Transcript Integrating Cisco Press Resources into the Academy Classroom

Switching Basics and Intermediate
Routing CCNA 3
Chapter 4
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Switching Concepts
Introduction
• Fifteen years ago, network designers used hubs
and bridges to build networks
• Now, switches and routers are the key
components in LAN design
• Problem with earlier network designs:
– Too many repeaters, hubs hurt performance due to
too many devices on the network segment
• Engineers began using bridges to segment
networks into multiple collision domains
• The bridge evolved into the modern switch
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Switching Concepts
Introduction
• Switches and bridges make nearly instantaneous
decisions on whether to forward or filter frames
• Switches are Layer 2 devices that increase available
bandwidth and reduce network congestion
• A switch can segment a network into microsegments,
which are segments with only a single host
– The number of collision domains is increased with
switches (a good thing)
– All hosts remain part of the same broadcast domain
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Introduction to LAN Switching
LAN Segmentation
• A network can be divided into smaller
units called segments
• Each segment uses carrier sense multiple
access with collision detection
(CSMA/CD)
• Each segment is its own collision domain
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Introduction to LAN Switching
LAN Segmentation
Bridges, Switches, and Routers Provide LAN Segmentation
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Introduction to LAN Switching
LAN Segmentation
• Segmentation significantly reduces network congestion on
each segment
• When data transmits within a segment, the bandwidth is
shared
– Fewer devices in a segment means each device has a larger share
of the bandwidth
• Bridges are Layer 2 devices that forward frames based on
Media Access Control (MAC) addresses
– They read the source MAC address to discover the devices on a
segment
– The source MAC addresses are used to build the bridge table
– Frames with a destination not in the table are flooded out all ports
except the port on which they were received
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Introduction to LAN Switching
LAN Segmentation
MAC Addresses Are Used for Communication Within
an Ethernet Collision Domain
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Introduction to LAN Switching
LAN Segmentation
Interface-to-MAC-Address Mapping
(from previous slide)
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Introduction to LAN Switching
LAN Segmentation
• Bridges increase latency by 10 to 30 percent
– A bridge is a store-and-forward device
– To check for errors in the frame, bridges examine the
Destination Address field and calculate the cyclic
redundancy check (CRC) in the Frame Check
Sequence (FCS) field before the frame is forwarded
• If a destination port is busy, bridges store the
frame temporarily
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Introduction to LAN Switching
LAN Segmentation
• Segmentation with bridges has these
characteristics:
– Provides fewer users per segment
– Stores, then forwards frames based on Layer 2
addresses
– Independent of Layer 3 protocols
– Increases latency on the network
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Introduction to LAN Switching
LAN Segmentation
• Routers provide network segmentation and add a
latency factor of 20 to 30 percent over a switched
network
– Routers operate at the network layer and use IP
addresses to determine best paths to destination
networks
– Routers require more software processing than
switches
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Introduction to LAN Switching
LAN Segmentation
• Segmentation with routers has these
characteristics:
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More manageable
Greater functionality
Multiple active paths to destination
Smaller broadcast domains
Operate at Layer 3
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Introduction to LAN Switching
LAN Segmentation
• Switches decrease bandwidth shortages and bottlenecks
• Switches segment LANs into microsegments, which
decreases the size of collision domains
• Segmentation with switches has these characteristics:
– Eliminates impact of collisions through microsegmentation
– Low latency and high frame-forwarding rates at each port
– Works with existing 802.3 (CSMA/CD) compliant network interface
cards (NICs) and cabling
• In a completely switched Ethernet LAN, source and
destination nodes act as if they are the only devices on the
network; maximizes bandwidth
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Introduction to LAN Switching
Basic Operations of a Switch
• Switches perform these two basic
operations:
– Switch data frames: process of receiving a
frame on one interface, selecting the correct
forwarding port, and forwarding the frame
– Maintain switch operations:
• Build and maintain forwarding tables
• Construct and maintain a loop-free topology across a
LAN
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Introduction to LAN Switching
Basic Operations of a Switch
Frame Generated by A is Destined for B
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Introduction to LAN Switching
Basic Operations of a Switch
MAC Address of A is Added to the MAC Address
Table and the Frame is Flooded
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Introduction to LAN Switching
Basic Operations of a Switch
Unicast Ethernet Frame Transmission Toward
Ingress Port
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Introduction to LAN Switching
Basic Operations of a Switch
Unicast Ethernet Frame Transmission From Egress Port
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Introduction to LAN Switching
Basic Operations of a Switch
• Switch characteristics (recap):
– Forwards frames based on MAC address table
– MAC address table maps MAC addresses to port
numbers
– Operate at Layer 2
– Entries into MAC address table are taken from source
addresses of ingress frames
– If destination MAC address is not in address table, frame
is flooded out all ports except ingress port
– Frame filtering is the process of sending unicast frames
to out a single port based on MAC address entries;
preserves bandwidth
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Introduction to LAN Switching
Ethernet Switch Latency
• Switch latency is the time from when a frame
enters a switch until it leaves
– Directly related to the configured switching
process and the volume of traffic
• Latency is measured in fractions of a second
– Network devices operate at incredibly high
speeds
– Every additional millisecond (ms) of delay
adversely affects network performance
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Introduction to LAN Switching
Layer 2 and Layer 3 Switching
• There are two primary methods of switching data
frames:
– Layer 2: Used by bridges and Layer 2 switches
– Layer 3: Used by routers and Layer 3 switches
• The difference between the two types is the type of
information inside the frame that determines the
correct output interface
– Layer 2 switching is based on MAC address information
– Layer 3 switching is based on network layer addresses
(IP) addresses
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Introduction to LAN Switching
Layer 2 and Layer 3 Switching
• The only major difference between the
packet-switching operation of a router and a
Layer 3 switch is the physical
implementation
– In general-purpose routers, packet switching
takes place in software
– In a Layer 3 switch, the switch does forwarding
with application-specific integrated circuit (ASIC)
hardware
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Introduction to LAN Switching
Layer 2 and Layer 3 Switching
• Characteristics of Layer 2 switching:
– Hardware-based switching
– Wire-speed performance
– Low latency
– Uses MAC addresses
– Low cost
– Switching table is contained in Content
Addressable Memory (CAM)
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Introduction to LAN Switching
Layer 2 and Layer 3 Switching
• Characteristics of Layer 3 switching:
– Hardware-based packet forwarding
– High-performance packet switching
– Low latency
– Lower per-routed-port cost
– Flow-based accounting
– Robust security
– Full QoS support
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Introduction to LAN Switching
Symmetric and Asymmetric Switching
• Symmetric and Asymmetric Switching differences
are based on how bandwidth is allocated:
– Symmetric switch provides connections between ports of
the same bandwidth
• Multiple simultaneous conversations increase throughput
– Asymmetric switching allows more bandwidth to be
dedicated to a server or backbone port to prevent a
bottleneck
• Memory buffering is required to keep frames in proper sequence
between ports with different data rates
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Introduction to LAN Switching
Symmetric and Asymmetric Switching
• An Ethernet switch might use one of two types of
buffering:
– In port-based memory buffering frames are in queues
linked to specific incoming ports
• A frame is transmitted to the outgoing port after all frames ahead
of it are transmitted
– In shared memory buffering, all frames are held in a
common memory shared by all ports on the switch
• The amount of memory is dynamically allocated
• Works well with asymmetric switching
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Introduction to LAN Switching
Frame Transmission Modes
• Ethernet switches operate in three primary modes:
– Store and forward: the entire frame is received,
destination and source addresses are checked, the CRC
is performed, filters are applied, and the frame is
forwarded
• High latency as entire frame must be received
– Cut-through: the destination address is read and the
switch immediately begins forwarding the frame
• Lowest latency, but does not do error-checking
– Fragment-free: switch reads first 64 bytes and begins
forwarding frame
• Can detect frame fragments and discard them
• Higher latency than cut-through
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Introduction to LAN Switching
Frame Transmission Modes
Frame Transmission Modes Include Cut-Through,
Store and Forward, and Fragment-Free
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Switch Operation Modes
Introduction
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•
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LAN switches are considered multiport bridges
No collision domain because of microsegmentation
Low latency level, high rate of speed for frame forwarding
Two devices connected to a switch form a virtual
connection within the switch
– This connection is established only when needed
• LAN switches create separate collision domains but all
hosts are on the switch are in the same broadcast domain
– Broadcasts to one node are received by all nodes
• Switching is performed in hardware instead of software
• Switches flood and forward traffic based on MAC
addresses
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Switch Operation Modes
Broadcast Domains
• Communication on a network occurs in three ways:
– Unicast transmission: one transmitter to one receiver
– Multicast transmission: one transmitter to a subset (group) of the
entire segment
– Broadcast transmission: one transmitter to all network receivers
• A Layer 2 broadcast has a destination MAC address of
FF:FF:FF:FF:FF:FF
– All devices must receive and process this frame
• A switch forwards a broadcast to each port except the
incoming port
– Broadcasting consumes networking bandwidth
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Switch Operation Modes
Broadcast Domains
Broadcast Domains Encompass Collision Domains
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Switch Operation Modes
Communication Between Switches and
Workstations
• Crossover cables can connect the following devices:
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Workstation to workstation
Switch to switch
Switch to hub
Hub to hub
Router to router
Router to PC
• Straight-through cables connect the following devices:
– Switch to router
– Switch to workstation or server
– Hub to workstation or server
• Many modern switches automatically adjust the port type
to support the type of cable attached
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Summary
• Segmentation divides a network into smaller units
– Reduces network congestion
– Enhances security
• CSMA/CD on each segment maintains traffic betweens
users
• Segmentation with a Layer 2 bridge is transparent to
users, but latency is significantly increased
• Routers provide segmentation of networks but can add 20
to 30 percent latency over a switched network
– A router operates at the network layer and uses IP addresses to
determine best paths
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Summary
• A switch can segment a network into microsegments
– Reduces the size of collision domains
– All hosts are still connected to the same broadcast domain
• Switching decreases congestion in Ethernet LANs
• Switching is the process of receiving frames on one
interface and delivering them out another interface
• Some routers now use Layer 3 switching to route packets;
switches user Layer 2 switching to forward frames
• A symmetric switch has ports with all the same bandwidth
• An asymmetric switch has ports of unlike bandwidth
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Summary
• A memory buffer is an area of memory where a switch
stores data
– It can use two methods to forward frames: port-based memory
buffering or shared memory buffering
• Three frame transmission modes are used to forward
frames:
– Store and forward receives the entire frame before forwarding;
highest latency
– Cut-through forwards the frame as soon as the destination MAC
address is read, decreasing latency
– Fragment-free reads the first 64 bytes (minimum Ethernet frame
size) before forwarding the frame; prevents the forwarding of
Ethernet collision fragments; medium latency
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