2.4 – Explain the operation of Cisco switches and basic switching concepts
2 – Implement a Small switched Network
2.4 – Explain the operation of Cisco switches and basic switching concepts
Switch Port Settings
A port on a switch needs to be configured with duplex settings that match the media type.
Later in this chapter, you will configure duplex settings. The Cisco Catalyst switches have
three settings:
- The auto option sets autonegotiation of duplex mode. With autonegotiation enabled,
the two ports communicate to decide the best mode of operation.
- The full option sets full-duplex mode.
- The half option sets half-duplex mode.
For Fast Ethernet and 10/100/1000 ports, the default is auto. For 100BASE-FX ports, the
default is full. The 10/100/1000 ports operate in either half- or full-duplex mode when they
are set to 10 or 100 Mbps, but when set to 1,000 Mbps, they operate only in full-duplex mode.
Switch MAC Address Table
Switches use MAC addresses to direct network communications through their switch fabric
to the appropriate port toward the destination node. The switch fabric is the integrated circuits
and the accompanying machine programming that allows the data paths through the
switch to be controlled. For a switch to know which port to use to transmit a unicast frame,
it must first learn which nodes exist on each of its ports.#
The following six steps describe the process used to populate the MAC address table on a
switch:
1. The switch receives a broadcast frame from PC1 on Port 1, as seen in Figure 2-4.
2. The switch enters the source MAC address and the switch port that received the frame
into the address table.
3. Because the destination address is a broadcast, the switch floods the frame to all ports,
except the port on which it received the frame.
4. The destination device replies to the broadcast with a unicast frame addressed to PC1.
5. The switch enters the source MAC address of PC2 and the port number of the switch
port that received the frame into the address table. The destination address of the frame
and its associated port are found in the MAC address table.
6. The switch can now forward frames between source and destination devices without
flooding, because it has entries in the address table that identify the associated ports.
Design Considerations for Ethernet/802.3 Networks
Bandwidth and Throughput
A major disadvantage of Ethernet 802.3 networks is collisions. Collisions occur when two
hosts transmit frames simultaneously. When a collision occurs, the transmitted frames are
corrupted or destroyed. The sending hosts stop sending further transmissions for a random
period, based on the Ethernet 802.3 rules of CSMA/CD.
Collision Domains
When expanding an Ethernet LAN to accommodate more users with more bandwidth
requirements, the potential for collisions increases. To reduce the number of nodes on a
given network segment, you can create separate physical network segments, called collision
domains.
Switches reduce collisions and improve bandwidth use on network segments because they
provide dedicated bandwidth to each network segment.
Broadcast Domains
Although switches filter most frames based on MAC addresses, they do not filter broadcast
frames. A collection of interconnected switches forms a single broadcast domain. Only a
Layer 3 entity, such as a router, or a virtual LAN (VLAN), can bound a Layer 2 broadcast
domain. Routers and VLANs are used to segment both collision and broadcast domains.
When a switch receives a broadcast frame, it forwards the frame to each of its ports, except
the incoming port where the switch received the broadcast frame. Each attached device recognizes
the broadcast frame and processes it. This leads to reduced network efficiency
because a portion of the available bandwidth is utilized in propagating the broadcast traffic.
When two switches are connected, the broadcast domain is increased.
Network Latency
Latency is the time that a frame or a packet takes to travel from the source to the destination.
Users of network-based applications experience latency when they have to wait many
minutes to access data stored in a data centre or when a website takes many minutes to load
in a browser. Latency has at least three sources.
First is the time it takes the source NIC to place voltage pulses on the wire and the time it
takes the destination NIC to interpret these pulses. This is sometimes called NIC delay.
Second is the actual propagation delay as the signal takes time to travel through the cable.
Typically, this is about 0.556 microseconds per 100 m for Cat 5 UTP. Longer cable and
slower nominal velocity of propagation (NVP) result in more propagation delay.
Third, latency is added based on network devices that are in the path between two devices.
These are either Layer 1, Layer 2, or Layer 3 devices.
Latency does not depend solely on distance and number of devices. For example, if three
properly configured switches separate two computers, the computers may experience less
54 LAN Switching and Wireless, CCNA Exploration Companion Guide
latency than if two properly configured routers separated them. This is because routers conduct
more complex and time-intensive operations. For example, a router must analyze
Layer 3 data, whereas switches just analyze the Layer 2 data. Because Layer 2 data is present
earlier in the frame structure than the Layer 3 data, switches can process the frame more
quickly. Switches also support the high transmission rates of voice, video, and data networks
by employing application-specific integrated circuits (ASIC) to provide hardware
support for many networking tasks. Additional switch features such as port-based memory
buffering, port level QoS, and congestion management, also help to reduce network latency.
Network Congestion
The primary reason for segmenting a LAN into smaller parts is to isolate traffic and to
achieve better use of bandwidth per user. Without segmentation, a LAN quickly becomes
clogged with traffic and collisions. The most common causes of network congestion are the
following:
- Increasingly powerful computer and network technologies: Today, CPUs, buses,
and peripherals are much faster and more powerful than those used in early LANs;
therefore, they can send more data at higher rates through the network, and they can
process more data at higher rates.
- Increasing volume of network traffic: Network traffic is now more common because
remote resources are necessary to carry out basic work. Additionally, broadcast messages,
such as address resolution queries sent out by ARP, can adversely affect endstation
and network performance.
- High-bandwidth applications: Software applications are becoming richer in their
functionality and are requiring more and more bandwidth. Desktop publishing, engineering
design, video on demand (VoD), electronic learning (e-learning), and streaming
video all require considerable processing power and speed.
Layer 2 and Layer 3 Switching
In this section, you review the concept of Layer 2 switching and learn about Layer 3
switching.
A Layer 2 LAN switch performs switching and filtering based only on the OSI data link
layer (Layer 2) MAC address. A Layer 2 switch is completely transparent to network protocols
and user applications. Recall that a Layer 2 switch builds a MAC address table that it
uses to make forwarding decisions.
A Layer 3 switch, such as a Catalyst 3560 with an IP Services image, functions similarly to
a Layer 2 switch, such as a Catalyst 2960, but instead of using only the Layer 2 MAC
address information for forwarding decisions, a Layer 3 switch can also use IP address
information.
Instead of learning only which MAC addresses are associated with each of its ports, a Layer
3 switch can also learn which IP addresses are associated with its interfaces. This allows the
Layer 3 switch to direct traffic throughout the network based on IP address information.
Layer 3 switches are also capable of performing Layer 3 routing functions, reducing the
need for dedicated routers on a LAN. Because Layer 3 switches have specialized switching
hardware, they can typically route data as quickly as they can switch data.
It should be emphasized that Layer 3 switches do not completely replace the need for
routers on a network. Routers perform additional Layer 3 services that Layer 3 switches are
not capable of performing. Routers are also capable of performing packet-forwarding tasks
not found on Layer 3 switches, such as establishing remote access connections to remote
networks and devices. Dedicated routers are more flexible in their support of WAN interface
cards (WIC), making them the preferred, and sometimes only, choice for connecting to
a WAN. Layer 3 switches can provide basic routing functions in a LAN and reduce the need
for dedicated routers.
