Transcript Integrating Cisco Press Resources into the Academy Classroom

Networking Basics CCNA 1
Chapter 8
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Ethernet Switch Operations
Layer 2 Bridging and Switching Operations
• Earliest networking devices were repeaters and
hubs
• Multiple LAN segments could be connected to
make larger LANs, within 5-4-3 design rules
• As it became apparent that reducing size of
collision domains was important, bridges were
created
• Bridges are aware of Ethernet framing and
Layer 2 MAC addressing (IEEE 802.3)
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Ethernet Switch Operations
Layer 2 Bridging and Switching Operations
• Bridges extend LAN distances, without some of
the negative effects of repeaters and hubs
• Bridges were typically much more expensive
than repeaters and hubs (were usually a PC
running software to perform the bridging
function)
• Bridges usually had only two interfaces, where
hubs had multiple ports
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Ethernet Switch Operations
Layer 2 Bridging and Switching Operations
• Next major step in LAN devices was the LAN
switch
– Does the same thing as a bridge
– Instead of using software, process could be done with
a chip (sometimes called application-specific
integrated circuits – ASICs)
– Switches have more interfaces than bridges, are
smaller, and do the same work faster
– As switch prices fell, bridges disappeared from the
market
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Ethernet Switch Operations
The Forwarding and Filtering Decision
• Repeaters and hubs simply react to the
incoming signal
– make no decisions and require no
programming logic
– Receive, regenerate and send signal out all
ports except the one on which it was received
• Bridges implemented their logic in
software
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Ethernet Switch Operations
The Forwarding and Filtering Decision
• Switches implement their logic in hardware
– Run much faster than bridges
– Cisco makes switches that can forward
hundreds of millions of Ethernet frames per
second
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Ethernet Switch Operations
The Forwarding and Filtering Decision
• Filtering and forwarding logic
– Examine incoming signal; interpret as 0s and
1s (OSI Layer 1 standards)
– Interpret the received bits based on Ethernet
framing rules; find MAC destination address in
frame (OSI Layer 2 standards, IEEE 802.3
MAC sublayer)
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Ethernet Switch Operations
The Forwarding and Filtering Decision
• Filtering and forwarding logic (continued)
– Examine table that maps MAC addresses with
corresponding interfaces
• Find table entry that matches the destination
MAC address of frame
• If frame came in on a different interface than the one listed on
the table, process is called forwarding the frame
• If the frame came in on the same interface as the one it was
received on, discard it (this is called filtering)
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Ethernet Switch Operations
The Forwarding and Filtering Decision
• The table a bridge or switch refers to may
be called:
– Bridging table
– Switching table
– MAC address table
– Forwarding table
– Content Addressable Memory (CAM) table
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Ethernet Switch Operations
A Bridge Filtering Decision Based on the CAM
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Ethernet Switch Operations
A Bridge Forwarding Decision Based on the CAM
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Ethernet Switch Operations
Learning CAM Table Entries and Flooding
Unknown Unicasts
• Switches and bridges learn entries in the CAM
dynamically
• They use this logic:
– Examine the source MAC address of the frame and
the interface on which it was received
– Add that source MAC address and corresponding
interface to the table
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Ethernet Switch Operations
Learning
CAM
Table
Entries:
One
Switch
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Ethernet Switch Operations
Learning CAM Table Entries: Two Switches
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Ethernet Switch Operations
Handling Unknown Unicasts
• Switches typically learn CAM entries for all
working devices on the LAN as soon as those
devices start sending data
• Sometimes a switch receives a frame that does
not have a CAM entry – this is an unknown
unicast frame
• The switch sends the unknown unicast frame
out all ports, a process called flooding
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Ethernet Switch Operations
Forwarding Broadcasts and Multicasts
• Unicast frame has a destination MAC address
of a single NIC or interface
• Broadcast frames are sent to a destination MAC
address of FFFF.FFFF.FFFF.FFFF and are
delivered to all devices on the LAN
• Multicast frames are sent to one of a range of
MAC addresses
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Ethernet Switch Operations
Flooding
Unknown
Unicasts
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Ethernet Switch Operations
Forwarding Broadcasts and Multicasts
• Multicast addresses provide a way to send
certain frames to a subset of devices
– Streaming video
• Some low-end switches flood multicasts like
broadcasts
• Higher-end switches allow multicasting, making
the process more efficient
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Ethernet Switch Operations
Different Forwarding Behavior for Multicasts
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Ethernet Switch Operations
The Cisco Switch CAM
• All switches and bridges use some table that
lists the MAC address and port through which
each MAC address can be reached
• Cisco calls this the CAM (Content Addressable
Memory)
• The MAC address is input into the memory and
CAM instantly outputs the table entry
• This process occurs quickly, every time,
regardless of table size
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Ethernet Switch Operations
Switch Internal Processing
• The amount of time it takes for a frame to
progress through a network from one device to
another is called latency
• Some factors that affect latency cannot be
improved, such as propagation delay (the
amount of time it takes for electricity to go from
one end of the network to another)
• Other types of delay vary with network
conditions; frames may be waiting in a buffer
(queuing delay)
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Ethernet Switch Operations
Switch Internal Processing – Factors that
Impact Latency
• The finite speed that signals can travel
(propagation delay)
• Circuit delays caused by electronics
• Software delays caused by software decisions
being made
• Delays caused by frame contents and location
of the frame switching decisions
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Ethernet Switch Operations
Store-and-Forward Switching
• Switch receives entire frame before forwarding it
• Advantages of store-and forward switching
– FCS field is at end of frame; frame can be checked
for an error
– Can check for rare error in which the 802.3 Length
field does not match the Data field length
– Can forward between ports running at different
speeds (asymmetric switching)
• Disadvantage
– More latency than other switching types
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Ethernet Switch Operations
Cut-Through Switching
• Destination MAC address is located at beginning of
Ethernet frame
• Advantage of cut-through switching
– Once destination MAC address is read, switch can
begin forwarding frame
– Less latency than store-and-forward
• Disadvantages of cut-through switching
– Cannot check FCS; may forward frames with errors
– Forwards before some legitimate collisions have
occurred
– Only works with symmetric switching
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Ethernet Switch Operations
Fragment-Free Switching
• Overcomes a problem that cut-through
switching has: cut-through is too fast
– Collisions should occur while a frame’s first
64 bytes are being transmitted
– Cut-through switching often begins
transmitting before 64 bytes are received
– Cut-through switching can forward collision
fragments
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Ethernet Switch Operations
Fragment-Free Switching
• Fragment-free switching waits until it has
received first 64 bytes to begin
transmitting
• Ensures switch does not forward frames
that have collided
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Ethernet Switch Operations
Cisco Enterprise Switch – Internal
Processing Paths
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Ethernet Switch Operations
Spanning Tree Protocol
• Most LAN design include redundant physical
paths
• A trunk is a link between two switches;
sometimes called a backbone link
• Spanning tree protocol (STP) prevents
switching loops from the logic used to forward
unknown unicast and broadcast frames
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Ethernet Switch Operations
Typical Enterprise Campus Building Block
Design, with Redundancy
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Ethernet Switch Operations
The
Problem
That STP
Solves:
Switching
Loops
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Ethernet Switch Operations
The Problem That STP Solves: Switching Loops
• In previous slide, if PC1 sends a broadcast, it
goes around LAN in both directions
• Each switch broadcast the frame(s) out every
port (except the one on which it was received)
• This process continues for a long time,
continuing until no other traffic can be sent over
the LAN: a “broadcast storm”
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Ethernet Switch Operations
STP Protocol: STP Blocking
• STP makes some ports quit forwarding or receiving
frames
• An interface that is not allowed to process traffic by STP
is considered to be in an STP blocking state
• In the figure that follows, SW3’s port 1 is in a blocking
state – it receives the broadcast frame but ignores it
• STP causes the LAN to use particular paths and leaves
others idle and unused
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Ethernet Switch Operations
IEEE 802.1D STP Interface States
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Ethernet Switch Operations
IEEE 802.1D STP Interface States
• The forwarding and blocking states are the
most common, because a working network
interface stabilizes into one of these states
• Failed interfaces stabilize into a disabled state
• Listening and learning states are used to solve
problems with CAM tables
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Ethernet Switch Operations
Stable STP Topology and Switch CAMs in a
Three-Switch Network
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Ethernet Switch Operations
Changing the CAM with the Listening and
Learning States
• The topology can fail when a trunk fails or when a new
trunk comes up
• STP determines the topology by having switches send
bridge protocol data units (BPDUs) to each other
• BPDUs and the Spanning Tree Algorithm (STA) are part
of the IEEE 802.1D standard
• Information learned allows the switches to determine
the topology and decide which interfaces should
forward and which should block frames
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Ethernet Switch Operations
Changing the CAM with the Listening and
Learning States
• The listening and learning states are used by STP
when it needs to transition to a new topology
• An STP topology refers to the topology of the network
when each interface is in one of three stable states
• STP remains in the stable topology until something
happens
– A trunk goes down (perhaps cut)
– The network engineer shuts down a trunk
– A new switch is added
– An interface fails
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Ethernet Switch Operations
Changing the CAM with the Listening and
Learning States
• Switches use listening and learning states as
interim states when transitioning an interface
for two reasons:
– For the switches’ CAM table entries to time out
(during the listening state)
– For the switches to relearn the MAC addresses and
(possibly different) interfaces used to reach the MAC
addresses
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Ethernet Switch Operations
A New STP Topology After a Failure
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LAN Design: Collision Domains and
Broadcast Domains
Collision Domains
• A collision domain is a set of LAN interfaces for
which a frame sent out any two of these
interfaces, at the same time, would cause a
collision
• Hubs repeat signals out interfaces and do not
consider CSMA/CD logic, so any frames sent
simultaneously will collide
• The terms shared bandwidth and shared media
refer to the fact that the devices in a hubbed
network share the same media and bandwidth
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LAN Design: Collision Domains and
Broadcast Domains
One Collision Domain with One 10BASE-T Hub
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LAN Design: Collision Domains and
Broadcast Domains
Large/Long Collision Domains
• The 5-4-3 (or 5-4-3-2-1) Rule for 10BASE-T
networks
–
–
–
–
5 segments of network media
4 repeaters or hubs at most
3 links at most, between two end-user devices
If 5 segments exist between two end-user devices, 2
segments must not have any end-user devices
connected to them
– It’s all 1 large collision domain
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LAN Design: Collision Domains and
Broadcast Domains
One Collision Domain with Multiple
10BASE-T Hubs
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LAN Design: Collision Domains and
Broadcast Domains
Large/Long Collision Domains
• The 5-4-3-2-1 rule for 10BASE-T restrictions
are required due to the round-trip time of the
collision domain
• Within one collision domain, all the devices
share the 10 Mbps of bandwidth
• Within one collision domain, a (practically)
simultaneous transmission of a frame by two or
more PCs results in a collision
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LAN Design: Collision Domains and
Broadcast Domains
Large/Long Collision Domains
• The more PCs in a collision domain, the less
efficient it is
• The more frames, the more collisions
• The more collisions, the more time sent waiting
to resend frames
• Once a LAN reaches about 30-40% of
bandwidth utilization, the number of collisions
increases dramatically
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LAN Design: Collision Domains and
Broadcast Domains
High LAN Utilization Resulting in Much Higher
Percentage of Collisions
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LAN Design: Collision Domains and
Broadcast Domains
Large/Long Collision Domains
• Large collision domains should not be
used for the following reasons:
– Shared bandwidth – as the size of the
collision domain grows, each device has less
available bandwidth
– Higher utilization – the more devices in a
single collision domain, the better the chance
of a collision and of driving the utilization rate
higher
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LAN Design: Collision Domains and
Broadcast Domains
Creating Many Small Collision Domains
• Significantly reduces the negative effects of a
large collision domain
• Process of breaking a LAN into multiple
collision domains is called segmentation
• Switches, bridges, and routers can segment
LANs into multiple collision domains
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LAN Design: Collision Domains and
Broadcast Domains
Two LANs with Many Small Collision Domains
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LAN Design: Collision Domains and
Broadcast Domains
Creating Many Small Collision Domains
• Benefits of segmenting 10BASE-T LANs:
– Design rules (5-4-3-2-1) apply to each individual
collision domain
– With smaller collision domains, reaching the point of
utilization where performance is degraded is less
likely
– Each domain gets its own bandwidth, so fewer
devices are sharing the available bandwidth
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LAN Design: Collision Domains and
Broadcast Domains
Creating Many Small Collision Domains
• When switches are used on the LAN, the terms
switched LAN and switched bandwidth are
used
– Each switch port connects to a separate collision
domain
– Connecting a single end-user device to each switch
port is a process called microsegmentation
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LAN Design: Collision Domains and
Broadcast Domains
Creating Many Small Collision Domains
• Microsegments meet the requirements to allow
full duplex
– Full duplex gives twice the bandwidth
– A 24 port 10BASE-T hub shares 10 Mbps of
bandwidth among 24 ports
– A 24 port 10BASE-T switch gives each port 20 Mbps
of bandwidth
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LAN Design: Collision Domains and
Broadcast Domains
Main Benefits of Using Many Small
Collision Domains
• Collision domain design rules are easier to
achieve
• Smaller collision domains reduce the probability
of LAN overutilization
• Each collision domain gets its own separate
switched bandwidth
• With a collision domain consisting of only two
interfaces/NICs, full duplex can be used
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LAN Design: Collision Domains and
Broadcast Domains
How Switches and Bridges Prevent
Collisions
• Switches reduce or prevent collisions by
buffering or queuing frames
• Repeaters and hubs do not perform buffering
• Bridges, switches and routers follow CSMA/CD
rules if not using full duplex
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LAN Design: Collision Domains and
Broadcast Domains
Switch Buffering Example
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LAN Design: Collision Domains and
Broadcast Domains
Layer 2 Broadcast Domains
• A broadcast domain is:
– The set of LAN interfaces (including NICs
and network device interfaces) for which a
broadcast frame sent by one device with be
forwarded to all other interfaces in that same
broadcast domain
– Bridges and switches forward broadcasts
– Routers do not forward broadcasts
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LAN Design: Collision Domains and
Broadcast Domains
One Router
Creating
Two
Broadcast
Domains
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LAN Design: Collision Domains and
Broadcast Domains
Performance Impact of Multicast and
Broadcast Domains
• PC NICs see all frames on the LAN
• PC NICs can ignore unicast frames not for
them
• PC NICs must send multicast and broadcast
frames to their CPU for processing, which
affects PC performance
• This is less of an issue today with fewer
proprietary network protocols doing broadcasts
and with more powerful processors
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LAN Design: Collision Domains and
Broadcast Domains
NIC Giving Broadcasts and Multicasts to the CPU
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LAN Design: Collision Domains and
Broadcast Domains
More Broadcasts, Less CPU Capacity for
End-User Work
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LAN Design: Collision Domains and
Broadcast Domains
The Impact of Broadcasts and
Multicasts Today
• Biggest risk is in wasting CPU cycles
from multicasts
– Switches flood multicasts just like broadcasts
– LAN engineers must enable multicast
optimization tools in switches to prevent
switches from flooding multicasts to every
device in the LAN
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LAN Design: Collision Domains and
Broadcast Domains
The Impact of Broadcasts and Multicasts
Today
• Broadcasts such as RIP and ARP don’t cause
problems in today’s networks, but did in the
past when networks were slower
– ARP remembers the info it learns, so an individual
PC might not send one ARP per minute
– RIP broadcasts may be sent by routers and UNIX
workstations; now most UNIX workstations have it
turned off by default so these are no longer an issue
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LAN Design: Collision Domains and
Broadcast Domains
Identifying Networking Devices by OSI Layer
• Repeaters and hubs are Layer 1 devices
• Bridges and switches are Layer 2 devices
• Routers are Layer 3 devices
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LAN Design: Collision Domains and
Broadcast Domains
Sample Network with Collision Domains
and Broadcast Domains Shown
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LAN Design: Collision Domains and
Broadcast Domains
Data Flow with Layer 1, Layer 2, and Layer 3
Devices
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LAN Design: Collision Domains and
Broadcast Domains
The Ambiguous Term Segment
• Three main uses of the term segment
– LAN concepts – a segment is a collision domain
– LAN (physical) – in a LAN using a bus topology, a
segment is a continuous electrical circuit, often
connected to other segments with repeaters
– TCP – the process of taking a large piece of data
and breaking it into smaller pieces; one of those
pieces
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Summary
• Bridges and switches work the same way
regarding basic forwarding, learning, flooding
and STP
– They build forwarding tables by examining the
source MAC addresses of incoming frames
– They make filtering and forwarding decisions by
looking at the destination MAC address of the frame
and comparing it to the table
– They flood broadcast frames and also flood multicast
frames, unless optimization features have been
enabled
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Summary
• Switches differ from bridges
– They have much more powerful hardware
– They use content addressable memory (CAM) to
hold the switching table
– The CAM allows the switch to find a MAC address
and its associated port very quickly every time
• Latency is the time that passes as a frame or
packet is sent through the network
• Propagation delay is the time it takes for
electrical or optical energy to pass over the
cable, and contributes to latency
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Summary
• Three internal switch processing options:
– Cut-through switching begins forwarding the frame
as soon as the destination MAC address is read;
does not check FCS to determine if frame is good;
low latency
– Store-and-forward switching receives the entire
frame; does error-checking; necessary for
asymmetrical switching
– Fragment-free switching waits for the first 64 bytes to
be received before beginning forwarding; enables it
to detect normal collisions
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Summary
• Switches and bridges use Spanning Tree
Protocol (STP) to identify and block redundant
paths through the network; gives a logical path
with no loops
• A collision domain with a single device
connected to a switch port is called a
microsegment
– Microsegments use UTP cabling, allow the use of full
duplex
– With no collisions possible, CSMA/CD can be
disabled
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Summary
• Placing a large number of PCs in a collision domain
increases demand for bandwidth
– This increases possibility of collisions
– Breaking large collision domains into multiple
smaller collision domains reduces the chance of
collisions while adding bandwidth
– Separating LANs into more segments by using
bridges and switches creates additional collision
domains, one per bridge and switch port
• Broadcast domains are a set of devices in which if one
device sends a broadcast, all other devices receive the
broadcast; Layer 3 devices (routers) separate
broadcast domains
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