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Glossary

control direct VCC

One of three control connections defined by Phase I

LAN Emulation; a bidirectional virtual control connection (VCC) estab-
lished in ATM by an LEC to an LES. See also: control distribute VCC.

control distribute VCC

One of three control connections defined by

Phase 1 LAN Emulation; a unidirectional virtual control connection (VCC)
set up in ATM from an LES to an LEC. Usually, the VCC is a point-to-
multipoint connection. See also: control direct VCC.

convergence

The process required for all routers in an internetwork to

update their routing tables and create a consistent view of the network, using
the best possible paths. No user data is passed during a convergence time.

core block

If you have two or more switch blocks, the Cisco rule of thumb

states that you need a core block. No routing is performed at the core, only
transferring of data. It is a pass-through for the switch block, the server
block, and the Internet. The core is responsible for transferring data to and
from the switch blocks as quickly as possible. You can build a fast core with
a frame, packet, or cell (ATM) network technology.

Core layer

Top layer in the Cisco three-layer hierarchical model, which

helps you design, build, and maintain Cisco hierarchical networks. The Core
layer passes packets quickly to Distribution layer devices only. No packet fil-
tering should take place at this layer.

cost

An arbitrary value, based on hop count, bandwidth, or other calcu-

lation, that is typically assigned by a network administrator and used by the
routing protocol to compare different routes through an internetwork.
Routing protocols use cost values to select the best path to a certain destina-
tion: The lowest cost identifies the best path. Also known as "path cost."
See also: routing metric.

count to infinity

A problem occurring in routing algorithms that are slow

to converge where routers keep increasing the hop count to particular net-
works. To avoid this problem, various solutions have been implemented into
each of the different routing protocols. Some of those solutions include
defining a maximum hop count (defining infinity), route poisoning, poison
reverse, and split horizon.

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