Glossary W

V

VBR

Variable Bit Rate: A QoS class, as defined by the ATM Forum, for use in ATM networks that is subdivided into real time (RT) class and non-real time (NRT) class. RT is employed when connections have a fixed-time relationship between samples. Conversely, NRT is employed when connections do not have a fixed-time relationship between samples, but still need an assured QoS. Compare to: ABR and CBR.

VCC

Virtual Channel Connection: A logical circuit that is created by VCLs (virtual channel links). VCCs carry data between two endpoints in an ATM network. Sometimes called a virtual circuit connection.

VIP

(1) Versatile Interface Processor: An interface card for Cisco 7000 and 7500 series routers, providing multi-layer switching and running the Cisco IOS software. The most recent version of VIP is VIP2. (2) Virtual IP: A function making it possible for logically separated switched IP workgroups to run Virtual Networking Services across the switch ports of a Catalyst 5000.

virtual circuit

Abbreviated VC, a logical circuit devised to ensure end-to-end communication between two devices on a network. A virtual circuit can be permanent (PVC) or switched (SVC). Virtual circuits are used in Frame Relay and X.25. Known as 'virtual channels' in ATM. See also: PVC and SVC.

virtual ring

In an SRB network, a logical connection between physical rings, either local or remote.

VLAN

Virtual LAN: A group of devices on one or more logically segmented LANs (configured by use of management software), enabling devices to communicate as if attached to the same physical medium, when they are actually located on numerous different LAN segments. VLANs are based on logical instead of physical connections and thus are tremendously flexible.

VLAN database

A special mode in 2900XL and 3500XL series switches where the administrator creates VLANs.

VLSM

variable-length subnet mask: Helps optimize available address space and specify a different subnet mask for the same network number on various subnets. Also commonly referred to as 'subnetting a subnet.'

VPN

virtual private network: A method of encrypting point-to-point logical connections across a public network, such as the Internet. This allows secure communications across a public network.

VRRP

Virtual Router Redundancy Protocol: A standards-based alternative to Cisco's HSRP, providing a virtual router address for use as a host default gateway.

VTP

VLAN Trunk Protocol: Used to update switches in a switch-fabric about VLANs configured on a VTP server. VTP devices can be a VTP server, client, or transparent device. Servers update clients. Transparent devices are only local devices and do not share information with VTP clients. VTPs send VLAN information down trunked links only.

VTP modes

See: VTP.

VTP pruning

VLAN Trunk Protocol is used to communicate VLAN information between switches in the same VTP domain. VTP pruning stops VLAN update information from being sent down trunked links if the updates are not needed.