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www.ciscopress.com Networking Basics CCNA 1 Chapter 8
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Networking Basics CCNA 1Chapter 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
