Cisco CRS-1 - Carrier Routing System Router Getting Started Manual page 264

Glossary
Generic Route Encapsulation (an IP tunnel within IP). Tunneling protocol developed by Cisco and
GRE
adapted by the IETF as a standard protocol defined in several RFCs. GRE can encapsulate a wide
variety of protocol packet types inside IP tunnels, creating a virtual point-to-point link to Cisco routers
at remote points over an IP internetwork. By connecting multiprotocol subnetworks in a single-protocol
backbone environment, IP tunneling using GRE allows network expansion across a single-protocol
backbone environment.
graphical user interface. A user environment that uses pictorial and textual representations of the input and
GUI
the output of applications and the hierarchical or other data structure in which information is stored. Such
conventions as buttons, icons, and windows are typical, and many actions are performed using a pointing
device (such as a mouse). Microsoft Windows and the Apple Macintosh are prominent examples of
platforms using a GUI. See also CLI.
globally unique MAC address is the IEEE issued 48 bit MAC address. Cisco CRS-1 Series routers also
GUMA
use geographic MAC addresses internally.
H
High availability is defined as the continuous operation of systems. For a system to be available all
HA
components including application and database servers, storage devices, and the end-to-end network
need to provide continuous service.
high-level data link control. ISO communications protocol used in X.25 packet switching networks.
HDLC
HDLC provides error correction at the data link layer and contains the following subsets: LAPB, and
SDLC. See also LAPB and SDLC.
Multicast packet that is used by routers for neighbor discovery and recovery. Hello packets also indicate
hello packet
that a client is still operating and network-ready.
Protocol used by OSPF systems for establishing and maintaining neighbor relationships.
Hello protocol
A number system having 16 as its base. This number representation uses the digits 0–9, with their usual
hexadecimal
meaning, plus the letters A–F (or a–f) to represent hexadecimal digits with values of (decimal) 10 to
15. The right-most digit counts ones, the next counts multiples of 16, then 16^2 = 256, and so on.
Hexadecimal is more succinct than binary for representing bit-masks, machines addresses, and other
low-level constants but it is still reasonably easy to split a hex number into different bit positions. For
example, the top 16 bits of a 32-bit word are the first four hex digits.
Passage of a data packet between two network nodes (for example, between two routers). See also hop
hop
count.
Routing metric used to measure the distance between a source and a destination.
hop count
Hypertext Transfer Protocol. Used by web browsers and web servers to transfer files, such as text and
HTTP
graphic files. HTTP is the set of rules for exchanging files (text, graphic images, sound, video, and other
multimedia files) on the world wide web. Relative to the TCP/IP suite of protocols (which are the basis
for information exchange on the Internet), HTTP is an application protocol.
Cisco CRS-1 Series Carrier Routing System Getting Started Guide
GL-6