bridges and switches. The goal is to boost
performance for a workgroup or a backbone. Switches can be used
with hubs to provide the appropriate level of performance for
different users and servers. Another important characteristic
of a LAN switch is how it can allocate bandwidth on a per-port
basis. This will provide more bandwidth to vertical cabling,
uplinks, and servers. This type of switching is referred to as
asymmetric switching. Asymmetric switching provides switched
connections between ports of unlike bandwidth, such as a
combination of 10-Mbps and 100-Mbps ports. The desired capacity
of a vertical cable run is greater than that of a horizontal
cable run. By installing a LAN switch at the MDF and IDF, the
vertical cable run can manage the data traffic from the MDF to
the IDF. The horizontal runs between the IDF and the
workstations uses Category 5e UTP. No horizontal cable drop
should be longer than 100 meters. In a normal environment, 10
Mbps is adequate for the horizontal drop. Use asymmetric LAN
switches to allow for mixing 10-Mbps and 100-Mbps ports on a
single switch. The next task is to determine the number of 10
Mbps and 100 Mbps ports needed in the MDF and every IDF. This
can be determined by reviewing the user requirements for the
number of horizontal cable drops per room and the number of
total drops in any catchment area. This includes the number of
vertical cable runs. For example, suppose that user
requirements dictate four horizontal cable runs to be installed
to each room. The IDF services a catchment area of 18 rooms.
Therefore, four drops in each of the 18 rooms will equal 72 LAN
switch ports. (4x18=72) The size of a collision domain is
determined by how many hosts are physically connected to any
single port on the switch. This also affects how much network
bandwidth is available to any host. In an ideal situation,
there is only one host connected on a LAN switch port. The
collision domain would consist only of the source host and
destination host. The size of the collision domain would be
two. Because of the small size of this collision domain, there
should be virtually no collisions when any two hosts are
communicating with each other. Another way to implement LAN
switching is to install shared LAN hubs on the switch ports,
and connect multiple hosts to a single switch port. All hosts
connected to the shared LAN hub share the same collision domain
and bandwidth. Collisions would occur more frequently. Some
older switches, such as the Catalyst 1700, do not properly
support sharing the same collision domain and bandwidth. The
older switches do not maintain multiple MAC addresses mapped to
each port. As a result, there are many broadcasts and ARP
requests. Shared media hubs are generally used in a LAN switch
environment to create more connection points at the end of the
horizontal cable runs. This is an acceptable solution, but care
must be taken. Collision domains should be kept small and
bandwidth requirements to the host must be provided according
to the specifications gathered in the requirements phase of
the network design process.
Content 5.1
LAN Design 5.1.6 Layer 3 design A router is a
Layer 3 device and is considered one of the most powerful
devices in the network topology.Layer 3 devices can be used to
create unique LAN segments. Layer 3 devices allow communication
between segments based on Layer 3 addressing, such as IP
addressing. Implementation of Layer 3 devices allows for
segmentation of the LAN into unique physical and logical
networks. Routers also allow for connectivity to wide-area
networks (WANs), such as the Internet. Layer 3 routing
determines traffic flow between unique physical network
segments based on Layer 3 addressing. A router forwards data
packets based on destination addresses. A router does not
forward LAN-based broadcasts such as ARP requests. Therefore,
the router interface is considered the entry and exit point of
a broadcast domain and stops broadcasts from reaching other LAN
segments. Routers provide scalability because they serve as
firewalls for broadcasts. They can also provide scalability by
dividing networks into subnetworks, or subnets, based on Layer
3 addresses. When deciding whether to use routers or switches,
remember to ask, "What is the problem that is to be
solved?" If the problem is related to protocol rather than
issues of contention, then routers are the appropriate
solution. Routers solve problems with excessive broadcasts,
protocols that do not scale well, security issues, and network
layer addressing. Routers are more expensive and more difficult
to configure than switches. Figure shows an example of an
implementation that has multiple physical networks. All data
traffic from Network 1 destined for Network 2 has to go through
the router. In this implementation, there are two broadcast
domains. The two networks have unique Layer 3 network
addressing schemes. In a structured Layer 1 wiring scheme,
multiple physical networks are easy to create by patching the
horizontal cabling and vertical cabling into the appropriate
Layer 2 switch. This can be done using patch cables. This
implementation also provides robust security, because all
traffic in and out of the LAN must pass through the router.
Once an IP addressing scheme has been developed for a client,
it should be clearly documented. A standard convention should
be set for addressing important hosts on the network. This
addressing scheme should be kept consistent throughout the
entire network. Addressing maps provide a snapshot of the
network. Creating physical maps of the network helps to
troubleshoot the network. VLAN implementation combines Layer 2
switching and Layer 3 routing technologies to limit both
collision domains and broadcast domains. VLANs can also be used
to provide security by creating the VLAN groups according to
function and by using routers to communicate between VLANs. A
physical port association is used to implement VLAN assignment.
Ports P1, P4, and P6 have been assigned to VLAN 1. VLAN 2 has
ports P2, P3, and P5. Communication between VLAN 1 and VLAN 2
can occur only through the router. This limits the size of the
broadcast domains and uses the router to determine whether VLAN
1 can talk to VLAN 2.
Content 5.2
LAN Switches 5.2.1 Switched LANs, access layer
overview The construction of a LAN that satisfies the needs of
both medium and large-sized organizations is more likely to be
successful if a hierarchical design model is used. The use of a
hierarchical design model will make it easier to make changes
to the network as the organization grows. The hierarchical
design model includes the following three layers: - The
access layer provides users in workgroups access to the
network.
- The distribution layer provides policy-based
connectivity.
- The core layer provides optimal
transport between sites. The core layer is often referred to as
the backbone.
This hierarchical model applies to
any network design. It is important to realize that these three
layers may exist in clear and distinct physical entities.
However, this is not a requirement. These layers are defined to
aid in successful network design and to represent functionality
that must exist in a network. The access layer is the entry
point for user workstations and servers to the network. In a
campus LAN the device used at the access layer can be a switch
or a hub. If a hub is used, bandwidth is shared. If a switch
is used, then bandwidth is dedicated. If a workstation or
server is directly connected to a switch port, then the full
bandwidth of the connection to the switch is available to the
connected computer. If a hub is connected to a switch port,
bandwidth is shared between all devices connected to the hub.
Access layer functions also include MAC layer filtering and
microsegmentation. MAC layer filtering allows switches to
direct frames only to the switch port that is connected to the
destination device. The switch creates small Layer 2 segments
called microsegments. The collision domain can be as small as