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									HDLC - High Level Data Link Control



Protocol Overall Description:

Layer 2 of the OSI model is the data link layer. One of the most common layer 2
protocols is the HDLC protocol. In fact, many other common layer 2 protocols are
heavily based on HDLC, particularly its framing structure: namely, SDLC, SS#7, LAPB
,LAPD and ADCCP. The basic framing structure of the HDLC protocol is shown below:

HDLC uses zero insertion/deletion process (commonly known as bit stuffing) to ensure
that the bit pattern of the delimiter flag does not occur in the fields between flags. The
HDLC frame is synchronous and therefore relies on the physical layer to provide method
of clocking and synchronizing the transmission and reception of frames.

The HDLC protocol is defined by ISO for use on both point-to-point and multipoint
(multidrop) data links. It supports full duplex transparent-mode operation and is now
extensively used in both multipoint and computer networks.

HDLC Operation Modes:

HDLC has three operational modes:

   1. Normal Response Mode (NRM)
   2. Asynchronous Response Mode (ARM)
   3. Asynchronous Balanced Mode (ABM)

Protocol operation

The two basic functions in the protocol are link management and data transfer (which
includes error and flow control).

Link management

. Prior to any kind of transmission (either between two stations connected by a point to
point link or between a primary and secondary station a multidrop link) a logical
connection between the two communication parties must be established.

Data transfer

. In NRM all data (information frames) if transferred under the control of the primary
station. The unnumbered poll frame with the P bit set to 1 is normally used by the
primary to poll a secondary. If the secondary has no data to transmit, it returns an RNR
frame with the F bit set. If data is waiting, it transmits the data, typically as a sequence of
information                                                                             frames.
The two most important aspects associated with the data transfer phase are error control
and flow control. Essentially, error control uses a continues RQ procedure with either a
selective repeat or a go back N transmission strategy, while flow controls based on a
window mechanism.




IGRP
Introduction

This document introduces Interior Gateway Routing Protocol (IGRP). It has two
purposes. One is to form an introduction to the IGRP technology, for those who are
interested in using, evaluating, and possibly implementing it. The other is to give wider
exposure to some interesting ideas and concepts that are embodied in IGRP. Refer to
Configuring IGRP, The Cisco IGRP Implementation and IGRP Commands for
information on how to configure IGRP.

Goals for IGRP

The IGRP protocol allows a number of gateways to coordinate their routing. Its goals are
the following:

      Stable routing even in very large or complex networks. No routing loops should
       occur, even as transients.
      Fast response to changes in network topology.
      Low overhead. That is, IGRP itself should not use more bandwidth than what is
       actually needed for its task.
      Splitting traffic among several parallel routes when they are of roughly equal
       desirability.
      Taking into account error rates and level of traffic on different paths.

The current implementation of IGRP handles routing for TCP/IP. However, the basic
design is intended to be able to handle a variety of protocols.

No one tool is going to solve all routing problems. Conventionally the routing problem is
broken into several pieces. Protocols such as IGRP are called "internal gateway
protocols" (IGPs). They are intended for use within a single set of networks, either under
a single management or closely coordinated managements. Such sets of networks are
connected by "external gateway protocols" (EGPs). An IGP is designed to keep track of a
good deal of detail about network topology. Priority in designing an IGP is placed on
producing optimal routes and responding quickly to changes. An EGP is intended to
protect one system of networks against errors or intentional misrepresentation by other
systems, BGP is one such Exterior gateway protocol.. Priority in designing an EGP is on
stability and administrative controls. Often it is sufficient for an EGP to produce a
reasonable route, rather than the optimal route.
IGRP has some similarities to older protocols such as Xerox's Routing Information
Protocol, Berkeley's RIP, and Dave Mills' Hello. It differs from these protocols primarily
in being designed for larger and more complex networks. See the Comparison with RIP
section for a more detailed comparison with RIP, which is the most widely used of the
older generation of protocols.

Like these older protocols, IGRP is a distance vector protocol. In such a protocol,
gateways exchange routing information only with adjacent gateways. This routing
information contains a summary of information about the rest of the network. It can be
shown mathematically that all of the gateways taken together are solving an optimization
problem by what amounts to a distributed algorithm. Each gateway only needs to solve
part of the problem, and it only has to receive a portion of the total data.

The major alternative to IGRP is Enhanced IGRP (EIGRP) and a class of algorithms
referred to as SPF (shortest- path first). OSPF uses this concept. To learn more about
OSPF refer to OSPF Design Guide. OSPF These are is based on a flooding technique,
where every gateway is kept up to date about the status of every interface on every other
gateway. Each gateway independently solves the optimization problem from its point of
view using data for the entire network. There are advantages to each approach. In some
circumstances SPF may be able to respond to changes more quickly. In order to prevent
routing loops, IGRP has to ignore new data for a few minutes after certain kinds of
changes. Because SPF has information directly from each gateway, it is able to avoid
these routing loops. Thus it can act on new information immediately. However, SPF has
to deal with substantially more data than IGRP, both in internal data structures and in
messages between gateways.

EIGRP
The term you selected is being presented by searchNetworking.com, a TechTarget site for
Networking professionals.

 EIGRP (Enhanced Interior Gateway Routing Protocol) is a network protocol that lets
 routers exchange information more efficiently than with earlier network protocols.
EIGRP evolved from IGRP (Interior Gateway Routing Protocol) and routers using either
EIGRP and IGRP can interoperate because the metric (criteria used for selecting a route)
used with one protocol can be translated into the metrics of the other protocol. EIGRP
can be used not only for Internet Protocol (IP) networks but also for AppleTalk and
Novell NetWare networks.

Using EIGRP, a router keeps a copy of its neighbor's routing tables. If it can't find a route
to a destination in one of these tables, it queries its neighbors for a route and they in turn
query their neighbors until a route is found. When a routing table entry changes in one of
the routers, it notifies its neighbors of the change only (some earlier protocols require
sending the entire table). To keep all routers aware of the state of neighbors, each router
sends out a periodic "hello" packet. A router from which no "hello" packet has been
received in a certain period of time is assumed to be inoperative.
EIGRP uses the Diffusing-Update Algorithm (DUAL) to determine the most efficient
(least cost) route to a destination. A DUAL finite state machine contains decision
information used by the algorithm to determine the least-cost route (which considers
distance and whether a destination path is loop-free).


Border Gateway Protocol
Introduction

The Border Gateway Protocol (BGP) is an interautonomous system routing protocol. An
autonomous system is a network or group of networks under a common administration
and with common routing policies. BGP is used to exchange routing information for the
Internet and is the protocol used between Internet service providers (ISP). Customer
networks, such as universities and corporations, usually employ an Interior Gateway
Protocol (IGP) such as RIP or OSPF for the exchange of routing information within their
networks. Customers connect to ISPs, and ISPs use BGP to exchange customer and ISP
routes. When BGP is used between autonomous systems (AS), the protocol is referred to
as External BGP (EBGP). If a service provider is using BGP to exchange routes within an
AS, then the protocol is referred to as Interior BGP (IBGP). Figure 39-1 illustrates this
distinction.


Figure 39-1              External              and             Interior            BGP




BGP is a very robust and scalable routing protocol, as evidenced by the fact that BGP is
the routing protocol employed on the Internet. At the time of this writing, the Internet
BGP routing tables number more than 90,000 routes. To achieve scalability at this level,
BGP uses many route parameters, called attributes, to define routing policies and
maintain a stable routing environment.

In addition to BGP attributes, classless interdomain routing (CIDR) is used by BGP to
reduce the size of the Internet routing tables. For example, assume that an ISP owns the
IP address block 195.10.x.x from the traditional Class C address space. This block
consists of 256 Class C address blocks, 195.10.0.x through 195.10.255.x. Assume that the
ISP assigns a Class C block to each of its customers. Without CIDR, the ISP would
advertise 256 Class C address blocks to its BGP peers. With CIDR, BGP can supernet the
address space and advertise one block, 195.10.x.x. This block is the same size as a
traditional Class B address block. The class distinctions are rendered obsolete by CIDR,
allowing a significant reduction in the BGP routing tables.

BGP neighbors exchange full routing information when the TCP connection between
neighbors is first established. When changes to the routing table are detected, the BGP
routers send to their neighbors only those routes that have changed. BGP routers do not
send periodic routing updates, and BGP routing updates advertise only the optimal path
to a destination network.

BGP Attributes

Routes learned via BGP have associated properties that are used to determine the best
route to a destination when multiple paths exist to a particular destination. These
properties are referred to as BGP attributes, and an understanding of how BGP attributes
influence route selection is required for the design of robust networks. This section
describes the attributes that BGP uses in the route selection process:

      Weight
      Local preference
      Multi-exit discriminator
      Origin
      AS_path
      Next hop
      Community

Fiber Distributed Data Interface
Introduction

The Fiber Distributed Data Interface (FDDI) specifies a 100-Mbps token-passing, dual-
ring LAN using fiber-optic cable. FDDI is frequently used as high-speed backbone
technology because of its support for high bandwidth and greater distances than copper. It
should be noted that relatively recently, a related copper specification, called Copper
Distributed Data Interface (CDDI), has emerged to provide 100-Mbps service over
copper.              CDDI                   is             the              implementation
of FDDI protocols over twisted-pair copper wire. This chapter focuses mainly on FDDI
specifications and operations, but it also provides a high-level overview of CDDI.

FDDI uses dual-ring architecture with traffic on each ring flowing in opposite directions
(called counter-rotating). The dual rings consist of a primary and a secondary ring.
During normal operation, the primary ring is used for data transmission, and the
secondary ring remains idle. As will be discussed in detail later in this chapter, the
primary purpose of the dual rings is to provide superior reliability and robustness. Figure
8-1 shows the counter-rotating primary and secondary FDDI rings.


Figure 8-1:   FDDI     Uses    Counter-Rotating      Primary    and    Secondary     Rings




Standards

FDDI was developed by the American National Standards Institute (ANSI) X3T9.5
standards committee in the mid-1980s. At the time, high-speed engineering workstations
were beginning to tax the bandwidth of existing local-area networks (LANs) based on
Ethernet and Token Ring. A new LAN media was needed that could easily support these
workstations and their new distributed applications. At the same time, network reliability
had become an increasingly important issue as system managers migrated mission-critical
applications from large computers to networks. FDDI was developed to fill these needs.
After completing the FDDI specification, ANSI submitted FDDI to the International
Organization for Standardization (ISO), which created an international version of FDDI
that is completely compatible with the ANSI standard version.

FDDI Transmission Media

FDDI uses optical fiber as the primary transmission medium, but it also can run over
copper cabling. As mentioned earlier, FDDI over copper is referred to as Copper-
Distributed Data Interface (CDDI). Optical fiber has several advantages over copper
media. In particular, security, reliability, and performance all are enhanced with optical
fiber media because fiber does not emit electrical signals. A physical medium that does
emit electrical signals (copper) can be tapped and therefore would permit unauthorized
access to the data that is transiting the medium. In addition, fiber is immune to electrical
interference from radio frequency interference (RFI) and electromagnetic interference
(EMI). Fiber historically has supported much higher bandwidth (throughput potential)
than copper, although recent technological advances have made copper capable of
transmitting at 100 Mbps. Finally, FDDI allows 2 km between stations using multimode
fiber, and even longer distances using a single mode.
FDDI defines two types of optical fiber: single-mode and multimode. A mode is a ray of
light that enters the fiber at a particular angle. Multimode fiber uses LED as the light-
generating device, while single-mode fiber generally uses lasers.

Multimode fiber allows multiple modes of light to propagate through the fiber. Because
these modes of light enter the fiber at different angles, they will arrive at the end of the
fiber at different times. This characteristic is known as modal dispersion. Modal
dispersion limits the bandwidth and distances that can be accomplished using multimode
fibers. For this reason, multimode fiber is generally used for connectivity within a
building or a relatively geographically contained environment.

Single-mode fiber allows only one mode of light to propagate through the fiber. Because
only a single mode of light is used, modal dispersion is not present with single-mode
fiber. Therefore, single-mode fiber is capable of delivering considerably higher
performance connectivity over much larger distances, which is why it generally is used
for connectivity between buildings and within environments that are more geographically
dispersed.

Figure 8-2 depicts single-mode fiber using a laser light source and multimode fiber using
a light emitting diode (LED) light source.


Figure 8-2: Light Sources Differ for Single-Mode and Multimode Fibers




FDDI Specifications

FDDI specifies the physical and media-access portions of the OSI reference model. FDDI
is not actually a single specification, but it is a collection of four separate specifications,
each with a specific function. Combined, these specifications have the capability to
provide high-speed connectivity between upper-layer protocols such as TCP/IP and IPX,
and media such as fiber-optic cabling.

FDDI's four specifications are the Media Access Control (MAC), Physical Layer
Protocol (PHY), Physical-Medium Dependent (PMD), and Station Management (SMT)
specifications. The MAC specification defines how the medium is accessed, including
frame format, token handling, addressing, algorithms for calculating cyclic redundancy
check (CRC) value, and error-recovery mechanisms. The PHY specification defines data
encoding/decoding procedures, clocking requirements, and framing, among other
functions. The PMD specification defines the characteristics of the transmission medium,
including fiber-optic links, power levels, bit-error rates, optical components, and
connectors. The SMT specification defines FDDI station configuration, ring
configuration, and ring control features, including station insertion and removal,
initialization, fault isolation and recovery, scheduling, and statistics collection.

FDDI is similar to IEEE 802.3 Ethernet and IEEE 802.5 Token Ring in its relationship
with the OSI model. Its primary purpose is to provide connectivity between upper OSI
layers of common protocols and the media used to connect network devices. Figure 8-3
illustrates the four FDDI specifications and their relationship to each other and to the
IEEE-defined Logical Link Control (LLC) sublayer. The LLC sublayer is a component of
Layer 2, the MAC layer, of the OSI reference model.


Figure 8-3:   FDDI     Specifications   Map     to   the   OSI   Hierarchical    Model




CDDI
Abbreviation of Copper Data Distribution Interface, a network technology capable of
carrying data at 100 Mbps over unshielded twisted pair (UTP) cable. CDDI is a trade
name of Crescendo Communications (acquired by Cisco Systems in 1993) and
commonly used instead of the general term Twisted Pair Physical Layer Medium (TP-
PMD). TP-PMD is the general ANSI standard name for this FDDI -like service.

CDDI cable lengths are limited to 100 meters.

								
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