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DATA COMMUNICATIONS MANAGEMENT
DESIGNING LARGE-SCALE
LAN/WANS, PART I
John R. Vacca
INSIDE
Large-Scale Internetwork Protocol (IP) LAN/WANs; IBM Systems Network Architecture (SNA); ATM;
Packet Service; Dial-on-Demand Routing (DDR); ISDN; LAN/WAN System Design Solutions: Design Basics
INTRODUCTION
Every aspect of connecting computers together is encompassed by the
communication between two or more networks. Vastly disparate endsystem communication requirements are now being supported by
LAN/WANs. A LAN/WAN system requires many protocols and features to
permit scalability and manageability without constant manual intervention. Large-scale LAN/WAN systems can consist of the following three
distinct components:
1. campus networks, which consist of locally connected users (LANs) in
a building or group of buildings
2. remote connections, which link branch offices and single users (mobile users and telecommuters) to a local campus or the Internet
3. wide area networks (WANs), which connect campuses together
According to Cisco, an example of a typical enterprise LAN/WAN system is shown in Exhibit 1.
A challenging task is designing a LAN/WAN system. Designers must
realize that each of the three major components of a LAN/WAN system
have distinct design requirements in order to design reliable, scalable
systems. A LAN/WAN system that consists of only 80 meshed routing
nodes can pose complex problems
that lead to unpredictable results. AtPAYOFF IDEA
This article provides an overview of the technolotempting to optimize LAN/WAN sysgies available today to design large-scale
tems that feature hundreds of
LAN/WANs. Some of the key networking protothousands of nodes can pose even
cols and technologies applicable in today’s netmore complex problems.
works are discussed, and a basic understanding
of key LAN/WAN concepts is provided.
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EXHIBIT 1 — Example of a Typical LAN/WAN System
LAN/WAN system design is becoming more difficult, despite improvements in equipment performance and media capabilities. The trend is toward increasingly complex environments involving interconnection to
LAN/WANs outside any single enterprise’s dominion of control, multiple
media, and multiple protocols. Carefully designing LAN/WAN systems
can reduce the hardships associated with growth as a LAN/WAN environment evolves.
LARGE-SCALE INTERNETWORK PROTOCOL (IP) LAN/WANS
Now take a look at the technologies that are available today to design
large-scale LAN/WAN systems. The immediate focus will be on the following design implications of the Enhanced Interior Gateway Routing
Protocol (IGRP) and Open Shortest Path First (OSPF) protocol.
Enhanced Interior Gateway Routing Protocol (IGRP) Design
Developed by Cisco Systems, the Enhanced IGRP is a routing protocol.
Enhanced IGRP combines the advantages of link-state protocols, such as
OSPF, with the advantages of distance vector protocols, such as IGRP.
Enhanced IGRP uses the Diffusing Update Algorithm (DUAL) to achieve
convergence quickly.
OPEN SHORTEST PATH FIRST (OSPF) DESIGN
Developed for use in Internet Protocol (IP)-based internetworks,1 OSPF
is an Interior Gateway Protocol (IGP). OSPF (as an IGP) distributes routAuerbach Publications
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ing information between routers belonging to a single autonomous system (AS). An AS is a group of routers exchanging routing information via
a common routing protocol. The OSPF protocol is based on shortestpath-first, or link-state, technology.
The OSPF Working Group of the Internet Engineering Task Force
(IETF) developed the OSPF protocol. It was designed expressly for the
IP environment. This includes explicit support for IP subnetting and the
tagging of externally derived routing information. OSPF Version 2 is documented in Request for Comments (RFC) 1247.
Discussions that follow are divided into the following general topics:
•
•
•
•
•
IBM System Network Architecture (SNA)
ATM
Packet service
Dial-on-Demand Routing (DDR)
ISDN
IBM SYSTEMS NETWORK ARCHITECTURE (SNA)
This part of the article very briefly discusses:
• source-route bridging (SRB) design
• synchronous data link control (SDLC) and serial tunneling (STUN),
SDLC logiocal link control type 2 (SDLLC), and qualified logical link
control (QLLC) design
• advanced peer-to-peer networking (APPN) and data link switching
(DLSw+) design
Source-Route Bridging (SRB) Design
SRB technology was viewed as a local technology that would interconnect a few rings and terminate at a remote 3745 when IBM developed it
in the mid-1980s. When the scale exceeds what was originally intended
by IBM, the challenge for any SRB system occurs. This technology encounters problems when non-IBM protocols are required to coexist with
native Token Ring traffic. Source-route bridges were intended to be the
primary LAN/WAN system tool for creating an enterprisewide Token Ring
system. These bridges were never meant to scale to the level that many
customers require. Later in the article, further discussions address the
challenges of this environment and the ability of LAN/WAN designers to
successfully implement SRB2 within a large, multi-protocol topology.
SDLC, STUN, SDLLC, and QLLC Design
This part of the article briefly addresses some of the special requirements
for implementing routing technology within IBM System Network ArchiAuerbach Publications
© 2000 CRC Press LLC
tecture (SNA) environments. A LAN/WAN system within an SNA environment often involves making special accommodations for devices that
were not originally designed for connection to meshed systems. Upcoming in this article are three techniques designed to enable LAN/WAN systems capabilities for SNA-based LAN/WAN architectures; these are
described as follows:
1. SDLC via STUN
2. SDLLC implementation
3. QLLC conversion
In addition to the description of serial tunneling (STUN), synchronous
data link control (SDLC) over the Logical Link Control type 2 (LLC) protocol (SDLLC), and Qualified Logical Link Control (QLLC), focus will also be
on the following topics: router technology options, implementation guidelines, and configuration examples; and technology overview and issues.
Advanced Peer-to-Peer Networking (APPN) Design
A second generation of the Systems Network Architecture (SNA) from
IBM is advanced peer-to-peer networking (APPN). It moves SNA from a
hierarchical, mainframe-centric environment to a peer-to-peer environment. It also provides capabilities similar to other LAN protocols, such as
dynamic resource definition and route discovery.
Data Link Switching Plus (DLSw+) Design
Transporting Systems Network Architecture (SNA) and Network Basic Input/Output System (NetBIOS)3 traffic over a campus or wide area network (WAN) is known as DLSw+. The end systems can attach to the
network over fiber distributed data interface (FDDI), Token Ring, Ethernet, Synchronous Data Link Control (SDLC) protocol, or Qualified Logical Link Control (QLLC). (FDDI is supported on the Cisco 7000 series
only and requires Cisco IOS Release 11.2 or higher.) DLSw+ switches between diverse media and locally terminates the data links, keeping acknowledgments, keepalives,4 and polling off the WAN. Local termination
of data links also eliminates data-link control timeouts that can occur during transient LAN/WAN congestion or when rerouting around failed
links. Finally, DLSw+ provides a mechanism for dynamically searching a
LAN/WAN for SNA or NetBIOS resources and includes caching algorithms that minimize broadcast traffic.
ATM
Designed for the high-speed transfer of voice, video, and data through
public and private networks in a cost-effective manner, ATM is an evolvAuerbach Publications
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ing technology. ATM is based on the efforts to apply very large-scale integration (VLSI) technology to the transfer of data within public networks
by Study Group XVIII of the International Telecommunication Union
Telecommunication Standardization Sector (ITU-T, formerly the Consultative Committee for International Telegraph and Telephone [CCITT])
and the American National Standards Institute (ANSI). Officially, the ATM
layer of the Broadband Integrated Services Digital Network (BISDN)
model is defined by CCITT I.361. Current efforts to bring ATM technology to private networks and to guarantee interoperability between private
and public networks is being done by the ATM Forum, which was jointly
founded in 1991 by Cisco Systems, NET/ADAPTIVE, Northern Telecom,
and Sprint.
PACKET SERVICE
The area between cost and performance is the chief trade-off in linking
local area networks (LANs) and private wide area networks (WANs) into
packet-switching data network (PSDN) services. Packet-services is optimized by an ideal design. Service optimization does not necessarily
translate into picking the service mix that represents the lowest possible
tariffs. Successful packet-service implementations result from adhering to
two basic rules: (1) when implementing a packet-switching solution, be
sure to balance cost savings derived by instituting PSDN interconnections
with the computing community’s performance requirements; and (2)
build an environment that is manageable and that can scale up as more
WAN links are required.
Frame Relay Design
Scalability is one of the chief concerns when designing a Frame Relay implementation. A LAN/WAN system must be able to grow to accommodate
changes as the requirements for remote interconnections grow. The
LAN/WAN system must also provide an acceptable level of performance,
while minimizing maintenance and management requirements. Meeting
all these objectives simultaneously can be quite a balancing act.
DIAL-ON DEMAND ROUTING (DDR)
LAN/WAN connections across public switched telephone networks
(PSTNs) are provided by Dial-on-Demand Routing (DDR). Dedicated
WANs are typically implemented on more modern service provider options or leased lines. These options consist of Frame Relay, Switched
Multimegabit Data Service (SMDS),5 or ATM. Dial-on-Demand Routing.
They provide session control for wide-area connectivity over circuit
switched LAN/WAN, which in turn provides on-demand services and decreased network costs.
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DDR can be used over asynchronous serial interfaces, synchronous
serial interfaces, or Integrated Services Digital Network (ISDN) interfaces.
V.25bis and DTR dialing are used for synchronous modems, switched 56
channel service unit6/data service units7 (CSU/DSUs), or ISDN terminal
adapters (TAs). Asynchronous serial lines are available on the auxiliary
port on Cisco routers and on Cisco communication servers for connections to asynchronous modems. DDR is supported over ISDN using Basic
Rate Interface (BRI)8 and Primary Rate Interface (PRI)9 interfaces.
ISDN
The public switched telephone network (PSTN) has been transformed
into an Integrated Systems Digital Network (ISDN). Such widespread services as caller-ID and dialed-number delivery, 800 directory number
lookup, calling card services, and digital data services have been made
possible by the implementation of Signaling System 7 (SS7) in the PSTN
backbone. Using BRI and PRI services, ISDN call switching can be extended to customer premises equipment (CPE) and provide end-to-end
digital paths.
Before ISDN was available, data connectivity over the PSTN was via
plain old telephone service (POTS) using analog modems. Connectivity
over ISDN offers the LAN/WAN internetworking designer lower signal/noise ratios, increased bandwidth, reduced call setup time, and reduced latency.
LAN/WAN SYSTEM DESIGN SOLUTIONS: DESIGN BASICS
LAN/WAN system design can be a challenging task. For example, as previously mentioned, unpredictable results can occur from a LAN/WAN
system that consists of only 80 meshed routing nodes. This can pose
complex problems. Attempting to optimize LAN/WAN systems that feature thousands of nodes can pose even more complex problems.
LAN/WAN system design is becoming more difficult, despite improvements in equipment performance and media capabilities. The trend is toward increasingly complex environments involving interconnection to
LAN/WANs outside any single enterprise’s dominion of control, multiple
media, and multiple protocols. Carefully designing LAN/WAN systems
can reduce the hardships associated with growth as a networking environment evolves.
Understanding Basic LAN/WAN System Concepts
This part of the article covers the following basic LAN/WAN system concepts: LAN/WAN devices and switching.
LAN/WAN Devices. Cisco recommends four basic types of devices that
are available to designers faced with designing a LAN/WAN. Exhibit 2
summarizes these four LAN/WAN system devices.
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EXHIBIT 2 — A Summary of LAN/WAN System Devices
Device
Description
Hubs
(concentrators)
Hubs (concentrators) are used to connect multiple users to a single
physical device, which connects to the LAN/WAN. Hubs and
concentrators act as repeaters by regenerating the signal as it
passes through them.
Bridges are used to logically separate LAN/WAN segments within
the same network. They operate at the Open System
Interconnection (OSI)a data link layer (Layer 2) and are
independent of higher-layer protocols.
Switches are similar to bridges but usually have more ports.
Switches provide a unique LAN/WAN segment on each port,
thereby separating collision domains. Today, LAN/WAN designers
are replacing hubs in their wiring closets with switches to
increase their network performance and bandwidth while
protecting their existing wiring investments.
Routers separate broadcast domains and are used to connect
different LAN/WANs. Routers direct LAN/WAN traffic based on
the destination network layer address (Layer 3) rather than the
workstation data link layer or Media Access Control (MAC)b
address. Routers are protocol dependent.
Bridges
Switches
Routers
a
International standardization program created by ISO and ITU-T to develop standards
for data networking that facilitate multivendor equipment interoperability.
b Lower of the two sublayers of the data link layer defined by the IEEE.
To be able build LAN/WAN systems, data communications experts
generally agree that network designers are moving away from bridges
and concentrators and primarily using switches and routers. Consequently, this part of the article focuses primarily on the role of switches and
routers in LAN/WAN system design.
Switching. All switching and routing equipment perform two basic operations in today’s data communications. First of all, one has switching
data frames. This is generally a store-and-forward operation in which a
frame arrives an input media and is transmitted to output media. Second,
one has maintenance of switching operations. In this operation, switches
build and maintain switching tables and search for loops. Routers build
and maintain both routing tables and service tables. There are two methods of switching data frames: Layer 2 and Layer 3 switching.
The process of taking an incoming frame from one interface and delivering it out through another interface is called Layer 2 and Layer 3
switching. Switches (Layer 2 switches) use Layer 2 switching to forward
frames, and routers use Layer 3 switching to route a packet.
The type of information inside the frame that is used to determine the
correct output interface is the difference between Layer 2 and Layer 3
switching. Based on MAC address information, frames are switched with
Layer 2 switching. With Layer 3 switching, frames are switched based on
LAN/WAN layer information.
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Like Layer 3 switching, Layer 2 switching does not look inside a packet for LAN/WAN layer information. By looking at a destination MAC address within a frame, Layer 2 switching is performed. It looks at the
frame’s destination address and sends it to the appropriate interface if it
knows the destination address location. Layer 2 switching builds and
maintains a switching table that keeps track of which MAC addresses belong to each port or interface.
To learn the correct destination, if the Layer 2 switch does not know
where to send the frame, it broadcasts the frame out all its ports to the
LAN/WAN. The switch learns the location of the new address and adds
the information to the switching table when the frame’s reply is returned.
The manufacturer of the data communications equipment used determines the Layer 2 addresses. They are unique addresses that are derived
in two parts: the unique identifier and the manufacturing (MFG) code.
The MFG code is assigned to each vendor by the IEEE. The vendor assigns a unique identifier to each board it produces. Except for Systems
Network Architecture (SNA) networks, users have little or no control
over Layer 2 addressing because Layer 2 addresses are fixed with a device, whereas Layer 3 addresses can be changed. In addition, Layer 2 addresses assume a flat address space with universally unique addresses.
The LAN/WAN layer is where Layer 3 switching operates. The Layer 3
switching examines packet information and forwards packets based on
their LAN/WAN-layer destination addresses. Layer 3 switching also supports router functionality.
Layer 3 addresses are determined by the LAN/WAN administrator who
installs a hierarchy on the network for the most part. Layer 3 addressing
is used by protocols such as IP, Internetwork Packet Exchange (IPX),10
and AppleTalk.11 By creating Layer 3 addresses, a LAN/WAN administrator creates local areas that act as single addressing units (similar to
streets, cities, states, and countries), and assigns a number to each local
entity. If users move to another building, their end stations will obtain
new Layer 3 addresses, but their Layer 2 addresses remain the same.
Routers can adhere to and formulate a hierarchical addressing structure as they operate at Layer 3 of the OSI model. A logical addressing
structure can be tied to a physical infrastructure by a routed LAN/WAN.
For example, this can be done through TCP/IP subnets or IPX
LAN/WANs for each segment. Traffic flow in a switched (flat) LAN/WAN
is therefore inherently different from traffic flow in a routed (hierarchical)
LAN/WAN. Hierarchical LAN/WANs offer more flexible traffic flow than
flat networks because they can use the LAN/WAN hierarchy to determine
optimal paths and contain broadcast domains.
So, what are the implications of Layer 2 and Layer 3 switching? The
need for greater bandwidth in traditional shared-media environments has
been driven by the increasing power of desktop processors and the requirements of client/server and multimedia applications. These requireAuerbach Publications
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ments are prompting LAN/WAN designers to replace hubs in wiring
closets with switches.
LAN/WAN designers are now faced with increasing demands for intersubnet communication, although Layer 2 switches use microsegmentation 12 to satisfy the demands for more bandwidth and increased
performance. Cisco recommends that the traffic must go through a Layer
3 device every time a user accesses servers and other resources that are
located on different subnets.13
Cisco recommends that LAN/WAN designers can add Layer 3 capabilities throughout the network to relieve this bottleneck. They are implementing Layer 3 switching on edge devices to alleviate the burden on
centralized routers.
CONCLUSION AND SUMMARY
Today’s growing, fast-changing LAN/WAN systems are like growing communities — the traffic they create tends to cause congestion and delays.
To alleviate these problems, one can design higher-speed LAN/WAN
technologies in a network. This article provided an overview of some of
the key LAN and WAN technologies for creating a high-speed LAN/WAN
environment.
John Vacca is an information technology consultant and internationally known author based in Pomeroy, Ohio.
Since 1982, John has authored 29 books and more than 350 articles in the areas of Internet and intranet security,
programming, systems development, rapid application development, multimedia, and the Internet. John was also
a configuration management specialist, computer specialist, and the computer security official for the NASA
space station program (Freedom) and the International Space Station Program, from 1988 until his early retirement from NASA in 1995. John can be reached at [email protected].
Some of the material in this article has been reproduced by Auerbach Publications with the
permission of Cisco Systems, Inc. Copyright©2000 Cisco Systems, Inc. All Rights Reserved.
Notes
1. Collection of networks interconnected by routers and other devices that functions (generally) as a single
network.
2. SRB is evaluated within two contexts: IBM’s Systems Network Architecture (SNA) and NetBIOS.
3. API used by applications on an IBM LAN to request services from lower-level network processes. These
services might include session establishment and termination, and information transfer.
4. Message sent by one network device to inform another network device that the virtual circuit between
the two is still active.
5. High-speed, packet-switched, datagram-based WAN networking technology offered by the telephone
companies.
6. Digital interface device that connects end-user equipment to the local digital telephone loop.
7. Device used in digital transmission that adapts the physical interface on a DTE device to a transmission
facility such as T1 or E1.
8. ISDN interface composed of two B channels and one D channel for circuit-switched communication of
voice, video, and data.
9. ISDN interface to primary rate access.
10. NetWare network layer (Layer 3) protocol used for transferring data from servers to workstations.
11. Series of communications protocols designed by Apple Computer consisting of two phases. Phase 1,
the earlier version, supports a single physical network that can have only one network number and be
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in one zone. Phase 2 supports multiple logical networks on a single physical network and allows networks to be in more than one zone.
12. Division of a network into smaller segments, usually with the intention of increasing aggregate bandwidth to network devices.
13. In IP networks, a network sharing a particular subnet address. Subnetworks are networks arbitrarily
segmented by a network administrator in order to provide a multilevel, hierarchical routing structure
while shielding the subnetwork from the addressing complexity of attached networks. Sometimes
called a subnet. In OSI networks, a collection of ESs and ISs under the control of a single administrative
domain and using a single network access protocol.
Addresses of Entities Mentioned in this Article
1. Cisco Systems, Inc., 170 West Tasman Drive, San Jose, CA, 95134-1706.
2. International Telecommunication Union (ITU), Place des Nations, CH-1211 Geneva 20, Switzerland.
3. American National Standards Institute, Headquarters: 1819 L Street, NW, Washington, D.C. 20036.
4. ATM Forum, Worldwide Headquarters, 2570 West El Camino Real, Suite 304, Mountain View, CA 940401313.
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