Provisioning Hardware and Media for the LAN
Designing Cisco Networks
Author: Diane Teare
Publisher: Cisco Press (53)
It will take you approximately two hours to complete this chapter and
its exercises. Upon completion of this second chapter in Part
IV you will be able to do the following:
Recognize scalability constraints and issues for standard
Recommend Cisco products and LAN technologies that will meet
a customer's requirements for performance, capacity, and scalability in small-
to medium-sized networks.
Update the network topology drawing you created in the previous
chapter to include hardware and media.
This chapter includes some tables and other job aids that you will find
useful when completing the case studies at the end of the chapter. References
to some Web sites are also included; relevant information has been extracted
from these sites and is provided in the chapter. If you have access to the
Internet, you might want to visit the sites mentioned to obtain detailed information
related to specific topics. All the sites referenced in this chapter are also
listed in Appendix C, “Interesting WWW Links and
Other Suggested Readings.”
The Cisco Product Selection Tool,
available on Cisco's web site, is also referenced in the case studies at the
end of this chapter. An introduction to the tool is provided there, and you
are encouraged to try the tool if you have access to the Internet.
Follow these steps to complete this chapter:
Study the chapter content, including any tables and job aids
Review the case studies at the end of this chapter.
Complete the questions in each case study.
Review the answers provided by our internetworking experts
in Appendix B, “Answers to Chapter Questions, Case
Studies, and Sample CCDA Exam.”
Provisioning LAN media and hardware involves making many decisions,
including which devices to use and what media to use to interconnect these
devices. This chapter discusses these issues in the following sections:
Switching versus Routing in Network Design
Cisco's Catalyst Switches
Selecting Switches, Routers, Access Servers, and Other Cisco
Provisioning Network Media
internetworking devices for small- to medium-sized networks, you need to decide
when LAN switches are appropriate and when routers are appropriate. The sections
that follow provide some information on the different types of services offered
by routers and switches, and how these devices can be used in your design.
If you need internetworking services, routers are needed. Routers
offer the following services:
Broadcast firewalling or filtering
Communication between dissimilar LANs
Quality of Service (QoS) routing
Redundancy and load balancing
Traffic flow management
Multimedia group membership
Some services are becoming available on switches also. For example,
support for multimedia often requires a protocol such as the Internet Group
Management Protocol (IGMP), which allows workstations to join a group that
receives multimedia multicast packets. Cisco now allows Catalyst switches
to participate in this process by using the Cisco Group Management Protocol
(CGMP). One router will still be needed, but you will not need a router in
every department of a company because with CGMP, switches can communicate
with the router to determine whether any users attached to them are part of
a multicast group.
Switching and bridging sometimes result in nonoptimal routing of packets
because packets only travel on paths that are included in the Spanning
Tree. (Recall that the spanning tree is running in order to prevent broadcast
storms in a switched network). When routers are used, the routing
of packets can be controlled and designed for optimal paths. Cisco supports
improved routing and redundancy in switched environments by allowing one
instance of the spanning tree per VLAN.
In general, incorporating switches in small-to medium-sized network
designs will provide the following advantages:
The decision to use an internetworking device depends on which
problems you are trying to solve for your client. Recall from Chapter
2, “Analyzing Small- to Medium-Sized Business Networks”
that customer problems can be categorized as follows:
The decision to use routing or switching depends on the problem to be
solved. Figure 6-1 illustrates
these problems and their solutions, as detailed in the sections that follow.
6-1. The Decision to Use Routing or Switching Depends on the Problem
to be Solved
problems refer to an excessive number of collisions on Ethernet or long waits
for the token in Token Ring or FDDI. Media problems are caused by too many
devices on the media, all with a high load for the network segment. Media
problems can be solved by dividing a network into separate segments, using
one or more switches.
problems are caused by protocols that do not scale well: for example, protocols
that send an excessive number of broadcasts. Protocol problems can be solved
by dividing a network into separate segments, using one or more routers.
category of problems includes the need to offer voice and video network services.
These services may require much more bandwidth than is available on a customer's
network or backbone. Transport problems can be solved by using high-bandwidth
technologies, such as Fast Ethernet or ATM.
domain, known as a collision domain for Ethernet LANs, includes
all devices that share the same bandwidth. For example, when using switches
or bridges, everything associated with one port is a bandwidth domain.
A broadcast domain includes all devices that see
each other's broadcasts (and multicasts). For example, all the devices associated
with one port on a router are in the same broadcast domain.
Devices in the same bandwidth domain are also in the same broadcast
domain; however, devices in the same broadcast domain may be in different
All workstations within one bandwidth domain compete for the same LAN
bandwidth resource. All traffic from any host in the bandwidth domain is visible
to all the other hosts. In the case of an Ethernet collision domain, two stations
may transmit at the same time, causing a collision. The stations then have
to stop transmitting and try again at a later time, resulting in a delay in
transmitting the traffic.
All broadcasts from any host in the same broadcast domain are visible
to all other hosts in the same broadcast domain. Desktop protocols, such as
AppleTalk, NetBIOS, IPX, and IP, require broadcasts or multicasts for resource
discovery and advertisement. Hubs, switches, and bridges forward broadcasts
and multicasts to all ports. Routers do not forward these broadcasts or multicasts
to any ports. In other words, routers block broadcasts (destined for all
networks) and multicasts; they only forward unicast packets
(destined for a specific device) and directed broadcasts
(destined for all devices on a specific network).
When analyzing your customer's current network
and future needs, as discussed in Part III of this
book, determine whether bandwidth domains need to be segmented using switches.
As noted in Chapter 3, “Characterizing the
Existing Network,” Ethernet networks should be segmented if
the network utilization is above approximately 40 percent for long periods
of time. Token Ring and FDDI networks should be segmented if network utilization
is above approximately 70 percent for long periods of time.
If there are too many hosts on a LAN, broadcast radiation can cause
performance degradation. Broadcast radiation
refers to the way that broadcasts and multicasts radiate from the source to
all connected LANs in a flat network, causing all hosts on the LAN to do extra
processing. When broadcasts and multicasts are more than approximately 20
percent of the traffic on a LAN, performance degrades.
As seen in Chapter 4, “Determining New Customer
Requirements,” there are guidelines for an upper limit on the
number of workstations on a LAN or VLAN, before broadcast radiation overwhelms
the CPUs of the hosts. These limits are shown again in Table
6-1. Actual workstation limits depend on factors such as:
IP addressing constraints
Inter-VLAN routing requirements
Management and fault isolation constraints
Traffic flow characteristics
When connecting LANs or VLANs via one or more routers, you need to understand
the characteristics of the network traffic. It is also important to understand
the performance of the router(s) you plan to use in the network design. You
can find out more about router performance and capacity in the next chapter, “Provisioning
Hardware and Media for the WAN.”
Campus LANs are easiest to design when the traffic
obeys the 80/20 rule, which states that 80 percent of traffic is local to
a LAN or VLAN and only 20 percent of the traffic goes to a different LAN or VLAN. This
is the case when users primarily access departmental servers and the LANs
or VLANs are subdivided by department. However, with the emergence of server
farms and corporate web servers, the 80/20 rule does not always apply. In
these cases, it is important to provision bandwidth, switches, and routers
carefully to avoid congestion and poor performance.
Table 6-2 summarizes the discussion
of switching Versus routing.
Table 6-2. Summary of Considerations for Switches versus Routers
Media (LAN switches),
and Transport of large payload (Fast Ethernet and ATM switches) problems
Many, including filtering, addressing, connecting dissimilar
LANs, security, load balancing, policy and QoS routing, multimedia
High bandwidth, low cost, ease of configuration
Broadcast and Bandwidth Domains
broadcast domain and bandwidth (collision) domain
bandwidth (collision) domain
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Broadband, Second Edition
by George Abe
Introduces the topics surrounding high-speed networks
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