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.