Content Overview This module defines
the purpose of VLANS and describes how VLAN implementation can
simplify network management and troubleshooting and can improve
network performance. When VLANs are created, their names and
descriptions are stored in a VLAN database that can be shared
between switches. You will see how design considerations
determine which VLANs span all the switches in a network and
which VLANs remain local to a switch block. The configuration
components of this module describe how individual switch ports
may carry traffic for one or more VLANs, depending on their
configuration as access or trunk ports. This module explains
both why and how VLAN implementation occurs in an enterprise
network.
Content 2.1 Implementing Best
Practices for VLAN Topologies 2.1.1 Describing
Issues in a Poorly Designed Network A poorly designed
network has increased support costs, reduced service
availability, and limited support for new applications and
solutions. Less than optimal performance affects end users and
access to central resources. Here are some of the issues that
stem from a poorly designed network. - Failure
domains: One of the most important reasons to implement an
effective design is to minimize the extent of a network problem
when it occurs. When Layer 2 and Layer 3 boundaries are not
clearly defined, failure in one network area can have a
far-reaching effect.
- Broadcast domains:
Broadcasts exist in every network. Many applications and
many network operations require broadcasts to function
properly. To minimize the negative impact of broadcasts,
broadcast domains should have clearly defined boundaries and
include an optimal number of devices.
- Large amount
of unknown MAC unicast traffic: Cisco Catalyst switches
limit unicast frame forwarding to ports associated with the
specific unicast address. However, frames arriving for a
destination MAC address not recorded in the MAC table are
subsequently flooded out all switch ports, which is referred to
as an “unknown MAC unicast flooding.” Because this causes
excessive traffic on switch ports, network interface cards
(NICs) have to attend to a larger number of frames on the wire,
and security can be compromised as data is propagated on a wire
for which it was not intended.
- Multicast traffic on
ports where not intended: IP multicast is a technique that
allows IP traffic to be propagated from one source to a
multicast group identified by a single IP and MAC destination
group address pair. Similar to unicast flooding and
broadcasting, multicast frames are flooded out on all switch
ports. A proper design contains multicast frames.
- Difficulty in management and support: Because a
poorly designed network may be disorganized, poorly documented,
and lacking easily identifiable traffic flows, support,
maintenance, and problem resolution become time-consuming and
arduous tasks.
- Possible security vulnerabilities:
A poorly designed switched network with little attention to
security requirements at the access layer can compromise the
integrity of the entire network.
A poorly designed
network always has a negative impact and becomes a burden for
any organization in terms of support and related costs.
Content 2.1 Implementing Best Practices for
VLAN Topologies 2.1.2 Grouping Business
Functions into VLANs Hierarchical network addressing means
that IP network numbers are applied to the network segments or
VLANs in an orderly fashion that takes the network as a whole
into consideration. Blocks of contiguous network addresses are
reserved for, and configured on, devices in a specific area of
the network. Here are some benefits of hierarchical addressing.
- Ease of management and troubleshooting:
Hierarchical addressing groups network addresses contiguously.
Network management and troubleshooting are more efficient,
because a hierarchical IP addressing scheme makes problem
components easier to locate.
- Minimized
errors: Orderly network address assignment can minimize
errors and duplicate address assignments.
- Reduced
number of routing table entries: In a hierarchical
addressing plan, routing protocols are able to perform route
summarization, which allows a single routing table entry to
represent a collection of IP network numbers. Route
summarization makes routing table entries more manageable and
provides the following benefits:
- Reduced number of CPU
cycles when recalculating a routing table or sorting through
the routing table entries to find a match
- Reduced
router memory requirements
- Faster convergence after a
change in the network
- Easier troubleshooting
The Enterprise Composite Network Model (ECNM)
provides a modular framework for designing and deploying
networks. It also provides the ideal structure for overlaying a
hierarchical IP addressing scheme. Here are some guidelines to
follow. - Design the IP addressing scheme so that
blocks of 4, 8, 16, 32, or 64 contiguous network numbers can be
assigned to the subnets in a given building distribution and
access switch block. This approach allows each switch block to
be summarized into one large address block.
- At the
Building Distribution layer, continue to assign network numbers
contiguously out toward to the access layer devices.
- Have a single IP subnet correspond with a single VLAN.
Each VLAN is a separate broadcast domain.
- Subnet at
the same binary value on all network numbers, avoiding variable
length subnet masks when possible to minimize errors and
confusion when troubleshooting or configuring new devices and
segments.
For example, a business with approximately
250 employees is looking to move to the enterprise composite
network model. Figure shows the number of users in each
department. Six VLANs are required to accommodate one VLAN per
user community. Therefore, in following the guidelines of the
ECNM, six IP subnets are required. The business has decided to
use network 10.0.0.0 as its base address. The Sales Department
is the largest department, which requires a minimum of 102
addresses for its users. Therefore, a subnet mask of
255.255.255.0 (/24) is chosen, giving a maximum number of 254
hosts per subnet. It has been decided, for future growth, to
have one switch block per building as follows:
- Building A is allocated 10.1.0.0/16.
- Building B is
allocated 10.2.0.0/16.
- Building C is allocated
10.3.0.0/16.
Building A VLANs and IP Subnets
Figure shows the allocation of VLANs and IP subnets within
building A. Building B VLANs and IP Subnets Figure shows
the allocation of VLANs and IP subnets within building B.
Building C VLANs and IP Subnets Figure shows the
allocation of VLANs and IP subnets within building C. Some of
the currently unused VLANs and IP subnets would be used to
manage the network devices. If the company decides to implement
additional technologies, such as IP telephony, some of the
unused VLANs and IP subnets would be allocated to the voice
VLANs.
Content 2.1 Implementing Best
Practices for VLAN Topologies 2.1.3 Describing
Interconnection Technologies A number of technologies are
available to interconnect devices in the campus network. Some
of the more common technologies are listed here. The
interconnection technology selected depends on the amount of
traffic the link must carry. A mixture of copper and
fiber-optic cabling will likely be used, based on distances,
noise immunity requirements, security, and other business
requirements. - Fast Ethernet (100 Mbps
Ethernet): This LAN specification (IEEE 802.3u) operates at
100 Mbps over twisted-pair cable. The Fast Ethernet standard
raises the speed of Ethernet from 10 Mbps to 100 Mbps with
only minimal changes to the existing cable structure. A switch
with port functioning at both 10 and 100 Mbps can move frames