; This is a type of client/server AAA protocol. It was originally a Cisco proprietary protocol.

Ternary Content Addressable Memory (TCAM)

This is an area in multilayer switch memory. The TCAM holds the AT and the FIB and handles the more complex matching involved in Access Control Lists (ACLs), QOS, and routing tables.

Three-Way Handshake A three-way handshake takes place at the beginning of the TCP connection, before hosts can exchange data with one another. SYN, SYN/ACK, and ACK are the sequences.

Tool Command TCL is a scripting language available on many Cisco routers and switches.


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Language (TCL)

Transmission Control Protocol (TCP)

TCP is a Layer 4 protocol which keeps track of data flow. Once a TCP session is established, it is maintained until the sender or receiver terminates the session.

Transmission Control Protocol / Internet Protocol (TCP/IP)

It is a standard protocol stack that oversees communication among multiple devices within a network.

Transport Layer The fourth layer of the OSI is the transport layer. It provides end-to-end communication over a network by providing features like error detection, correction, and traffic flow control.

Triple Data Encryption Standard (3DES)

3DES is a cipher that applies the DES cipher algorithm three times to each data block.

Trunk In switching, a trunk carries traffic for multiple VLANs.

Tunnel Tunneling encapsulates datagrams for many purposes. For example, a tunnel can be used to connect “islands” of IPv6 over an IPv4 network.

Unicast A unicast transmission is sent to only one device.

User Datagram Protocol (UDP)

UDP is a Layer 4 protocol which is faster than TCP, but does not provide reliable transmission.

User EXEC mode When a Cisco router or switch boots, the IOS user interface is in user EXEC mode. This mode provides very few commands and > prompt indicates that the user EXEC mode is active.

Variable-Length Subnet Masking (VLSM)

Variable Length Subnet Masking allows the creation of many networks from a single, large network using subnet masks of different lengths. VLSM provides better route summarization and optimization of addressing.

Virtual Circuit (VC) As the name suggests, these are virtual connections, i.e., these are not physical circuits but logical connections between devices to meet the traffic flow requirement in a network.

Virtual LAN (VLAN) A single layer-2 network may be partitioned to create multiple distinct broadcast domains. These are called VLANs.

Virtual Private Network (VPN)

A Virtual Private Network (VPN) provides the security necessary to use the Internet as a WAN connection.

Virtual Router Redundancy Protocol (VRRP)

VRRP is a standards-based first-hop redundancy protocol. It is similar to HSRP.

Virtual Type Terminal (VTY)

VTY lines are used for Telnet or SSH access to networking devices. Remote access is most often used to manage networking devices.

VLAN Trunk Protocol (VTP)

VTP propagates the definition of VLANs on multiple switches. This is a Cisco proprietary protocol.

Voice over IP (VoIP) In VoIP, phone calls are digitized and sent over the IP network. They are sent in packets as data.

VTP client mode A VTP client sends and receives VTP updates on trunks. A client cannot create, change, or delete VLANs.

VTP Pruning VTP pruning is used to eliminate unused traffic in the network. It ensures that all the switches are aware of the VLAN traffic and delete the VLANs on a trunk to make the network congestion free.


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VTP server mode A VTP server can add, delete, and change the VLAN configuration information. This is the default mode and on receiving an update, the server processes it and sends it to other trunk ports.

VTP transparent mode VTP transparent switches do not share VLAN information.

Web VPN This is Cisco’s proprietary SSL VPN solution. Web VPN is used to provide secured access to websites and other web-based applications.

Wide Area Network (WAN)

Multiple LANs spread over a vast geography are connected to each other via a WAN. This provides a network for a large geography. Devices in a WAN can be connected using a third party public network, leased lines, or satellites. The four types of connections or circuits used in WAN are circuit-switched, cell-switched, packet-switched, and dedicated connections.

Wildcard Mask A wildcard mask is a mask used to define which bits must match a given IP address. It is the inverse of a subnet mask. Wildcard masks are used in ACLs and some routing protocols.

Windowing This is a process used by TCP to control the amount of data sent at once. This is to ensure that there is no overflow of data into a particular device. The sliding windowing technique is used to determine the number of segments that can be transmitted simultaneously.

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