Network Glossary U V W Y

U/L bit The second most significant bit in the most significant byte of an Ethernet MAC address, a value of binary 0 implies that the address is a Universally Administered Address (UAA) (also known as Burned-In Address [BIA]), and a value of binary 1 implies that the MAC address is a locally configured address.

UDLD See UniDirectional Link Detection.

unicast MAC address Ethernet MAC address that represents a single NIC or interface.

UniDirectional Link Detection A protection against problems caused by unidirectional links between two switches. Uses messaging between switches to detect the loop, err-disabling the port when the link is unidirectional.

Update (EIGRP) An EIGRP message that informs neighbors about routing information. Update messages require an Ack.

Update timer With RIP, the regular interval at which updates are sent. Each interface uses an independent timer, defaulting to 30 seconds.

UplinkFast Cisco-proprietary STP feature in which an access layer switch is configured to be unlikely to become Root or to become a transit switch. Also, convergence upon the loss of the switch’s Root Port takes place in a few seconds.

upstream router From one multicast router’s perspective, the upstream router is another router that has just forwarded a multicast packet to that router.

User Priority A 3-bit field in an 802.1Q header used for marking frames.

variance An integer setting for EIGRP and IGRP. Any FS route whose metric is less than this variance multiplier times the successor’s metric is added to the routing table, within the restrictions of the maximum-paths command.

variable-length subnet masking A strategy for subnetting a classful network for which masks/prefixes are different for some subnets of that one classful network.

VC See virtual circuit.

violate A category used by a policer to classify packets relative to the traffic contract. These packets are considered to be above the traffic contract in all cases.

virtual circuit A logical concept that represents the path over which frames travel between DTEs. VCs are particularly useful when comparing Frame Relay to leased physical circuits.

virtual IP address The IP address used by hosts as the default gateway in a VRRP configuration. This address is shared by two or more VRRP routers, much as HSRP works.

virtual LAN A group of devices on one or more LANs that are configured (using management software) so that they can communicate as if they were attached to the same wire, when, in fact, they are located on a number of different LAN segments. Because VLANs are based on logical instead of physical connections, they are extremely flexible.

virtual link With OSPF, the encapsulation of OSPF messages inside IP, to a router with which no common subnet is shared, for the purpose of either mending partitioned areas or providing a connection from some remote area to the backbone area.

Virtual Router Redundancy Protocol A standard (RFC 3768) feature by which multiple routers can provide interface IP address redundancy so that hosts using the shared, virtual IP address as their default gateway can still reach the rest of a network even if one or more routers fail.

Virtual Routing and Forwarding table In MPLS VPNs, an entity in a single router that provides a means to separate routes in different VPNs. The VRF includes per-VRF instances of routing protocols, a routing table, and an associated CEF FIB.

VLAN See virtual LAN.

VLAN Trunking Protocol A Cisco-proprietary protocol, used by LAN switches to communicate VLAN configuration.

VLSM See variable-length subnet masking.

VoFR See Voice over Frame Relay.

Voice over Frame Relay Defined in FRF.11, an FR VC that uses a slightly varied header, as compared with FRF.3 data VCs, to accommodate voice payloads directly encapsulated inside the Frame Relay LAPF header.

VPN label The innermost MPLS header in an packet traversing an MPLS VPN, with the label value identifying the forwarding details for the egress PE’s VRF associated with that VPN.

VRF table See Virtual Routing and Forwarding table.

VRRP See Virtual Router Redundancy Protocol.

VRRP Master router The router in a VRRP group that is currently actively forwarding IP packets. Conceptually the same as an HSRP Active router.

VTP See VLAN Trunking Protocol.

VTP pruning VTP process that prevents the flow of broadcasts and unknown unicast Ethernet frames in a VLAN from being sent to switches that have no ports in that VLAN.

WCCP See Web Cache Communication Protocol.

WCCP cluster A logical group of content engines running WCCP between them. The lead content engine determines the traffic distribution within the cluster, for optimum performance and scalability.

Web Cache Communication Protocol The protocol used by content engines to manage traffic flow between routers configured for WCCP and between content engines. WCCP takes advantage of the fact that many web pages (and other content) are regularly accessed by users in a given network. Therefore, routers can redirect content requests to a cache engine or a cluster of cache engines to improve response time and reduce WAN usage for cached content before new requests are made across the WAN.

weight A local Cisco-proprietary BGP setting that is not advertised to any peers. A larger value is considered to be better.

weighted fair queuing A Cisco IOS queuing tool most notable for its automatic classification of packets into separate per-flow queues.

weighted random early detection WRED is a method of congestion avoidance that works by dropping packets before the output queue becomes completely full. WRED can base its dropping behavior on IP Precedence or DSCP values to drop low-priority packets before high-priority packets.

weighted round-robin A queuing scheduler concept, much like CQ’s scheduler, in which queues are given some service in sequence. This term is often used with queuing in Cisco LAN switches.

weighted tail drop A method that creates three thresholds per egress queue in the Cisco 3560 switch. Traffic is divided into the three queues based on CoS value, and given different likelihoods (weight) for tail drop when congestion occurs based on which egress queue is involved.

well-known discretionary A characterization of a BGP path attribute in which all BGP implementations must support and understand the attribute (well known), but BGP Updates can either include the attribute or not depending on whether a related feature has been configured (discretionary).

well-known mandatory A characterization of a BGP path attribute in which all BGP implementations must support and understand the attribute (well known), and all BGP Updates must include the attribute (mandatory).

WEP See Wired Equivalent Privacy.

WFQ See weighted fair queuing.

Wi-Fi Protected Access A security standard that includes both TKIP and AES and was ratified by the Wi-Fi Alliance.

window Typically used by protocols that perform flow control (like TCP), a TCP window is the number of bytes that a sender can send before it must pause and wait for an acknowledgement of some of the yet-unacknowledged data.

Wired Equivalent Privacy The initial 802.11 common key encryption mechanism; vulnerable to hackers.

wireless LAN controller Controls access to the Internet in public wireless LANs.

Wireless LAN Threat Defense Solution An intrusion detection system that safeguards the wireless LAN from malicious and unauthorized access.

WLSE See Cisco Wireless LAN Solution Engine.

WPA Wi-Fi Protected Access. A security standard that includes both TKIP and AES and was ratified by the Wi-Fi Alliance.

WRED See weighted random early detection.

WRR See weighted round-robin.

WTD See weighted tail drop.

Yellow Alarm A T1 alarm state that occurs when a device receives a Yellow Alarm signal. This typically means that the device on the other end of the line is in a Red Alarm state.

Network Glossary T

T1 A name used for DS1 lines inside the North American TDM hierarchy.

T3 A name used for DS3 lines inside the North American TDM hierarchy.

TACACS+ A Cisco-proprietary protocol that defines how to perform authentication between an authenticator (for example, a router) and an authentication server that holds a list of usernames and passwords.

Tag Distribution Protocol The original MPLS protocol used to advertise the binding (mapping) information about each particular IP prefix and associated label. It is slightly different from LDP, but functionally equivalent. See also LDP.

tail drop An event in which a new packet arrives, needing to be placed into a queue, and the queue is full—so the packet is discarded.

Tc See Time Interval.

TDP See Tag Distribution Protocol.

Time Interval (Tc) Variable name for the time interval used by shapers and by CAR.

TCP code bits Single-bit fields in the TCP header. For example, the TCP SYN and ACK code bits are used during connection establishment.

TCP flags The same thing as TCP code bits. See TCP code bits.

TCP header compression The process of taking the IP and TCP headers of a packet, compressing them, and then uncompressing them on the receiving router.

TCP intercept A Cisco router feature in which the router works to prevent SYN attacks either by monitoring TCP connections flowing through the router, or by actively terminating TCP connection until the TCP connection is established and then knitting the client-side connection with a server-side TCP connection.

TCP SYN flood An attack by which the attacker initiates many TCP connections to a server, but does not complete the TCP connections, by simply not sending the third segment normally used to establish the connection. The server may consume resources and reject new connection attempts as a result.

TDM See time-division multiplexing.

TDM hierarchy The structure inside telcos’ original digital circuit build-out in the mid-1900s, based upon using TDM to combine and disperse smaller DS levels into larger levels, and vice versa.

Temporal Key Integrity Protocol An enhanced version of WEP that is part of the 802.11i standard and has an automatic key-update mechanism that makes it much more secure than WEP.

TKIP is not as strong as AES in terms of data protection.

terminal history The feature in a Cisco IOS device by which a terminal session’s previously typed commands are remembered, allowing the user to recall the old commands to the command line through a simple key sequence (for example, the up-arrow key).

time-division multiplexing The process of combining multiple synchronized input signals over a single medium by giving each signal its own time slot, and then breaking out those signals.

Time to Live A field in the IP header that is decremented at each pass through a Layer 3 forwarding device.

TKIP See Temporal Key Integrity Protocol.

token bucket A conceptual model used by shapers and policers to represent their internal logic.

ToS byte See Type of Service byte.

totally NSSA area A type of OSPF NSSA area for which neither external (type 5) LSAs are introduced, nor type 3 summary LSAs; instead, the ABRs originate and inject default routes into the area. External routes can be injected into a totally NSSA area.

totally stubby area A type of OSPF stub area for which neither external (type 5) LSAs are introduced, nor type 3 summary LSAs; instead, the ABRs originate and inject default routes into the area. External routes cannot be injected into a totally stubby area.

traffic contract In shaping and policing, the definition of parameters that together imply the allowed rate and bursts.

transient multicast group Multicast addresses that are not assigned by IANA.

transit network (OSPF) A network/subnet over which two or more OSPF routers have become neighbors, thereby being able to forward packets from one router to another across that network.

transit router (OSPF) A router that is allowed to receive a packet from an OSPF router and then forward the packet to another OSPF router.

transmit power The signal strength of the RF signal at the output of the radio card or access point transmitter, before being fed into the antenna. Measured in milliwatts, watts, or dBm.

Trap In the context of SNMP, the Trap command is sent by an SNMP agent, to a manager, when the agent wants to send unsolicited information to the manager. Trap is not followed by a Response message from the receiving SNMP manager.

Triggered Extensions to RIP for On-Demand Circuits Defined in RFC 2091, the extensions define how RIP may send a full update once, and then send updates only when routes change, when an update is requested, or when a RIP interface changes state from down to up.

triggered updates A routing protocol feature for which the routing protocol sends routing updates immediately upon hearing about a changed route, even though it may normally only send updates on a regular update interval.

trunking Also called VLAN trunking, a method (using either the Cisco ISL protocol or the IEEE 802.1Q protocol) to support carrying traffic between switches for multiple VLANs that have members on more than one switch.

TTL See Time to Live.

TTL scoping Controls the distribution of multicast traffic by checking the TTL values configured on the interfaces. It forwards the multicast packet only on those interfaces whose configured TTL value is less than or equal to the TTL value of the multicast packet.

Type of Service byte A 1-byte field in the IP header, originally defined by RFC 791 for QoS marking purposes.

Network Glossary S

SAFE Blueprint An architecture and set of documents that defines Cisco’s best recommendations for how to secure a network.

same-layer interaction The two computers use a protocol with which to communicate with the same layer on another computer. The protocol defined by each layer uses a header that is transmitted between the computers to communicate what each computer wants to do.

scheduler A queuing tool’s logic by which it selects the next packet to dequeue from its many queues.

sequence number (OSPF) In OSPF, a number assigned to each LSA, ranging from 0x80000001 and wrapping back around to 0x7FFFFFFF, which is used to determine which LSA is most recent.

sequence number A term used with WFQ for the number assigned to a packet as it is enqueued into a WFQ. WFQ schedules the currently lowest SN packet next.

Service Interworking The process, defined by FRF.5 and FRF.8, for combining ATM and FR technologies for an individual VC.

service policy A term referring to the MQC service-policy command, which is used to enable a policy map on an interface.

service set identifier Defines a particular wireless LAN. The SSID configured in the radio card must match the SSID in the access point before the station can connect with the access point.

Set In the context of SNMP, the Set command is sent by an SNMP manager, to an agent, requesting that the agent set a single identified variable to the stated value. The main purpose is to allow remote configuration and remote operation, such as shutting down an interface by using an SNMP Set of an interface state MIB variable.

SF See Superframe.

shaped mode The operating mode of shaped round-robin that provides a low-latency queue with policing.

shaped round-robin A packet-scheduling algorithm used in Cisco switches that provides similar behavior to CBWFQ in shared mode and polices in shaped mode.

shaping rate The rate at which a shaper limits the bits exiting the shaper.

shared distribution tree In PIM-SM, the path of the group traffic that flows from the RP to the routers that need the traffic. It is also called the root-path tree (RPT), because it is rooted at the RP.

shared mode The operating mode of shaped round-robin that provides behavior like CBWFQ with bandwidth allocated between different traffic classes by a relative amount rather than absolute percentage of the available bandwidth.

shortest-path tree switchover In the PIM-SM design, the process by which a PIM-SM router can build the SPT between itself and the source of a multicast group and take advantage of the most efficient path available from the source to the router as long as it has one directly connected group member. Once it builds an SPT, it sends a PIM-SM (S, G) RP-bit Prune toward the upstream router on the shared tree.

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

single-rate, two-color policer Policing in which a single rate is metered, and packets are placed into one of two categories (conform or exceed).

signal-to-noise ratio The difference between the measured signal power and the noise power that a particular receiver sees at a given time. Higher SNRs generally indicate better performance.

Slow Start A method for how a TCP sender grows its calculated CWND variable, thereby growing the allowed window for the connection. Slow Start grows CWND at an exponential rate.

Slow Start Threshold A calculated TCP variable, used along with the TCP CWND variable, to dictate a TCP sender’s behavior when it recognizes packet loss. As CWND grows after packet loss, the TCP sender increases CWND based on Slow Start rules, until CWND grows to be as high as the SSThresh setting, at which point TCP Congestion Avoidance logic is used. Essentially,

SSThresh is the threshold at which Slow Start logic ends.

SLSM See static length subnet masking.

SMI See Structure of Management Information.

Smoothed Round-Trip Time With EIGRP, a purposefully slowly changing measurement of round-trip time between neighbors, from which the EIGRP RTO is calculated.

smurf attack A style of attack in which an ICMP Echo is sent with a directed broadcast (subnet broadcast) destination IP address, and a source address of the host that is being attacked. The attack can result in the Echo reaching a large number of hosts, all of which reply by sending an Echo Reply to the host being attacked.

SN See sequence number.

SNMP agent A process on a computing device that accepts SNMP requests, responds with SNMP-structured MIB data, and initiates unsolicited Trap messages back to an SNMP management station.

SNMP manager A process on a computing device that issues requests for SNMP MIB variables from SNMP agents, receives and processes the MIB data, and accepts unsolicited Trap messages from SNMP agents.

SNR See signal-to-noise ratio.

socket A 3-tuple consisting of an IP address, port number, and transport layer protocol. TCP connections exist between a pair of sockets.

soft reconfiguration A BGP process by which a router reapplies routing policy configuration (route maps, filters, and the like) based on stored copies of sent and received BGP Updates.

software queue A queue created by Cisco IOS as a result of the configuration of a queuing tool.

solicited node multicast In IPv6, an address used in the Neighbor Discovery (ND) process. The format for these addresses is FF02::1:FF00:0000/104, and each IPv6 host must join the corresponding group for each of its unicast and anycast addresses. source DR A designated router that is directly connected with a source of the multicast group.

source registration In the PIM-SM design, the process by which a source DR, after it starts to receive the group traffic, encapsulates the multicast packets in the unicast packets and sends them to the RP.

source-based distribution tree Method by which a dense-mode routing protocol distributes multicast traffic from a source to all the segments of a network. Also called shortest-path tree (SPT), because it uses the shortest routing path from the source to the segments of the network.

source-specific addresses The range 232.0.0.0 through 232.255.255.255 that is allocated by IANA for SSM destination addresses and is reserved for use by source-specific applications and protocols.

source-specific multicast IGMPv3 is designed to support source filtering. IGMPv3 allows a host to indicate interest in receiving multicast packets only from specific source addresses, or from all but specific source addresses, sent to a particular multicast destination address.

sparse-mode protocol A multicast routing protocol that forwards the multicast traffic only when requested by a downstream router.

Spanning Tree Protocol Defined in IEEE 802.1d, a protocol used on LAN bridges and switches to dynamically define a logical network topology that allows all devices to be reached, but prevents the formation of loops.

SPF algorithm The algorithm used by OSPF and IS-IS to compute routes based on the LSDB.

SPF calculation The process of running the SPF algorithm against the LSDB, with the result being the determination of the current best route(s) to each subnet.

split horizon Instead of advertising all routes out a particular interface, the routing protocol omits the routes whose outgoing interface field matches the interface out which the update would be sent.

spread spectrum A technology that enables frequency reuse. Two variants exist: frequency hopping (FHSS) and direct sequence (DSSS). Both techniques spread the signal power over a relatively wide portion of the frequency spectrum over time, to reduce interference between systems.

SRR See shaped round-robin.

SRTT See Smoothed Round-Trip Time.

SSID See service set identifier.

SSM See source-specific multicast.

SSThresh See Slow Start Threshold.

stateful autoconfiguration A method of obtaining an IPv6 address that uses DHCPv6. See also stateless autoconfiguration.

stateless autoconfiguration A method used by an IPv6 host to determine its own IP address, without DHCPv6, by using NDP and the modified EUI-64 address format. See also stateful autoconfiguration.

static length subnet masking A strategy for subnetting a classful network for which all masks/prefixes are the same value for all subnets of that one classful network.

sticky learning In switch port security, the process whereby the switch dynamically learns the MAC address(es) of the device(s) connected to a switch port, and then adds those addresses to the running configuration as allowed MAC addresses for port security.

storm control A Cisco switch feature that permits limiting traffic arriving at switch ports by percentage or absolute bandwidth. Separate thresholds are available per port for unicast, multicast, and broadcast traffic.

STP See Spanning Tree Protocol.

straight-through cable Copper cable with RJ-45 connectors in which the wire at pin 1 on one end is connected to pin 1 on the other end; the wire at pin 2 is connected to pin 2 on the other end; and so on.

strict priority A queuing scheduler’s logic by which, if a particular queue has packets in it, those packets always get serviced next.

Structure of Management Information The SNMP specifications, standardized in RFCs, defining the rules by which SNMP MIB variables should be defined.

stub area An OSPF area into which external (type 5) LSAs are not introduced by its ABRs; instead, the ABRs originate and inject default routes into the area.

stub network (OSPF) A network/subnet to which only one OSPF router is connected.

stub router (EIGRP) A router that should not be used to forward packets between other routers. Other routers will not send Query messages to a stub router.

stub router (OSPF) A router that should either permanently or temporarily not be used as a transit router. Can wait a certain time after OSPF process start, or after BGP notifies OSPF that BGP has converged, before ceasing to be a stub router.

stuck-in-active The condition in which a route has been in an EIGRP active state for longer than the router’s Active timer.

sub-AS The term referring to a group of iBGP routers in a confederation, with the group members being assigned a hidden ASN for the purposes of loop avoidance.

subnet A subset of a classful IP network, as defined by a subnet mask, which used to address IP hosts on the same Layer 2 network in much the same way as a classful network is used.

subnet broadcast address A single address in each subnet for which packets sent to this address will be broadcast to all hosts in the subnet. It is the highest numeric value in the range of IP addresses implied by a subnet number and prefix/mask.

subnet ID 16 bits between the interface ID and global routing prefix in an IPv6 global address, used for subnet assignment inside an enterprise.

subnet mask A dotted-decimal number used to help define the structure of an IP address. The binary 0s in the mask identify the host portion of an address, and the binary 1s identify either the combined network and subnet part (when thinking classfully) or the network prefix (when thinking classlessly).

subnet number A dotted-decimal number that represents a subnet. It is the lowest numeric value in the range of IP addresses implied by a subnet number and prefix/mask.

subnet zero When subnetting a class A, B, or C address, the subnet for which all subnet bits are binary 0.

successor route With EIGRP, the route to each destination for which the metric is the lowest of all known routes to that network.

summary route A route that is created to represent one or more smaller component routes, typically in an effort to reduce the size of routing and topology tables.

Superframe An early T1 framing standard.

superior BPDU Jargon used by STP mostly when discussing the root election process; refers to a Hello with a lower bridge ID. Sometimes refers to a Hello with the same bridge ID as another, but with better values for the tiebreakers in the election process.

supplicant The 802.1X driver that supplies a username/password prompt to the user and sends/receives the EAPoL messages.

SVC See switched virtual circuit.

switched interface An interface on a Cisco IOS–based switch that is treated as if it were an interface on a switch.

switched virtual circuit A VC that is set up dynamically when needed. An SVC can be equated to a dial-on-demand connection in concept.

synchronization In BGP, a feature in which BGP routes cannot be considered to be a best route to reach an NLRI unless that same prefix exists in the router’s IP routing table as learned via some IGP.

Network Glossary R

radio management aggregation Reduces the bandwidth necessary for radio management information, such as access point status messages, that is sent across the network by eliminating redundant management information.

RADIUS A protocol, defined in RFC 2865, that defines how to perform authentication between an authenticator (for example, a router) and an authentication server that holds a list of usernames and passwords.

Rapid Per-VLAN Spanning Tree Plus The combination of PVST+ and Rapid Spanning Tree. It provides subsecond convergence time and is compatible with PVST+ and MSTP. Rapid Spanning Tree Protocol Defined in IEEE 802.1w, a specification to enhance the 802.1d standard to improve the speed of STP convergence.

RARP See Reverse ARP.

RD See Reported distance or Route Distinguisher.

Ready To Send On a serial cable, the pin lead set by the DTE to tell the DCE that the DTE wants to send data.

receiver’s advertised window In TCP, a TCP host sets the TCP header’s Window field to the number of bytes it allows the other host to send before requiring an acknowledgement. In effect, the receiving host, by stating a particular window size, grants the sending host the right to send that number of bytes in a single window.

Red Alarm A T1 alarm state that occurs when a device has detected a local LOF/LOS/AIS condition. The device in Red alarm state then sends a Yellow alarm signal.regular expression A list of interspersed alphanumeric literals and metacharacters that are used to apply complex matching logic to alphanumeric strings. Often used for matching AS_PATHs in Cisco routers.

Reliable Transport Protocol A protocol used for reliable multicast and unicast transmissions. Used by EIGRP.

remaining bandwidth A CBWFQ and LLQ term referring to the bandwidth on an interface that is neither reserved nor allocated via a priority command.

remote label In MPLS, a term used to define a label that an LSR learned from a neighboring LSR.

rendezvous point In the PIM-SM design, the central distribution point to which the multicast traffic is first delivered from the source designated router.

Reply (EIGRP) An EIGRP message that is used by neighbors to reply to a query. Reply messages require an Ack.

reported distance With EIGRP, the metric (distance) of a route as reported by a neighboring router.

request-to-send/clear-to-send A mechanism that counters collisions caused by hidden nodes. If enabled, the station or access point must first send an RTS frame and receive a CTS frame before sending each data frame.

Report Suppression mechanism When a Query is received from a router, each host randomly picks a time between 0 and the Maximum Response Time period to send a Report. When the host with the smallest time period first sends the Report, the rest of the hosts suppress their reports.

Response In the context of SNMP, the Response command is sent by an SNMP agent, back to a manager, in response to any of the three types of Get requests, or in response to a Set request. It is also used by a manager in response to a received Inform command from another SNMP manager. The Response holds the value(s) of the requested variables.

Retransmission Timeout With EIGRP, a timer started when a reliable (to be acknowledged) message is transmitted. For any neighbor(s) failing to respond in its RTO, the RTP protocol causes retransmission. RTO is calculated based on SRTT.

Reverse ARP A standard (RFC 903) protocol by which a LAN-attached host can dynamically broadcast a request for a server to assign it an IP address. See also ARP.

RF channel The specific frequency subband on which the radio card or access point is operating. The RF channel is set in the access point or ad hoc stations.

RGMP See Router-Port Group Management Protocol.

RID See router ID.

ROMMON An alternative software loaded into a Cisco router, used for low-level debugging and for password recovery.

Root Guard Cisco-proprietary STP feature in which a switch port monitors for incoming superior Hellos, and reacts to a superior Hello to prevent any switch connected to that port from becoming root.

root port The single port on each nonroot switch upon which the best Hello BPDU is received.

Route Distinguisher A 64-bit extension to the BGP NLRI field, used by MPLS for the purpose of making MPLS VPN customer routes unique in spite of the possibility of overlapping IPv4 address spaces in different customer networks.

route map A configuration tool in Cisco IOS that allows basic programming logic to be applied to a set of items. Often used for decisions about what routes to redistribute, and for setting particular characteristics of those routes—for instance, metric values.

route poisoning The process of sending an infinite-metric route in routing updates when that route fails.

route redistribution The process of taking routes known through one routing protocol and advertising those routes with another routing protocol.

route reflector A BGP feature by which a router learns iBGP routes, and then forwards them to other iBGP peers, reducing the required number of iBGP peers while also avoiding routing loops.

route reflector client A BGP router that, unknown to it, is aided by a route reflector server to cause all iBGP routers in an AS to learn all eBGP-learned prefixes.

route reflector non-client A BGP router in an AS that uses route reflectors, but that is not aided by any RR server.

route reflector server A BGP router that forwards iBGP-learned routes to other iBGP routers.

Route Tag field A field within a route entry in a routing update, used to associate a generic number with the route. It is used when passing routes between routing protocols, allowing an inter-mediate routing protocol to pass information about a route that is not natively defined to that intermediate routing protocol. Frequently used for identifying certain routes for filtering by a downstream routing process.

Route Target In MPLS VPNs, a 64-bit Extended Community path attribute attached to a BGP route for the purpose of controlling into which VRFs the route is added.

routed interface An interface on a Cisco IOS–based switch that is treated as if it were an interface on a router.

Router Advertisement In IPv6, a Router Advertisement message used by an IPv6 router to send information about itself to nodes and other routers connected to that router.

router ID The 32-bit number used to represent an OSPF router.

Router-Port Group Management Protocol A Cisco-proprietary Layer 2 protocol that enables a router to communicate to a switch which multicast group traffic the router does and does not want to receive from the switch.

routing black hole A problem that occurs when an AS does not run BGP on all routers, with synchronization disabled. The routers running BGP may believe they have working routes to reach a prefix, and forward packets to internal routers that do not run BGP and do not have a route to reach the prefix.

RP See rendezvous point.

RPF check Designed to solve the problems of multicast duplication and multicast routing loops. For every multicast packet received, a multicast router examines its source IP address, consults its unicast routing table, determines which interface it would use to go in the reverse direction toward the source IP address, compares it with the interface on which the packet was received, and, if they match, accepts the packet and forwards it; otherwise, the router drops the packet.

RPVST+ See Rapid Per-VLAN Spanning Tree Plus.

RSTP See Rapid Spanning Tree Protocol.

RT See Route Target.

RTO See Retransmission Timeout.

RTP See Reliable Transport Protocol.

RTP header compression The process of taking the IP, UDP, and RTP headers of a voice or video packet, compressing them, and then uncompressing them on the receiving router.

RTS See Ready To Send.

RTS/CTS See request-to-send/clear-to-send.

RXBOOT An alternative software loaded into a Cisco router, used for basic IP connectivity—most useful when Flash memory is broken and you need IP connectivity to copy a new IOS image into Flash memory.

Network Glossary Q

quartet A set of four hex digits listed in an IPv6 address. Each quartet is separated by a colon.

querier election When multiple routers are connected to a subnet, only one should be sending IGMP queries. It is called a querier. IGMPv1 does not have any rules for electing a querier. In IGMPv2 and IGMPv3, a router with the lowest interface IP address on the subnet is elected as a querier.

Query (EIGRP) An EIGRP message that is used to ask neighboring routers to verify their route to a particular subnet. Query messages require an Ack.

query scope (EIGRP) The characterization of how far EIGRP Query messages flow away from the router that first notices a failed route and goes active for a particular subnet.

queue starvation A possible side effect of a scheduler that performs strict-priority scheduling of a queue, which can result in lower-priority queues getting little or no service.

QV See quantum value.

QoS pre-classification A process used in routers that are encrypting traffic to permit egress QoS actions to be taken on traffic that is being encrypted on that router. QoS pre-classification keeps a copy of each packet to be encrypted in memory long enough to take the appropriate egress QoS actions on that traffic as it leaves that router, because the encrypted traffic cannot be inspected for QoS actions.

quantum value The number of bytes in a queue that are removed per cycle in MDRR. Similar to byte count in the custom queuing (CQ) scheduler.

Network Glossary P

P router See provider router.

PAgP See Port Aggregation Protocol.

PAP See Password Authentication Protocol.

partial SPF calculation An SPF calculation for which a router does not need to run SPF for any LSAs inside its area, but instead runs a very simple algorithm for changes to LSAs outside its own area.

partial update A routing protocol feature by which the routing update includes only routes that have changed, rather than include the entire set of routes.

passive (EIGRP) A state for a route in an EIGRP topology table that indicates that the router believes that the route is stable, and it is not currently looking for any new routes to that subnet.

passive mode FTP Defines a particular behavior for FTP regarding the establishment of TCP data connections. In passive mode, an FTP server uses the FTP PORT command, over the FTP control connection, to tell the FTP client the port on which the server will be listening for a new data connection. The client allocates an unused port, and initiates a connection to the FTP server’s earlier-declared port.

passive scanning Each 802.11 station passively monitors each RF channel for a specific amount of time and listens for beacons. Stations use the signal strengths of found beacons to determine the access point or ad hoc network with which to attempt association.

Password Authentication Protocol An Internet standard authentication protocol that uses clear-text passwords and a two-way handshake to perform authentication over a PPP link.

PAT See Port Address Translation.

path attribute A term generally describing characteristics about BGP paths that are advertised in BGP Updates.

payload compression See Layer 2 payload compression.

PCM See pulse code modulation.

PDU See protocol data unit.

PE See provider edge.

peak information rate In two-rate policing, the second and higher rate defined to the policer.

peer group In BGP, a configuration construct in which multiple neighbors’ parameters can be configured as a group, thereby reducing the length of the configuration. Additionally, BGP performs routing policy logic against only one set of Updates for the entire peer group, improving convergence time.

penultimate hop popping An MPLS VPN term referring to the more efficient choice of popping the outer label at the second-to-last (penultimate) LSR, which then prevents the egress PE from having to perform two LFIB lookups to forward the packet.

Per-Hop Behavior With DiffServ, a DSCP marking and a related set of QoS actions applied to packets that have that marking.

permanent multicast group The multicast addresses assigned by IANA.

permanent virtual circuit A predefined VC. A PVC can be equated to a leased line in concept.

Per-VLAN Spanning Tree Plus A Cisco-proprietary STP implementation, created many years before IEEE 802.1s and 802.1w, that speeds convergence and allows for one STP instance for each VLAN.

PHB See Per-Hop Behavior.

PHP See penultimate hop popping.

PIM Hello message Sent by a PIM router, by default every 30 seconds, on every interface on which PIM is configured to discover neighbors, establish adjacency, and maintain adjacency.

PIM-DM See Protocol Independent Multicast dense-mode routing protocol.

PIM-SM See Protocol Independent Multicast sparse-mode routing protocol.

PIM-SM (S,G) RP-bit Prune When a PIM-SM router switches from RPT to SPT, it sends a PIM-SM Prune message for the source and the group with the RP bit set to its upstream router on the shared tree. RFC 2362 uses the notation PIM-SM (S, G) RP-bit Prune for this message.

PIR See peak information rate.

point coordination function An optional contention-free 802.11 access protocol that requires the access point to poll wireless stations before they are able to send frames. Not commonly implemented.

Point-to-Point Protocol An Internet standard serial data-link protocol, used on synchronous and asynchronous links, that provides data-link framing, link negotiation, Layer 3 interface features, and other functions.

poison reverse With RIP, the advertisement of a poisoned route out an interface, when that route was formerly not advertised out that interface due to split horizon rules.

policing rate The rate at which a policer limits the bits exiting or entering the policer.

policy map A term referring to the MQC policy-map command and its related subcommands, which are used to apply QoS actions to classes of packets.

policy routing Cisco IOS router feature by which a route map determines how to forward a packet, typically based on information in the packet other than the destination IP address.

Port Address Translation A NAT term describing the process of multiplexing TCP and UDP flows, based on port numbers, to a small number of public IP addresses. Also called NAT overloading.

Port Aggregation Protocol A Cisco-proprietary messaging protocol used to negotiate the dynamic creation of PortChannels (EtherChannels) and to choose which ports can be placed into an EtherChannel.

port security A switch feature that limits the number of allowed MAC addresses on a port, with optional limits based on the actual values of the MAC addresses.

PortFast Cisco-proprietary STP feature in which a switch port, known to not have a bridge or switch attached to it, transitions from disabled to forwarding state without using any intermediate states.

power-save mode A mechanism for conserving battery power in wireless stations. The access point buffers data frames destined to sleeping stations, which wake periodically to learn from information in the beacon frame whether or not data frames are waiting for transmission. The radio card receives applicable data frames and then goes back to sleep.

PPP See Point-to-Point Protocol.

PQ See priority queue and priority queuing.

prefix A numeric value between 0 and 32 (inclusive) that defines the number of beginning bits in an IP address for which all IP addresses in the same group have the same value. Alternative:The number of binary 1s beginning a subnet mask, written as a decimal value between 0 and 32, used as a more convenient form of representing the subnet mask.

prefix list A Cisco IOS configuration tool that can be used to match routing updates based on a base network address, a prefix, and a range of possible masks used inside the values defined by the base network address and prefix.

priority (OSPF) An administrative setting, included in Hellos, that is the first criteria for electing a DR. The highest priority wins, with values from 1–255, with priority 0 meaning a router cannot become DR or BDR.

priority queue Jargon referring to any queue that receives priority service, often used for queues in an LLQ configuration that have the priority command configured.

priority queuing A Cisco IOS queuing tool most notable for its scheduler, which always services the high-priority queue over all other queues.

private addresses RFC 1918-defined IPv4 network numbers that are not assigned as public IP address ranges, and are not routable on the Internet. Intended for use inside enterprise networks.

private AS A BGP ASN whose value is between 64,512 and 65,535. These values are not assigned for use on the Internet, and can be used for private purposes, typically either within confederations or by ISPs to hide the ASN used by some customers.

private VLAN A Cisco switch feature that allows separation of ports as if they were in separate VLANs, while allowing the use of a single IP subnet for all ports.

process switching A Layer 3 forwarding path through a router that does not optimize the forwarding path through the router.

promiscuous port With private VLANs, a port that can send and receive frames with all other ports in the private VLAN.

protocol data unit A generic term that refers to the data structure used by a layer in a layered network architecture when sending data.

Protocol Independent Multicast dense-mode routing protocol PIM-DM is a method of routing multicast packets that depends on a flood-and-prune approach. PIM Dense Mode gets its name from the assumption that there are many receivers of a particular multicast group, close together (from a network perspective). Does not depend on any particular unicast routing protocol to perform its multicast functions.

Protocol Independent Multicast sparse-mode routing protocol PIM-SM is a method of routing multicast packets that requires some intelligence in the network about the locations of receivers so that multicast traffic is not flooded into areas with no receivers. PIM Sparse Mode gets its name from the assumption that relatively few receivers of a particular multicast group, widely scattered (from a network perspective), want to receive that multicast traffic. Does not depend on any unicast routing protocol to perform its multicast functions.

provider edge An MPLS VPN term referring to any LSR that connects to customers to support the forwarding of unlabeled packets, as well as connecting to the MPLS network to support labeled packets, thereby making the LSR be on the edge between the provider and the customer.

provider router An MPLS VPN term referring to an LSR that has no direct customer connections, meaning that the P router does not need any visibility into the VPN customer’s IP address space.

proxy ARP A router feature used when a router sees an ARP request searching for an IP host’s MAC, when the router believes the IP host could not be on that LAN because the host is in another subnet. If the router has a route to reach the subnet where the ARP-determined host resides, the router replies to the ARP request with the router’s MAC address.

Prune Override On a multiaccess network, when a PIM-DM or PIM-SM router receives a Prune message, it starts a 3-second timer. If it receives a Join message on the multiaccess network from another router before the timer expires, it considers the message as an override to the previously received Prune message and continues forwarding the group traffic on the LAN interface; otherwise, it stops forwarding the traffic on the LAN interface.

pruning See VTP pruning.

public wireless LAN A wireless LAN that offers connections to the Internet from public places, such as airports, hotels, and coffee shops.

pulse code modulation An early standard from AT&T for encoding analog voice as a digital signal for transmission over a TDM network. PCM requires 64 kbps, and is the basis for the DS0 speed.

PVC See permanent virtual circuit.

PVST+ See Per-VLAN Spanning Tree Plus.

Network Glossary O

OAM See Operation, Administration, and Maintenance.

OFDM See orthogonal frequency division multiplexing.

offset list A Cisco IOS configuration tool for RIP and EIGRP for which the list matches routes in routing updates, and adds a defined value to the sent or received metric for the routes. The value added to the metric is the offset.

one-time password Defined in RFC 2289, a mechanism by which a shared key and a secret key together feed into a hash algorithm, creating a password that is transmitted over a network. Because the shared key is not reused, the hash value is only valid for that individual authentication attempt.

OOF See Out of Frame.

Operation, Administration, and Maintenance A term referring to the processes and bits in the data stream used to manage the Telco TDM hierarchy.

optional nontransitive A characterization of a BGP path attribute in which BGP implementations are not required to support the attribute (optional), and for which if a router receives a route with such an attribute, the router should remove the attribute before advertising the route
(nontransitive).

optional transitive A characterization of a BGP path attribute in which BGP implementations are not required to support the attribute (optional), and for which if a router receives a route with such an attribute, the router should forward the attribute unchanged (transitive).

ORIGIN A BGP path attribute that implies how the route was originally injected into some router’s BGP table.

ORIGINATOR_ID Used by RRs to denote the RID of the iBGP neighbor that injected the NLRI into the AS.

orthogonal frequency division multiplexing A technology that sends a high-speed data stream over multiple subcarriers simultaneously. It is highly immune to multipath interference. 802.11a and 802.11g specify the use of OFDM.

OTP See one-time password.

Out of Frame A T1 alarm state that occurs when the receiver can no longer consistently identify the frame. See LOF.

outer label An MPLS term referring to the first of several labels when an MPLS-forwarded packet has multiple labels (a label stack).

Outside Global address A NAT term describing an IP address representing a host that resides outside the enterprise network, with the address being used in packets outside the enterprise network.

Outside Local address A NAT term describing an IP address representing a host that resides outside the enterprise network, with the address being used in packets inside the enterprise network.

overlapping VPN An MPLS term describing designs in which one or more MPLS customer sites can be reached from multiple other VPNs.

overloading Another term for Port Address Translation. See PAT

Network Glossary N

NA See Neighbor Advertisement.

NAT See Network Address Translation.

NAT-PT See Network Address Translation-Protocol Translation.

native VLAN The one VLAN on an 802.1Q trunk for which the endpoints do not add the 4-byte 802.1Q tag when transmitting frames in that VLAN.

NBAR See Network Based Application Recognition.

NCP See Network Control Protocol.

ND See Neighbor Discovery Protocol.

neighbor (EIGRP) With EIGRP, a router sharing the same primary subnet, with which Hellos are exchanged, parameters match, and with which routes can be exchanged.

neighbor (OSPF) Any other router, sharing a common data link, with which a router exchanges Hellos, and for which the parameters in the Hello pass the parameter-check process.

Neighbor Advertisement In IPv6, the Neighbor Discovery message used by an IPv6 node to send information about itself to its neighbors.

Neighbor Discovery Protocol The protocol used in IPv6 for many functions, including address autoconfiguration, duplicate address detection, router, neighbor, and prefix discovery, neighbor address resolution, and parameter discovery.

Neighbor Solicitation In IPv6, the Neighbor Discovery message used by an IPv6 node to request information about a neighbor or neighbors.

neighbor state A state variable kept by a router for each known neighbor or potential neighbor.

Neighbor Type In BGP, either external BGP (eBGP), confederation eBGP, or internal BGP(iBGP). The term refers to a peer connection, and whether the peers are in different ASs(eBGP), different confederation sub-ASs (confederation eBGP), or in the same AS (iBGP).

nested policy maps An MQC configuration style by which one policy map calls a second policy map. For example, a shaping policy map can call an LLQ policy map to implement LLQ for packets shaped by CB Shaping.

Network Address Translation Defined in RFC 1631, a method of translating IP addresses in headers with the goal of allowing multiple hosts to share single public IP addresses, thereby reducing IPv4 public address depletion.

Network Address Translation-Protocol Translation As defined in RFCs 2765 and 2766, a method of translating between IPv4 and IPv6 that removes the need for hosts to run dual protocol stacks. NAT-PT is an alternative to tunneling IPv6 over an IPv4 network, or vice versa.

network allocation vector A time value that each wireless station must set based on the duration value found in every 802.11 frame. The time value counts down and must be equal to zero before a station is allowed to access the wireless medium. The result is a collision-avoidance mechanism.

Network Based Application Recognition A Cisco IOS feature that performs deep packet inspection to classify packets based on application layer information.

Network Control Protocol The portions of PPP focused on features that are related to specific Layer 3 protocols.

network layer reachability information A BGP term referring to an IP prefix and prefix length.

Network Time Protocol An Internet standard (RFC 1305) that defines the messages and modes used for IP hosts to synchronize their time-of-day clocks.

network type (OSPF) A characteristic of OSPF interfaces that determines whether a DR election is attempted, whether or not neighbors must be statically configured, and the default Hello and Dead timer settings.

Next Hop field With a routing update, or routing table entry, the portion of a route that defines the next router to which a packet should be sent to reach the destination subnet. With routing protocols, the Next Hop field may define a router other than the router sending the routing update.

NEXT_HOP A BGP path attribute that lists the next-hop IP address used to reach an NLRI.

NLPID Network Layer Protocol ID is a field in the RFC 2427 header that is used as a Protocol Type field in order to identify the type of Layer 3 packet encapsulated inside a Frame Relay frame.

NLRI See network layer reachability information.

no drop A WRED process by which WRED does not discard packets during times in which a queue’s minimum threshold has not been passed.

NO_ADVERT A reserved value for the BGP COMMUNITY path attribute that implies that the route should not be advertised to any other peer.

NO_EXPORT A reserved value for the BGP COMMUNITY path attribute that implies that the route should not be advertised outside the local AS.

NO_EXPORT_SUBCONFED The RFC 1997 name for the reserved COMMUNITY path attribute known to Cisco IOS as LOCAL_AS. (See LOCAL_AS.)

not-so-stubby area A type of OSPF stub area that, unlike stub areas, can inject external routes into the NSSA area.

NS See Neighbor Solicitation.

NSSA See not-so-stubby area.

NTP See Network Time Protocol.

NTP broadcast client An NTP client that assumes that a server will send NTP broadcasts, removing the requirement for the client to have the NTP server’s IP address preconfigured.

NTP client mode An NTP mode in which an NTP host adjusts its clock in relation to an NTP server’s clock.

NTP server mode An NTP mode in which an NTP host does not adjust its clock, but in which it sends NTP messages to clients so that the clients can update their clocks based on the server’s clock.

NTP symmetric active mode An NTP mode in which two or more NTP servers mutually synchronize their clocks.

Network Glossary M

Management Information Base The definitions for a particular set of data variables, with those definitions following the SMI specifications. See also SMI.

man-in-the-middle attack A characterization of a network attack in which packets flow to the attacker, and then out to the true recipient. As a result, the user continues to send data, increasing the chance that the attacker learns more and better information.

map class An FRTS configuration construct, configured with the map-class frame-relay global configuration command.

mark probability denominator Used by WRED to calculate the maximum percentage of packets discarded when the average queue depth falls between the minimum and maximum thresholds.

marking down Jargon referring to a policer action through which, instead of discarding an out-of-contract packet, the policer marks a different IPP or DSCP value, allowing the packet to continue on its way, but making the packet more likely to be discarded later.

MaxAge An OSPF timer that determines how long an LSA can remain in the LSDB without having heard a reflooded copy of the LSA.

Maxage timer An STP timer that dictates how long a switch should wait when it ceases to hear Hellos.

maximum reserved bandwidth A Cisco IOS interface setting, as a percentage between 1 and 99, that defines how much of the interface’s bandwidth setting may be allocated by a queuing tool. The default value is 75 percent.

Maximum Response Time After a host receives an IGMP Query, the amount of time (default, 10 seconds) the host has to send the IGMP Report.

Maximum Segment Size A TCP variable that defines the largest number of bytes allowed in a TCP segment’s Data field. The calculation does not include the TCP header. With a typical IP MTU of 1500 bytes, the resulting default MSS would be 1460. TCP hosts must support an MSS of at least 536 bytes

maximum threshold WRED compares this setting to the average queue depth to decide whether packets should be discarded. All packets are discarded if the average queue depth rises above this maximum threshold.

maximum transmission unit An IP variable that defines the largest size allowed in an IP packet, including the IP header. IP hosts must support an MTU of at least 576 bytes.

MD5 See Message Digest 5.

MD5 hash A term referring to the process of applying the Message Digest 5 (MD5) algorithm to a string, resulting in another value. The original string cannot be easily computed even when the hash is known, making this process a strong method for storing passwords.

MDRR See Modified Deficit Round-Robin.

Measured Round-Trip Time A TCP variable used as the basis for a TCP sender’s timer defining how long it should wait for a missing acknowledgement before resending the data.

Message Digest 5 A method of applying a mathematical formula, with input including a private key, the message contents, and sometimes a shared text string, with the resulting digest being included with the message. The sender and the receiver perform the same math to allow authentication and to prove that no intermediate device changed the message contents.

metric With routing protocols, the measurement of favorability that determines which entry will be installed in a routing table if more than one router is advertising that exact network and mask.

MIB See Management Information Base.

MIB walk In SNMP, the process of a manager using successive GetNext and GetBulk commands to discover the exact MIB structure supported by an SNMP agent. The process involves the manager asking for each successive MIB leaf variable.

MIB-I The original standardized set of generic SNMP MIB variables, defined in RFC 1158.

MIB-II The most recent standardized set of generic SNMP MIB variables, defined in RFC 1213 and updated in RFCs 2011 through 2013.

mincir See minimum CIR.

minimum CIR Jargon referring to the minimum value to which adaptive shaping will lower the shaping rate.

minimum threshold WRED compares this setting to the average queue depth to decide whether packets should be discarded. No packets are discarded if the average queue depth falls below this minimum threshold.

MLD See Multicast Listener Discovery.

MLP See Multilink PPP.

MLP LFI The PPP function for fragmenting packets, plus interleaving delay-sensitive later-arriving packets between the fragments of the first packet.

MLS See Multilayer Switching.

Modified Deficit Round-Robin A Cisco 12000 series router feature that combines the key features of LLQ and CQ to provide similar congestion-management features.

modified tail drop A WFQ term referring to its drop logic, which is similar to tail-drop behavior.

Modular QoS CLI The common set of IOS configuration commands that is used with each QoS feature whose name begins with “Class-Based.”

MOSPF See Multicast Open Shortest Path First.

MPD See mark probability denominator.

MPLS Experimental (EXP) A 3-bit field in an MPLS header used for marking frames.

MPLS TTL propagation The MPLS feature by which an ingress E-LSR copies the IP packet’s IP TTL field into the MPLS header’s TTL field.

MPLS unicast IP routing The simplest MPLS application, involving the advertisement of an IGP to learn IP routes, and LDP or TDP to advertise labels.

MPLS VPNs An MPLS application that allows the MPLS network to connect to multiple different IP networks, with overlapping IP addresses, and provide IP connectivity to those multiple networks.

MQC See Modular QoS CLI.

MRT See Maximum Response Time.

MRTT See Measured Round-Trip Time.

MSS See Maximum Segment Size.

MST See Multiple Spanning Trees.

MTU See maximum transmission unit.

MULTI_EXIT_DISC (MED) A BGP path attribute that allows routers in one AS to set a value and advertise it into a neighboring AS, impacting the decision process in that neighboring AS. A smaller value is considered better. Also called the BGP metric.

multi-action policing In MQC and CB Policing, a configuration style by which, for one category of packets (conform, exceed, or violate), more than one marking action is defined for a single category. For example, marking DSCP and DE.

multicast A type of IPv4 and IPv6 traffic designed primarily to provide one-to-many connectivity but unlike broadcast, has the capability to control the scope of traffic distribution.

multicast IP address range IP multicast address range from 224.0.0.0 through 239.255.255.255.

multicast IP address structure The first 4 bits of the first octet must be 1110. The last 28 bits are unstructured.

Multicast Listener Discovery The IPv6 protocol used for the discovery of which hosts are listening for which multicast IP addresses for IPv6.

multicast MAC address A 48-bit address that is calculated from a Layer 3 multicast address by using 0x0100.5E as the multicast vendor code (OUI) for the first 24 bits, always binary 0 for the 25th bit, and copying the last 23 bits of the Layer 3 multicast address.

Multicast Open Shortest Path First A multicast routing protocol that operates in dense mode and depends on the OSPF unicast routing protocol to perform its multicast functions.

multicast scoping The practice of defining boundaries that determine how far multicast traffic will travel in your network.

multicast state information The information maintained by a router for each multicast entry in its multicast routing table, such as incoming interface, outgoing interface list, Uptime timer, Expire timer, etc.

multicasting Sending a message from a single source or multiple sources to selected multiple destinations across a Layer 3 network in one data stream.

Multilayer Switching A process whereby a switch, when making a forwarding decision, uses not only Layer 2 logic but other OSI layer equivalents as well.

Multilink PPP A PPP feature used to load balance multiple parallel links at Layer 2 by fragmenting frames, sending one frame over each of the links in the bundle, and reassembling them at the receiving end of the link.

multipath An issue whereby parts of the RF signal take different paths from the source to the destination, which causes direct and reflected signals to reach the receiver at different times, and corresponding bit errors.

Multiple Spanning Trees Defined in IEEE 802.1s, a specification for multiple STP instances when using 802.1Q trunks.

Network Glossary J K L

Join/Prune message Sent by a PIM router to its upstream router to either request that the upstream router forward the group traffic or stop forwarding the group traffic that is currently being forwarded. If a PIM router wants to start receiving the group traffic, it lists the group address under the Join field. If it wants the upstream router to stop forwarding the group traffic, it lists the group address under the Prune field.

joining a group The process of installing a multicast application; also referred to as launching an application.

K value EIGRP (and IGRP) allows for the use of bandwidth, load, delay, MTU, and link reliability; the K values refer to an integer constant that includes these five possible metric components. Only bandwidth and delay are used by default, to minimize recomputation of metrics for small changes in minor metric components.

label binding In MPLS, the mapping of an IP prefix and a label, which is then advertised to neighbors using LDP.

Label Distribution Protocol The RFC-standard MPLS protocol used to advertise the binding (mapping) information about each particular IP prefix and associated label. See also TDP.

Label Forwarding Information Base An MPLS data structure used for forwarding labeled packets. The LFIB lists the incoming label, which is compared to the incoming packet’s label, along with forwarding instructions for the packet.

Label Switch Router An MPLS term referring to any device that can forward packets that have MPLS labels.

label switched path The combination of MPLS labels and links over which a packet will be forwarded over an MPLS network, from the point of ingress to the MPLS network to the point of egress.

LACP See Link Aggregation Control Protocol.

LAPF See Link Access Procedure for Frame-Mode Bearer Services.

Layer 2 payload compression The process of taking the payload inside a Layer 2 frame, including the headers of Layer 3 and above, compressing the data, and then uncompressing the data on the receiving router.

Layer 2 protocol tunneling Another name for 802.1Q-in-Q. See 802.1Q-in-Q.

Layer x PDU The PDU used by a particular layer of a networking model, with x defining the layer.

LCP See Link Control Protocol.

LDP See Label Distribution Protocol.

Lead Content Engine The content engine in a WCCP cluster, which determines how traffic will be distributed within the cluster.

learning state An 802.1d STP transitory port state in which the port does not send or receive frames, but does learn the source MAC addresses from incoming frames.

LFI See Link Fragmentation and Interleaving.

LFIB See Label Forwarding Information Base.

limiting query scope (EIGRP) An effort to reduce the query scope with EIGRP, using route summarization or EIGRP stub routers.

line coding See encoding.

Link Access Procedure for Frame-Mode Bearer Services An ITU standard Frame Relay header, including the DLCI, DE, FECN, and BECN bits in the LAPF header, and a frame check in the LAPF trailer.

Link Aggregation Control Protocol Defined in IEEE 802.1AD, defines a messaging protocol used to negotiate the dynamic creation of PortChannels (EtherChannels) and to choose which ports can be placed into an EtherChannel.

Link Control Protocol The portion of PPP focused on features that are unrelated to any specific Layer 3 protocol.

Link Fragmentation and Interleaving The process of breaking a frame into pieces, sending some of the fragments, and then sending all or part of a different packet, all of which is done to reduce the delay of the second packet.

link-local An address type in IPv6 networks that is used only on the local link and never beyond that scope.

Link-State Acknowledgment A type of OSPF packet used to acknowledge LSU packets.

link-state advertisement The OSPF data structure that describes topology information.

link-state database The data structure used by OSPF to hold LSAs.

link-state routing protocol Any routing protocol that uses the concept of using the SPF algorithm with an LSDB to compute routes.

Link-State Update A type of OSPF packet, used to communicate LSAs to another router.

listening state An 802.1d STP transitory port state in which the port does not send or receive frames, and does not learn MAC addresses, but does wait for STP convergence and for CAM flushing by the switches in the network.

LLQ See low-latency queuing.

LMI See Local Management Interface.

local computation An EIGRP router’s reaction to an input event, leading to the use of a feasible successor or going active on a route.

local label In MPLS, a term used to define a label that an LSR allocates and then advertises to neighboring routers. The label is considered “local” on the router that allocates and advertises the label.

LOCAL_AS A reserved value for the BGP COMMUNITY path attribute that implies that the route should not be advertised outside the local confederation sub-AS.

Local Management Interface The Frame Relay protocol used between a DCE and DTE to manage the connection. Signaling messages for SVCs, PVC Status messages, and keepalives are all LMI messages.

LOCAL_PREF A BGP path attribute that is communicated throughout a single AS to signify which route of multiple possible routes is the best route to be taken when leaving that AS. A larger value is considered to be better.

LOF See Loss of Frame.

Loop Guard Protects against problems caused by unidirectional links between two switches. Watches for loss of received Hello BPDUs, in which case it transitions to a loop-inconsistent state instead of transitioning to a forwarding state.

loopback circuitry A feature of Ethernet NICs. When the NIC transmits an electrical signal, it “loops” the transmitted electrical current back onto the receive pair. By doing so, if another NIC transmits a frame at the same time, the NIC can detect the overlapping received electrical signals, and sense that a collision has occurred.

LOS Loss of Signal. A T1 alarm state that occurs when the receiver has not received any pulses of either polarity for a defined time period.

Loss of Frame A T1 alarm state that occurs when the receiver can no longer consistently identify the frame.

low-latency queuing A Cisco IOS queuing tool that uses MQC configuration commands, reserves a minimum bandwidth for some queues, provides high-priority scheduling for some queues, and polices those queues to prevent starvation of lower-priority queues during interface congestion.

LSA See link-state advertisement.

LSA flooding The process of successive neighboring routers exchanging LSAs such that all routers have an identical LSDB for each area to which they are attached.

LSA type (OSPF) A definition that determines the data structure and information implied by a particular LSA.

LSAck See Link-State Acknowledgment.

LSDB See link-state database.

LSP See label switched path.

LSP segment A single label and link that is part of a complete LDP. See also label switched path.

LSR See Label Switch Router.

LSRefresh Link-State Refresh. A timer that determines how often the originating router should reflood an LSA, even if no changes have occurred to the LSA.

LSU See Link-State Update.

LxPDU See Layer x PDU.

LZS The Lempel Ziv STAC compression algorithm is used in Frame Relay networks to define dynamic dictionary entries that list a binary string from the compressed data and an associated smaller string that represents it during transmission—thereby reducing the number of bits used to send data.

Network Glossary I

I/G bit The most significant bit in the most significant byte of an Ethernet MAC address, its value implies that the address is a unicast MAC address (binary 0) or not (binary 1).

iBGP See Internal BGP.

IEEE 802.1X An IEEE standard that, when used with EAP, provides user authentication before their connected switch port allows the device to fully use the LAN.

IGMP See Internet Group Management Protocol.

IGMP snooping A method for optimizing the flow of multicast IP packets passing through a LAN switch. The switch using IGMP snooping examines IGMP messages to determine which ports need to receive traffic for each multicast group.

IGMPv1 Host Membership Query A message sent by the multicast router, by default every 60 seconds, on each of its LAN interfaces to determine whether any host wants to receive multicast traffic for any group.

IGMPv1 Host Membership Report A message that each host sends, either in response to a router Query message or on its own, to all multicast groups for which it would like to receive multicast traffic.

IGMPv2 Group-Specific Query A message sent by a router, after receiving a Leave message from a host, to determine whether there are still any active members of the group. The router uses the group address as the destination address.

IGMPv2 Host Membership Query A message sent by a multicast router, by default every 125 seconds, on each of its LAN interfaces to determine whether any host wants to receive multicast traffic for any group.

IGMPv2 Host Membership Report A message sent by each host, either in response to a router Query or on its own, to all multicast groups for which it would like to receive multicast traffic.

IGMPv2 Leave A message sent by a host when it wants to leave a group, addressed to the All Multicast Routers address 224.0.0.2.

IGMPv3 Host Membership Query A message sent by a multicast router, by default every 125 seconds, on each of its LAN interfaces to determine whether any host wants to receive multicast traffic for any group.

IGMPv3 Host Membership Report A message sent by each host, either in response to a router query or on its own, to all multicast groups for which it would like to receive multicast traffic. The destination address on the Report is 224.0.0.22, and a host can specify the source address(es) from which it would like to receive the group traffic.

InARP See Inverse ARP.

Inform In the context of SNMP, the Inform command is sent by an SNMP manager to communicate a set of variables, and their values, to another SNMP manager. The main purpose is to allow multiple managers to exchange MIB information, and work together, without requiring each manager to individually use Get commands to gather the data.

infrastructure mode A wireless LAN that includes the use of access points. Infrastructure mode connects wireless users to a wired network and allows wireless users to roam throughout a facility between different access points. All 802.11 data frames in an infrastructure wireless LAN travel through the access point.

ingress PE An E-LSR in an MPLS VPN network whose role in a particular discussion is to receive unlabeled packets over customer links and then forward the packets as labeled packets into the MPLS network.

inner label An MPLS term referring to the MPLS label just before the IP header. Also called the VPN label when implementing MPLS VPNs.

input event Any occurrence that could change a router’s EIGRP topology table, including a received Update or Query, a failed interface, or the loss of a neighbor.

Inside Global address A NAT term describing an IP address representing a host that resides inside the enterprise network, with the address being used in packets outside the enterprise network.

Inside Local address A NAT term describing an IP address representing a host that resides inside the enterprise network, with the address being used in packets inside the enterprise network.

inspection rule A set of parameters for CBAC to perform in its traffic inspection process.

interface ID 64 bits at the end of an IPv6 global address, used to uniquely identify each host in a subnet.

Inter-Switch Link Cisco-proprietary VLAN trunking protocol.

internal BGP Refers to how a router views a BGP peer relationship, in which the peer is in the same AS.

internal DSCP A term used with Cisco LAN switches, referring to a DSCP value used when making QoS decisions about a frame. This value may not be the actual DSCP value in the IP header encapsulated inside the frame.

internal router (OSPF) A router that is not an ABR or ASBR in that all of its interfaces connect to only a single OSPF area.

Internet Group Management Protocol A communication protocol between hosts and a multicast router by which routers learn of which multicast groups’ packets need to be forwarded onto a LAN.

Invalid timer With RIP, a per-route timer that increases until the router receives a routing update that confirms the route is still valid, upon which the timer is reset to 0. If the updates cease, the Invalid timer will grow, until reaching the timer setting (default 180 seconds), after which the route is considered invalid.

Inverse ARP Defined in RFC 1293, this protocol allows a Frame Relay–attached device to react to a received LMI “PVC up” message by announcing its Layer 3 addresses to the device on the other end of the PVC.

IPCP See IP Control Protocol.

IP Control Protocol The portion of PPP focused on negotiating IP features—for example, TCP or RTP header compression.

IP forwarding The process of forwarding packets through a router. Also call IP routing.

IP PBX A component that interfaces with a phone using IP and provides connections to the Public Switched Telephone Network (PSTN).

IP Precedence A 3-bit field in the first 3 bits of the ToS byte in the IP header, used for QoS marking.

IP prefix list See prefix list.

IP routing The process of forwarding packets through a router. Also called IP forwarding.

IP Source Guard A switch feature that examines incoming frames, comparing the source IP and MAC addresses to the DHCP snooping binding database, filtering frames whose addresses are not listed in the database for the incoming interface.

IPv4 Version 4 of the IP protocol, which is the generally deployed version worldwide (at publication), and uses 32-bit IP addresses.

IPv6 Version 6 of the IP protocol, which uses 128-bit IP addresses.

ISATAP An IPv6/IPv4 tunneling method that is designed for transporting IPv6 packets within a site where a native IPv6 infrastructures is not available.

ISL See Inter-Switch Link.

isolated VLAN With private VLANs, a secondary VLAN in which the ports can send and receive frames only with promiscuous ports in the primary VLAN.